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  1. 1. ZJ BRAKES 5-1 BRAKES CONTENTS page pageABS BRAKE DIAGNOSIS . . . . . . . . . . . . . . . . . . . 3 MASTER CYLINDER—BRAKE BOOSTER—ABS SYSTEM OPERATION . . . . . . . . . . . . . . . . 10 COMBINATION VALVE—HCU . . . . . . . . . . . . . 18BRAKE FLUID—BRAKE BLEEDING— PARKING BRAKES . . . . . . . . . . . . . . . . . . . . . . . 51 BRAKELINES/HOSES . . . . . . . . . . . . . . . . . . . 15 REAR DISC BRAKES . . . . . . . . . . . . . . . . . . . . . 41BRAKE PEDAL AND BRAKELIGHT SWITCH . . . 61 SERVICE BRAKE DIAGNOSIS . . . . . . . . . . . . . . . 4DISC BRAKE ROTOR SERVICE . . . . . . . . . . . . . 48 SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . 63FRONT DISC BRAKES . . . . . . . . . . . . . . . . . . . . 33 SPEED SENSORS—TONE WHEELS—GENERAL INFORMATION . . . . . . . . . . . . . . . . . . 1 ACCELERATION SWITCH—ECU . . . . . . . . . . . 29 GENERAL INFORMATION INDEX page pageABS Brakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Brake Warning Lights . . . . . . . . . . . . . . . . . . . . . . . 2Brake Changes for 1995 . . . . . . . . . . . . . . . . . . . . . 1 Brakelining Material . . . . . . . . . . . . . . . . . . . . . . . . 2Brake Fluid/Lubricants/Cleaning Solvents . . . . . . . . . 2 Parking Brake Mechanism . . . . . . . . . . . . . . . . . . . . 1Brake Safety Precautions . . . . . . . . . . . . . . . . . . . . 2 Wheel Brake Components . . . . . . . . . . . . . . . . . . . . 1WHEEL BRAKE COMPONENTS BRAKE CHANGES FOR 1995 All Grand Cherokee models are equipped with A different master cylinder, power brake booster,power assist four-wheel disc brakes. Antilock (ABS) and HCU are used in the 1995 Grand Cherokee ABSbrakes are also standard equipment on all models. system. Single piston, disc brake calipers are used front The master cylinder reservoir has a single filler capand rear. Ventilated disc brake rotors are used at the and is no longer interconnected with the HCU. Thefront and solid rotors are used at the rear. new HCU has built-in accumulators. The pedal travel Power brake assist is supplied by a vacuum oper- sensor has been eliminated which means the powerated, dual diaphragm power brake booster. brake booster is different as a result. The master cylinder used for all applications has The rear disc brakeshoes and caliper bracket arean aluminum body and nylon reservoir with single new for 1995. The bracket ledges are machined to ac-filler cap. commodate the changed brakeshoes. The outboard A combination valve is used for all applications. brakeshoes now have anti-rattle springs and the in-The valve contains a pressure differential switch and board shoe has a wear warning strip attached.rear brake proportioning valve. The rear disc splash shield is now secured to the caliper bracket with two factory installed rivets. Al-ABS BRAKES though the rivets have to be drilled out to separate The antilock system is an electronically operated the shield from the bracket, the rivets do not have toall wheel brake control system. The system is de- be replaced afterward. Refer to the service proce-signed to prevent wheel lockup during periods of dures in the parking brake section.high wheel slip when braking. The antilock electronic control system is separate PARKING BRAKE MECHANISMfrom other electrical circuits in the vehicle. A sepa- The parking brake mechanism consists of a cablerate electronic control unit (ECU) is used for the ABS operated, dual shoe, drum brake mechanism. Thesystem. brake shoes operate within a drum cast into the rear disc brake rotor. The shoes are mounted on a splash
  2. 2. 5-2 BRAKES ZJshield attached to the caliper bracket and rear axle CAUTION: Never use gasoline, kerosene, methyl ortube flange. Parking brake adjustment is controlled isopropyl alcohol, paint thinner, or any fluid con-by a cable tensioner mechanism. taining mineral oil to clean the system components. These fluids damage rubber cups and seals. If sys-BRAKE WARNING LIGHTS tem contamination is suspected, check the fluid for All Grand Cherokee models have two brake warn- dirt, discoloration, or separation into distinct layers.ing lights. A red light is used for the service and Drain and flush the system with new brake fluid ifparking brake system. An amber light is used for the contamination is suspected.ABS system. Both lights are in the instrument clus-ter. The red light alerts the driver if a pressure differ- BRAKE SAFETY PRECAUTIONSential exists between the front and rear hydraulicsystems. The red light also alerts the driver when WARNING: ALTHOUGH FACTORY INSTALLEDthe parking brakes are applied. BRAKELINING ON GRAND CHEROKEE MODELS IS The amber antilock light only illuminates when an MADE FROM ASBESTOS FREE MATERIALS, SOMEantilock system fault occurs. AFTER MARKET BRAKELINING MAY CONTAIN AS- Both lights illuminate for about 1-2 seconds at en- BESTOS. THIS SHOULD BE TAKEN INTO ACCOUNTgine start; this occurs as part of a normal bulb check. WHEN SERVICING A VEHICLE WITH PRIOR BRAKE SERVICE. WEAR A RESPIRATOR WHEN CLEANINGBRAKELINING MATERIAL BRAKE COMPONENTS AS ASBESTOS FIBERS CAN Factory installed front and rear brakelining on BE A HEALTH HAZARD. NEVER CLEAN BRAKEGrand Cherokee models, is made from organic mate- COMPONENTS WITH COMPRESSED AIR. USE Arials combined with metallic particles. The brakelin- VACUUM CLEANER SPECIFICALLY DESIGNED FORing material does not contain asbestos. REMOVING BRAKE DUST. IF A VACUUM CLEANERBRAKE FLUID/LUBRICANTS/CLEANING SOLVENTS IS NOT AVAILABLE, CLEAN THE PARTS WITH WA- Recommended brake fluid is Mopar brake fluid, or TER DAMPENED SHOP RAGS. DO NOT CREATEequivalent meeting SAE J1703 and DOT 3 stan- DUST BY SANDING BRAKELINING. DISPOSE OFdards. ALL DUST AND DIRT SUSPECTED OF CONTAINING Use Mopar multi-mileage grease to lubricate drum ASBESTOS FIBERS IN SEALED BAGS OR CON-brake pivot pins and rear brakeshoe contact points TAINERS. FOLLOW ALL RECOMMENDED SAFETYon the support plates. Use GE 661 or Dow 111 sili- PRACTICES PRESCRIBED BY THE OCCUPATIONALcone grease, or multi-mileage grease on caliper bush- SAFETY AND HEALTH ADMINISTRATION (OSHA)ings and slide pins. AND THE ENVIRONMENTAL PROTECTION AGENCY Use Mopar Brake Cleaner, or fresh brake fluid to (EPA), FOR HANDLING AND DISPOSAL OF PROD-clean or flush brake system components. These are UCTS CONTAINING ASBESTOS.the only cleaning materials recommended.
  3. 3. ZJ ABS BRAKE DIAGNOSIS 5-3 ABS BRAKE DIAGNOSIS INDEX page pageABS Diagnostic Connector . . . . . . . . . . . . . . . . . . . 3 General Information . . . . . . . . . . . . . . . . . . . . . . . . 3ABS Warning Light Display . . . . . . . . . . . . . . . . . . . 3 Normal Operating Conditions . . . . . . . . . . . . . . . . . . 3Antilock ECU and HCU Diagnosis . . . . . . . . . . . . . . 3 Wheel/Tire Size and Input Signals . . . . . . . . . . . . . . 3DRB Scan Tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3GENERAL INFORMATION hicle wheels and tires should all be the same size The DRB scan tool is required for all ABS diagno- and type. However, the Jeep ABS system is designedsis. The scan tool is used to identify ABS circuit to function with a compact spare tire installed.faults. Once a circuit fault has been identified, refer to the NORMAL OPERATING CONDITIONSappropriate chassis/body diagnostic manual for indi-vidual component testing. Sound Levels The hydraulic control unit pump and solenoidABS WARNING LIGHT DISPLAY valves may produce some sound as they cycle on and The amber antilock light illuminates at startup as off. This is a normal condition and should not be mis-part of the system self check feature. The light illu- taken for faulty operation. Under most conditions,minates for 1-3 seconds then goes off as part of the pump and solenoid valve operating sounds will not benormal bulb check routine. audible. An ABS circuit fault is indicated when the amberlight remains on after startup, or illuminates at any Vehicle Response In Antilock Modetime during vehicle operation. During antilock braking, the hydraulic control unit Verify that a fault is actually related to the ABS solenoid valves cycle rapidly in response to antilocksystem before making repairs. For example, if the electronic control unit signals.red light illuminates but the ABS light does not, the The driver will experience a pulsing sensationproblem is related to a service brake component and within the vehicle as the solenoids decrease, hold, ornot the ABS system. Or, if neither light illuminates increase pressure as needed. Brake pedal pulsing willbut a brake problem is noted, again, the problem is also be noted and is a normal condition.with a service brake component and not with theABS system. Steering ResponseABS DIAGNOSTIC CONNECTOR A modest amount of steering input is required dur- The ABS diagnostic connector is located inside the ing extremely high deceleration braking, or whenvehicle. The connecter is the access point for the braking on differing traction surfaces. An example ofDRB scan tool. differing traction surfaces would be when the left The connector is blue or black in color and is a side wheels are on ice and the right side wheels are6-way type. The connector is under the carpet at the on dry pavement.forward end of the console just under the IP center. Owner Induced FaultsDRB SCAN TOOL Driving away with the parking brakes still applied ABS diagnosis is performed with the DRB scan will cause warning light illumination. Pumping thetool. Refer to the DRB scan tool manual for test brake pedal will also generate a system fault and in-hookup and procedures. Diagnosis information is pro- terfere with ABS system operation.vided in the Chassis Diagnostic Procedures Manualfor Jeep Grand Cherokee Models. ANTILOCK ECU AND HCU DIAGNOSIS An ECU or HCU fault can only be determinedWHEEL/TIRE SIZE AND INPUT SIGNALS through testing with the DRB scan tool. Do not re- Antilock system operation is dependant on accurate place either component unless a fault is actually in-signals from the wheel speed sensors. Ideally, the ve- dicated.
  4. 4. 5-4 SERVICE BRAKE DIAGNOSIS ZJ SERVICE BRAKE DIAGNOSIS INDEX page pageBrake Component Inspection . . . . . . . . . . . . . . . . . . 5 Low Pedal . . . . . . . . . . . . . . . . . . . . . .... . . . . . . 5Brake Drag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Parking Brake Diagnosis . . . . . . . . . . . .... . . . . . . 7Brake Fade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Pedal Falls Away . . . . . . . . . . . . . . . . .... . . . . . . 5Brake Fluid Contamination . . . . . . . . . . . . . . . . . . . 6 Pedal Pulsation (Non-ABS Brakes Only) ... . . . . . . 6Brake Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Power Brake Booster Check Valve Test .... . . . . . . 8Brake Pull . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Power Brake Booster Vacuum Test . . . .... . . . . . . 9Brakes Do Not Hold After Driving Through Deep Preliminary Brake Check . . . . . . . . . . . .... . . . . . . 4 Water Puddles . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Rear Brake Grab . . . . . . . . . . . . . . . . .... . . . . . . 6Contaminated Brakelining . . . . . . . . . . . . . . . . . . . . 6 Road Test . . . . . . . . . . . . . . . . . . . . . . .... . . . . . . 5Diagnosis Procedures . . . . . . . . . . . . . . . . . . . . . . . 4 Service Brake Warning Light Operation .... . . . . . . 5General Information . . . . . . . . . . . . . . . . . . . . . . . . 4Hard Pedal or High Pedal Effort . . . . . . . . . . . . . . . 5 Spongy Pedal . . . . . . . . . . . . . . . . . . . .... . . . . . . 5GENERAL INFORMATION (2) Check condition of tires and wheels. Damaged The diagnosis information in this section covers the wheels and worn, damaged, or underinflated tires canvehicle service brake components which include: cause pull, shudder, tramp and a condition similar to• disc brake calipers grab.• disc brakeshoes (3) If complaint was based on noise when braking,• disc brake rotors check suspension components. Jounce front and rear• parking brake mechanism of vehicle and listen for noise that might be caused• master cylinder by loose, worn, or damaged suspension or steering• combination valve components.• power brake booster (4) Inspect brake fluid level and condition. Correct• brake pedal and brakelight switch fluid level is to FULL mark on reservoir. Remember• red brake warning light that fluid level will decrease slightly as normal brakelining wear occurs. If fluid level is abnormallyDIAGNOSIS PROCEDURES low, look for leaks at calipers, wheel cylinders, brake- Service brake diagnosis involves determining if the lines and master cylinder.problem is related to a mechanical, hydraulic or vacuum (5) Check fluid condition.operated part. A preliminary check, road testing and (a) Fluid should be reasonably clear and free ofcomponent inspection are needed to determine a prob- foreign material. Note that brake fluid tends tolem cause. darken over time. This is normal and should Road testing will either verify proper brake opera- not be mistaken for contamination. If fluid istion or confirm the existence of a problem. Compo- reasonably clear and free of foreign material,nent inspection will, in most cases, identify the it is OK.actual part causing a problem. (b) If fluid is highly discolored, or appears to con- The first diagnosis step is the preliminary check. This tain foreign material, drain out a sample with ainvolves inspecting fluid level, parking brake action, clean suction gun. Pour sample in a glass containerwheel and tire condition, checking for obvious leaks or and note condition described in step (c).component damage and testing brake pedal response. A (c) If fluid separates into layers, or obviously con-road test will confirm or deny the existence of a prob- tains oil or substance other than brake fluid, sys-lem. The final diagnosis procedure involves road test tem seals and cups will have to be replaced andanalysis and a visual inspection of brake components. hydraulic system flushed. (6) Check parking brake operation. Verify freePRELIMINARY BRAKE CHECK movement and full release of cables and foot pedal or (1) If amber antilock light is illuminated, refer to hand lever. Also note if vehicle was being operatedAntilock Brake System Diagnosis. However, if the with parking brake partially applied.red warning light is illuminated, or if no warning (7) If components checked appear OK, proceed tolight is illuminated, continue with diagnosis. Road Test.
  5. 5. ZJ SERVICE BRAKE DIAGNOSIS 5-5ROAD TEST and fluid level. If a problem is confirmed, inspect the (1) If red light is not on, proceed to step (3). wheel brake hydraulic system. (2) If red light is on, proceed as follows: The amber antilock warning light illuminates only (a) See if parking brakes are applied. Release when an ABS circuit fault has occurred. Refer to the them if necessary and proceed to step (b). Antilock Brake Diagnosis section. (b) Note if brake pedal is abnormally low. If pedal is definitely low and red light is on, check PEDAL FALLS AWAY front and rear brake hydraulic circuits for leak. Do A brake pedal that falls away under steady foot not continue with road test. Inspect and re- pressure is generally the result of a system leak. The pair hydraulic components as needed. leak point could be at a brakeline, fitting, hose, or (3) Check brake pedal response with transmission caliper. Internal leakage in the master cylinderin Neutral and engine running. caused by worn or damaged piston cups, may also be (a) If pedal remains firm under steady foot pres- the problem cause. sure, proceed to step (4). If leakage is severe, fluid will be evident at or (b) If pedal falls away, problem is in master cyl- around the leaking component. However internal inder, or HCU on ABS models. Do not road test leakage in the master cylinder may not be physically vehicle; repair as necessary instead. evident. Refer to the cylinder test procedure in this (4) During road test, make normal and firm brake section.stops in 25-40 mph range. Note faulty brake opera-tion such as hard pedal, pull, grab, drag, noise, fade, LOW PEDALpedal pulsation, etc. If a low pedal is experienced and the red light is (5) Inspect brake components after road test and not on, worn lining and rotors are the most likelyrefer to problem diagnosis information for causes of cause.various brake conditions. If the red warning light is on, a system leak has occurred. A leak at a front or rear caliper, brakeline,BRAKE COMPONENT INSPECTION or brake hose will activate the differential pressure Fluid leak points and dragging brake units can switch in the combination valve. The switch willusually be located without removing any components. shuttle forward or rearward depending on where theThe area around a leak point will be wet with fluid. leak is. Switch movement in either direction willThe components at a dragging brake unit (wheel, complete the electrical circuit to the red warningtire, rotor) will be quite warm or hot to the touch. light causing the light to illuminate. Other brake problem conditions will require compo-nent removal for proper inspection. Raise the vehicle SPONGY PEDALand remove the necessary wheels for better visual ac- A spongy pedal is caused by air in the brake hy-cess. draulic system. Brake bleeding will be necessary to During inspection, pay particular attention to purge the air.heavily rusted/corroded brake components (e.g. ro- Air enters the system through leak points and whentors, caliper pistons, cables, brakelines, etc.). the master cylinder reservoir runs dry as a result of a Heavy accumulations of rust may be an indicator of leak. Allowing the cylinder to run dry during brakerust and corrosion damage to a brake component. It bleeding, will also allow air into the system.is wise to remove surface rust in order to accuratelydetermine the depth of rust penetration and damage. HARD PEDAL OR HIGH PEDAL EFFORT Light surface rust is normal and not a major con- A hard pedal or high pedal effort may be due to lin-cern (as long as it is removed, or neutralized). How- ing that is water soaked, contaminated, glazed, or se-ever, heavy rust buildup, especially on high mileage verely worn.vehicles, may actually cover structural damage to The power brake booster, or booster check valvesuch important components as: brakelines, rotors, or could also be faulty. Loss of brake boost will cause abrake booster. hard pedal and high effort. Test the booster and valve as described in this section.SERVICE BRAKE WARNING LIGHT OPERATION The red warning light illuminates when the park- BRAKE DRAGing brakes are applied and when there is a leak in Brake drag occurs when the lining is in constantthe front or rear wheel brake hydraulic circuit. It will contact with the rotor. Drag can occur at one wheel,also illuminate at startup as part of a bulb check. all wheels, fronts only, or rears only. It is a product of If the light comes on, first verify that the parking incomplete brakeshoe release. Drag can be minor orbrakes are fully released. Then check pedal action severe enough to overheat the linings and rotors.
  6. 6. 5-6 SERVICE BRAKE DIAGNOSIS ZJ Brake drag also has a direct effect on fuel economy. If stops. If pulsation occurs during light to moderateundetected, minor brake drag can be misdiagnosed as brake stops, a standard brake part is either loose, oran engine or transmission/torque converter problem. worn beyond tolerance. Minor drag will usually cause slight surface char-ring of the lining. It can also generate hard spots in BRAKE PULLrotors from the overheat-cool down process. In most A front pull condition that only occurs when thecases, the rotors, wheels and tires are quite warm to brakes are applied, could result from contaminatedthe touch after the vehicle is stopped. lining in one caliper, seized caliper piston, binding Severe drag can char brakelining all the way caliper, loose caliper, loose or corroded slide pins, im-through. It can also distort and score rotors to the proper brakeshoes, or a damaged rotor.point of replacement. The wheels, tires and brake A worn, damaged wheel bearing or suspension com-components will be extremely hot. In severe cases, ponent are further causes of pull. A damaged frontthe lining may generate smoke as it chars from over- tire (bruised, ply separation) can also cause constantheating. pull that is magnified when the brakes are applied. Common causes of brake drag are: A common and confusing pull condition is where di-• seized or sticking caliper piston rection of pull changes after a few brake stops. The• caliper binding (on bushings, bolts, bracket) cause is a combination of brake drag followed by fade• incorrect length caliper mounting bolts (too long) at the dragging brake unit.• loose or damaged wheel bearing As the dragging brake overheats, efficiency is so re-• loose caliper mounting bracket duced that fade occurs. If the opposite brake unit is• misassembled components still functioning normally, its braking effect is magni-• mispositioned brakelight switch fied. This causes pull to switch direction in favor of• binding brake pedal the brake unit that is functioning normally.• master cylinder internal fault When diagnosing a change in pull condition, re- If brake drag occurs at all wheels, the problem may member that pull will return to the original directionbe related to a blocked master cylinder compensator if the dragging brake unit is allowed to cool downport or faulty power booster (binds-does not release). (and is not seriously damaged).The condition will worsen as system component tem- REAR BRAKE GRABperature increases. Rear grab is usually caused by contaminated lin- An improperly mounted or adjusted brakelight ing, bent or binding shoes, or improperly assembledswitch can prevent full brake pedal return. The re- components.sult will be the same as if the cylinder compensatorports are blocked. In this case, the brakes would be BRAKES DO NOT HOLD AFTER DRIVING THROUGHpartially applied all the time causing drag (which DEEP WATER PUDDLESwill become more severe as component temperature This condition is generally caused by water soakedincreases). lining. If the lining is only wet, it can be dried by driving with the brakes lightly applied for a mile orBRAKE FADE two. However, if the lining is also dirty, shoe removal Brake fade is a product of overheating caused by and cleaning with brake cleaner will be necessary.brake drag. However, brake overheating and subse-quent fade can also be caused by riding the brake CONTAMINATED BRAKELININGpedal, making repeated high deceleration stops in a Brakelining contaminated by water is salvageable.short time span, or constant braking on steep roads. The lining can either be air dried or dried using heat.Refer to the Brake Drag information in this section In cases where brakelining is contaminated by oil,for causes. grease, or brake fluid, the lining should be replaced. Replacement is especially necessary when fluids/lu-PEDAL PULSATION (NON-ABS BRAKES ONLY) bricants have actually soaked into the lining mate- Pedal pulsation is caused by brake parts that are rial. However, grease or dirt that gets onto the liningloose, or out of tolerance limits. This type of pulsa- surface (from handling) during brake repairs, can betion is experienced every time the brakes are applied. cleaned off. Simply spray the lining surface clean Disc brake rotors with excessive lateral runout or with Mopar brake cleaner.thickness variation, or out of round brake drums arethe primary causes of pulsation. BRAKE FLUID CONTAMINATION On vehicles with ABS brakes, remember that pedal There are two basic causes of brake fluid contami-pulsation is normal during antilock mode brake nation. The first involves allowing dirt, debris, or other liquid materials to enter the cylinder reservoir
  7. 7. ZJ SERVICE BRAKE DIAGNOSIS 5-7when the cap or cover is off. The second involves top- Loud brake squeak, squeal, scraping, or grindingping off, or filling the reservoir with non-recom- sounds are a sign of severely worn brake lining. Ifmended fluid. the lining has worn completely through in spots, met- Brake fluid contaminated with only dirt, or debris al-to-metal contact occurs. If the condition is allowedusually retains a normal appearance. Generally, the to continue, rotors can become scored severelyforeign material will remain suspended in the fluid enough to require replacement.and be visible. The fluid and foreign material can beremoved from the reservoir with a suction gun but Thump/Clunkonly if the brakes have not been applied. If the Thumping or clunk noises during braking are fre-brakes are applied after contamination, system flush- quently not caused by brake components. In manying will be required. The master cylinder will also cases, such noises are caused by loose or damagedhave to be flushed or replaced if the contaminants steering, suspension, or engine components. However,cannot be removed. Foreign material lodged in the calipers that bind on the slide surfaces can generatereservoir compensator/return ports can cause brake a thump or clunk noise. In addition, worn out, im-drag by restricting fluid return after brake applica- properly adjusted, or improperly assembled reartion. brakeshoes can also produce a thump noise. Brake fluid contaminated by a non-recommendedfluid, may appear highly discolored, milky, oily look- Chatter/Shuddering, or foamy. In some cases, the fluid may even ap- Brake chatter is usually caused by loose or wornpear to contain sludge. However, be advised that components, or glazed/burnt lining. Rotors with hardbrake fluid will darken in time and occasion- spots can also contribute to chatter. Additional causesally be cloudy in appearance. These are normal of chatter are out-of-tolerance rotors, brake lining notconditions and should not be mistaken for con- securely attached to the shoes, loose wheel bearingstamination. and contaminated brake lining. If some type of oil has been added to the system,the fluid will separate into distinct layers. To verify BRAKELINING CONTAMINATIONthis, drain off a sample with a clean suction gun. Brakelining contamination is a product of leakingThen pour the sample into a glass container and ob- calipers or wheel cylinders, driving through deep wa-serve fluid action. If the fluid separates into distinct ter puddles, or lining that has become covered withlayers, it is definitely contaminated. grease and grit during repair. The only real correction for contamination, is todisassemble and flush the entire hydraulic system WHEEL AND TIRE PROBLEMSand replace all seals. Some conditions attributed to brake components may actually be caused by a wheel or tire problem.BRAKE NOISE A damaged wheel can cause shudder, vibration and pull. A worn or damaged tire can also cause pull.Squeak/Squeal Severely worn tires with little or no tread left can Factory installed brakelining is made from as- produce a grab-like condition as the tire loses and re-bestos free materials. These materials have dif- covers traction.ferent operating characteristics than previous Flat-spotted tires can cause vibration and wheelbrakelining material. Under certain conditions, tramp and generate shudder during brake operation.asbestos free lining may generate some squeak, A tire with internal damage such as a severe bruisegroan or chirp noise. This noise is considered or ply separation can cause pull and vibration.normal and does not indicate a problem. Theonly time inspection is necessary, is when noise PARKING BRAKE DIAGNOSISbecomes constant or when grinding, scrapingnoises occur. Adjustment Mechanism The rear brakeshoes are equipped with wear warn- Parking brake adjustment is controlled by aing tabs. The tabs contact the rotors when the lining cable tensioner. Once the tensioner is adjustedwears to replacement thickness. A constant scraping, at the factory, it should not require further at-squeak type noise will occur at contact. tention. However, there are two instances when Constant brake squeak or squeal may be due to lin- adjustment will be required. The first is when aings that are wet or contaminated with brake fluid, new tensioner, or cables have been installed.grease, or oil. Glazed linings and rotors with hardspots will produce squeak. Dirt and foreign material And the second, is when the tensioner and ca-embedded in the brake lining will also cause squeak/ bles are disconnected for access to other brakesqueal. components.
  8. 8. 5-8 SERVICE BRAKE DIAGNOSIS ZJParking Brake Switch And Warning Light Illumination Parking brake adjustment and parts replacement The parking brake switch is in circuit with the red procedures are described in the Parking Brake sec-warning light in the dash. The switch will cause the tion.light to illuminate only when the parking brakes areapplied. If the light remains on after parking brake POWER BRAKE BOOSTER CHECK VALVE TESTrelease, the switch or wires are faulty, or cable ten- (1) Disconnect vacuum hose from check valve.sioner adjustment is incorrect. (2) Remove check valve and valve seal from If the red light comes on while the vehicle is in mo- booster (Fig. 1). Inspect seal for cuts, cracks, tears,tion and brake pedal height decreases, a fault has oc- and replace if necessary.curred in the front or rear brake hydraulic system. (3) Hand operated vacuum pump can be used for test (Fig. 2).Parking Brake problem Causes (4) Apply 15-20 inches vacuum at large end of In most cases, the actual cause of an improperly check valve (Fig. 1).functioning parking brake (too loose/too tight/wont (5) Vacuum should hold steady. If gauge on pumphold), can be traced to a parking brake component. indicates any vacuum loss, valve is faulty and must The leading cause of improper parking brake be replaced.operation, is excessive clearance between theparking brakeshoes and the shoe braking sur-face. Excessive clearance is a result of liningand/or drum wear, drum surface machinedoversize, or inoperative adjuster components. Excessive parking brake lever travel (sometimesdescribed as a loose lever or too loose condition), isthe result of worn brakeshoes, improper brakeshoeadjustment, or improperly assembled brake parts. A ‘‘too loose’’ condition can also be caused by inop-erative or improperly assembled parking brakeshoeparts. A condition where the parking brakes do not hold,will most probably be due to a wheel brake compo-nent. Items to look for when diagnosing a parking brakeproblem, are:• rear brakeshoe wear Fig. 1 Vacuum Check Valve And Seal Location• drum surface (in rear rotor) machined oversize• front cable not secured to lever• rear cable not attached to lever• rear cable seized• parking shoes reversed• parking brake strut not seated in shoes• parking brake lever not seated• parking brake lever bind• cam and lever worn or misassembled• adjuster screws seized• adjuster screws reversed Fig. 2 Typical Hand Operated Vacuum Pump
  9. 9. ZJ SERVICE BRAKE DIAGNOSIS 5-9POWER BRAKE BOOSTER VACUUM TEST (1) Connect a vacuum gauge to the booster checkvalve with a short length of hose and a T-fitting (Fig.3). (2) Start and run engine at idle speed for oneminute. (3) Clamp hose shut between vacuum source andcheck valve (Fig. 3). (4) Stop engine and observe vacuum gauge. (5) If vacuum drops more than one inch HG (33millibars) within 15 seconds, booster diaphragm orcheck valve is faulty. Fig. 3 Booster Vacuum Test Connections
  10. 10. 5 - 10 ABS SYSTEM OPERATION ZJ ABS SYSTEM OPERATION INDEX page pageABS Diagnostic Connector . . . . . . . . . . . . . . . . . . 11 Hcu Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 13ABS Operation in Antilock Braking Mode . . . . . . . . 12 Hydraulic Control Unit (HCU) . . . . . . . . . . . . . . . . . 10ABS Operation in Normal Braking Mode . . . . . . . . 12 Ignition Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . 12ABS System Power-Up and Initialization . . . . . . . . 12 System Description . . . . . . . . . . . . . . . . . . . . . . . . 10Acceleration Switch . . . . . . . . . . . . . . . . . . . . . . . . 11 System Relays . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Acceleration Switch Operation . . . . . . . . . . . . . . . . 14 System Warning Light . . . . . . . . . . . . . . . . . . . . . . 12Combination Valve . . . . . . . . . . . . . . . . . . . . . . . . 12 Wheel Speed Sensor Operation . . . . . . . . . . . . . . 13ECU Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Wheel Speed Sensors . . . . . . . . . . . . . . . . . . . . . . 11Electronic Control Unit (ECU) . . . . . . . . . . . . . . . . 11SYSTEM DESCRIPTION • electronic control unit (ECU) The Jeep antilock brake system (ABS) is an elec- • wheel speed sensors and axle shaft tone ringstronically operated, all wheel brake control system. • acceleration switchThe system is designed to prevent wheel lockup and • main relay, pump motor relay,maintain steering control during periods of high • pump motor speed sensorwheel slip when braking. Preventing wheel lockup • ABS warning lightand is accomplished by modulating fluid pressure tothe wheel brake units. HYDRAULIC CONTROL UNIT (HCU) The hydraulic system is a three channel design. The hydraulic control unit (HCU) consists of aThe front wheel brakes are controlled individually valve body, pump body, accumulators, pump motor,and the rear wheel brakes in tandem (Fig. 1). The and wire harnesses (Fig. 2).ABS electrical system is separate from other electri- The pump body, motor, and accumulators are com-cal circuits in the vehicle. A specially programmed bined into an assembly attached to the valve body.electronic control unit (ECU) operates the system The accumulators store the fluid released for ABScomponents. mode operation. The pump provides the fluid volume needed for antilock braking and a DC-type motor runs the pump. The pump/motor is controlled by the ECU. The valve body contains solenoid valves. The sole- noid valves modulate brake pressure during antilock braking. The valves are controlled by the ECU. The HCU provides three channel pressure control to the front and rear brakes. One channel controls the rear wheel brakes in tandem. The two other channels control the front wheel brakes individually. During antilock braking, the solenoid valves are opened and closed as needed. The valves are not static. They are cycled rapidly and continuously to modulate pressure and control wheel slip and decel- eration. At the start of antilock braking, pedal height will decrease as the initial volume of fluid is released to the accumulators. The accumulators then provide the fluid needed for initial operation. The pump motor runs continuously until the ABS mode brake stop is completed. The 1995 master cylinder and HCU are different Fig. 1 Jeep AntiLock System from previous models. The new HCU has built-in ac- cumulators that store the fluid released for antilock ABS system components include: braking. The new master cylinder has a single filler• hydraulic control unit (HCU) cap and the reservoir and HCU are no longer inter-
  11. 11. ZJ ABS SYSTEM OPERATION 5 - 11connected by hoses. The 1995 ABS components arenot interchangeable with prior year ABS compo-nents. Fig. 2 Antilock Hydraulic Control Unit (HCU) Fig. 3 Anti-Lock ECUELECTRONIC CONTROL UNIT (ECU) An electronic control unit (ECU) operates the ABSsystem (Fig. 3). The ECU is separate from other ve-hicle electrical circuits. ECU voltage source isthrough the ignition switch in the Run position. The ECU is located in the engine compartment. Itis mounted on the driver side inner fender panel. The ECU contains dual microprocessors. A logicblock in each microprocessor receives identical sensorsignals which are processed and compared simulta-neously. The ECU contains a self check program that illu-minates the ABS warning light when a system faultis detected. Faults are stored in a diagnostic programmemory and are accessible with the DRB scan tool. ABS faults remain in memory until cleared, or un-til after the vehicle is started approximately 50 Fig. 4 Wheel Speed Sensorstimes. Stored faults are not erased if the battery is ABS DIAGNOSTIC CONNECTORdisconnected. ABS circuit diagnosis is performed with the DRB scan tool. The access point for the scan tool is the di-WHEEL SPEED SENSORS agnostic connector. A speed sensor is used at each wheel. The sensors The connector is under the carpet at the forwardconvert wheel speed into an electrical signal. This end of the console just under the IP center. The con-signal is transmitted to the antilock ECU. nector is black or blue in color and is a 6-way type. A gear type tone ring serves as the trigger mecha-nism for each sensor. The tone rings are mounted atthe outboard ends of the front and rear axle shafts. ACCELERATION SWITCH Different sensors are used at the front and rear An acceleration switch (Fig. 5), provides an addi-wheels (Fig. 4). The front/rear sensors have the same tional vehicle deceleration reference during 4-wheelelectrical values but are not interchangeable. drive operation. The switch is monitored by the an-
  12. 12. 5 - 12 ABS SYSTEM OPERATION ZJtilock ECU at all times. The switch reference signal ABS SYSTEM POWER-UP AND INITIALIZATIONis utilized by the ECU when all wheels are deceler- battery voltage is supplied to the ECU ignition ter-ating at the same rate. minal when the ignition switch is turned to Run po- sition. The ECU performs a system initialization procedure at this point. Initialization consists of a static and dynamic self check of system electrical components. The static check occurs immediately after the igni- tion switch is turned in Run position. The dynamic check occurs when vehicle road speed reaches ap- proximately 10 kph (6 mph). During the dynamic check, the ECU briefly cycles the pump to verify op- eration and the HCU solenoids are checked. If an ABS component exhibits a fault during initial- ization, the ECU illuminates the amber warning light and registers a fault code in the microprocessor memory. ABS OPERATION IN NORMAL BRAKING MODE The ECU monitors wheel speed sensor inputs con- tinuously while the vehicle is in motion. However, the ECU will not activate any ABS components as Fig. 5 Acceleration Switch long as sensor inputs and the acceleration switch in- dicate normal braking.SYSTEM RELAYS During normal braking, the master cylinder, power The ABS system has two relays, which are the booster and wheel brake units all function as theymain and motor pump relays. The motor pump relay would in a vehicle without ABS. The HCU pump andis used for the motor pump only. The main relay is solenoids are not activated.used for the solenoid valves. The main relay is con-nected to the ECU at the power control relay termi- ABS OPERATION IN ANTILOCK BRAKING MODEnal. The pump motor relay starts/stops the pump The purpose of the antilock system is to preventmotor when signaled by the ECU. wheel lockup during periods of high wheel slip. Pre- venting lockup helps maintain vehicle braking actionIGNITION SWITCH and steering control. The antilock ECU and warning light are in standby The antilock ECU activates the system whenevermode with the ignition switch in Off or Accessory po- sensor signals indicate periods of high wheel slip.sition. No operating voltage is supplied to the system High wheel slip can be described as the point wherecomponents. wheel rotation begins approaching zero (or lockup) A 12 volt power feed is supplied to the ECU and during braking. Periods of high wheel slip may occurwarning light when the ignition switch is in the Run when brake stops involve high rates of vehicle decel-position. eration. The antilock system prevents lockup during highSYSTEM WARNING LIGHT slip conditions by modulating fluid apply pressure to The amber ABS warning light is in circuit with the the wheel brake units.ECU and operates independently of the red brake Brake fluid apply pressure is modulated accordingwarning light. to wheel speed, degree of slip and rate of decelera- The ABS light indicates antilock system condition. tion. A sensor at each wheel converts wheel speedThe light illuminates for 1-2 seconds at start-up as into electrical signals. These signals are transmittedpart of a bulb check routine. The light goes out when to the ECU for processing and determination ofthe self test program determines system operation is wheel slip and deceleration rate.normal. The ABS system has three fluid pressure control channels. The front brakes are controlled separatelyCOMBINATION VALVE and the rear brakes in tandem (Fig. 1). A speed sen- A combination valve is used with the ABS system. sor input signal indicating high slip conditions acti-The valve contains a front/rear brake pressure differ- vates the ECU antilock program.ential valve and switch and a rear brake proportion- Two solenoid valves are used in each antilock con-ing valve. trol channel. The valves are all located within the
  13. 13. ZJ ABS SYSTEM OPERATION 5 - 13HCU valve body and work in pairs to either increase, Pressure Holdhold, or decrease apply pressure as needed in the in- Both solenoid valves are closed in the pressure holddividual control channels. cycle (Fig. 7). Fluid apply pressure in the control The solenoid valves are not static during antilock channel is maintained at a constant rate. The ECUbraking. They are cycled continuously to modulate maintains the hold cycle until sensor inputs indicatepressure. Solenoid cycle time in antilock mode can be a pressure change is necessary. The pump does notmeasured in milliseconds. run during the pressure hold cycle.HCU OPERATIONNormal Braking During normal braking, the HCU solenoid valves,pump, accumulators, and motor are not activated.The master cylinder and power booster operate thesame as a vehicle without ABS brakes.Antilock Pressure Modulation Solenoid valve pressure modulation occurs in threestages which are: pressure increase, pressure hold,and pressure decrease. The valves are all containedin the valve body portion of the HCU.Pressure Decrease The outlet valve is opened and the inlet valve isclosed during the pressure decrease cycle (Fig. 6). A pressure decrease cycle is initiated when speedsensor signals indicate high wheel slip at one ormore wheels. At this point, the ECU opens the outletvalve, which also opens the return circuit to the ac-cumulators. Fluid pressure is allowed to bleed off (de- Fig. 7 Pressure Hold Cyclecrease) as needed to prevent wheel lock. Pressure Increase Once the period of high wheel slip has ended, the The inlet valve is open and the outlet valve isECU closes the outlet valve and begins a pressure in- closed during the pressure increase cycle (Fig. 8). Thecrease or hold cycle as needed. pressure increase cycle is used to counteract unequal wheel speeds. This cycle controls re-application of fluid apply pressure due to changing road surfaces or wheel speed. WHEEL SPEED SENSOR OPERATION Wheel speed input signals are generated by a sen- sor and tone ring at each wheel. The sensors, which are connected directly to the ECU, are mounted on brackets attached to the front steering knuckles and rear brake support plates. The sensor triggering devices are the tone rings which are similar in appearance to gears. The tone rings are located on the outboard end of each front/ rear axle shaft. The speed sensors generate a signal whenever a tone ring tooth rotates past the sensor pickup face. The wheel speed sensors provide the input signal to the ECU. If input signals indicate ABS mode brak- ing, the ECU causes the HCU solenoids to decrease, hold, or increase fluid apply pressure as needed. The HCU solenoid valves are activated only when wheel speed input signals indicate that a wheel is Fig. 6 Pressure Decrease Cycle approaching a high slip, or lockup condition. At this
  14. 14. 5 - 14 ABS SYSTEM OPERATION ZJ these signals for degree of deceleration and wheel slip. If signals indicate normal braking, the solenoid valves are not activated. However, when incoming signals indicate the approach of wheel slip, or lockup, the ECU cycles the solenoid valves as needed. ACCELERATION SWITCH OPERATION The ECU monitors the acceleration switch at all times. The switch assembly contains three mercury switches that monitor vehicle ride height and decel- eration rates (G-force). Sudden, rapid changes in ve- hicle and wheel deceleration rate, triggers the switch sending a signal to the ECU. The switch assembly provides three deceleration rates; two for forward braking and one for rearward braking. ECU OPERATION The antilock ECU controls all phases of antilock operation. It monitors and processes input signals from the system sensors. It is the ECU that activates the solenoid valves to modulate apply pressure during antilock braking. Fig. 8 Pressure Increase Cycle The ECU program is able to determine which wheelpoint, the ECU will cycle the appropriate wheel con- control channel requires modulation and which fluidtrol channel solenoid valves to prevent slip or lockup. pressure modulation cycle to use. The ECU cycles the The wheel sensors provide speed signals whenever solenoid valves through the pressure decrease, holdthe vehicle wheels are rotating. The ECU examines and increase phases.
  15. 15. ZJ BRAKE FLUID—BRAKE BLEEDING—BRAKELINES/HOSES 5 - 15 BRAKE FLUID—BRAKE BLEEDING—BRAKELINES/HOSES INDEX page pageABS Brake Bleeding . . . . . . . . . . . . . . . . . . . . . . . 15 Correct Brake Fluid Level . . . . . . . . . . . . . . . . . . . 15Brakelines and Hoses . . . . . . . . . . . . . . . . . . . . . . 16 Importance of Clean Brake Fluid . . . . . . . . . . . . . . 15Checking Brake Fluid for Contamination . . . . . . . . 15 Recommended Brake Fluid . . . . . . . . . . . . . . . . . . 15RECOMMENDED BRAKE FLUID rial in the fluid, or non-recommended fluids will Recommended brake fluid is Mopar brake fluid, or cause system malfunctions.an equivalent quality fluid meeting SAE J1703 and Clean the reservoir and cap thoroughly beforeDOT 3 standards. checking level or adding fluid. Cap open lines and Brake fluid used in the ABS system must not only hoses during service to prevent dirt entry.meet SAE/DOT standards, it must be exceptionally Dirt or foreign material entering the ABS hydraulicclean as well. Never use substandard fluid, fluid system through the reservoir opening will circulatenot meeting SAE/DOT standards, reclaimed within the system. Dirt or foreign material in thefluid, or fluid from containers that have been system can lead to component malfunction. Alwaysleft open for lengthy periods. clean the reservoir exterior before checking fluid level or adding fluid. Use clean, fresh fluid only toCORRECT BRAKE FLUID LEVEL top off, or refill the system. Correct fluid level is marked on each the side ofthe master cylinder reservoir (Fig. 1). CHECKING BRAKE FLUID FOR CONTAMINATION Preferred fluid level is to the FULL mark. Accept- Oil in the fluid will cause brake system rubberable fluid level is between the ADD and FULL seals to soften and swell. The seals may also becomemarks. porous and begin to deteriorate. If fluid level is below the ADD mark, the brake hy- If fluid contamination is suspected, drain off a sam-draulic system should be checked for leaks. ple from the master cylinder. A suction gun or similarCAUTION: Clean the reservoir cap and exterior thor- device can be used for this purpose.oughly before checking fluid level. Do not allow any Empty the drained fluid into a glass container.dirt or foreign material to enter the reservoir while Contaminants in the fluid will cause the fluid to sep-checking fluid level. Such materials can interfere arate into distinct layers. If contamination has oc-with solenoid valve operation causing an ABS mal- curred, the system rubber seals, hoses and cups mustfunction. be replaced and the system thoroughly flushed with clean brake fluid. ABS BRAKE BLEEDING ABS brake bleeding requires use of the DRB scan tool. The procedure involves performing a conven- tional bleed, followed by use of the scan tool to cycle and bleed the HCU pump and solenoids. A second conventional bleed procedure is then required to en- sure that all air is purged from the system. BRAKE BLEED PROCEDURE (1) If new master cylinder is to be installed, bleed cylinder on bench before installation in vehicle. Refer to procedure in section covering master cylinder ser- vice. (2) Wipe master cylinder reservoir and cap clean. This avoids having dirt or foreign material fall into Fig. 1 Reservoir Fluid Level Indicator Marks reservoir.IMPORTANCE OF CLEAN BRAKE FLUID (3) Fill reservoir with Mopar, or equivalent quality The ABS system brake fluid must be kept clean brake fluid meeting SAE J1703 and DOT 3 stan-and free of any type of contamination. Foreign mate- dards.
  16. 16. 5 - 16 BRAKE FLUID—BRAKE BLEEDING—BRAKELINES/HOSES ZJ (4) Perform conventional brake bleed as described (a) Connect scan tool to ABS diagnostic connec-in steps (5) and (6). tor. Connector is under carpet at front of console, (5) Bleed master cylinder and combination valve at just under instrument panel center bezel.brakeline fittings. Have helper operate brake pedal (b) Select CHASSIS SYSTEM, followed bywhile bleeding cylinder and valve. TEVES ABS BRAKES, then BLEED BRAKES. When scan tool displays TEST COMPLETE, dis- connect scan tool and proceed to next step. (8) Repeat conventional bleed procedure described in steps (5) and (6). (9) Top off master cylinder fluid level and verify proper brake operation before moving vehicle. BRAKELINES AND HOSES Metal brakelines and rubber front brake hoses (Figs. 3 and 4), should be inspected periodically and replaced if damaged. The HCU lines with the braided, flexible sections should be replaced if damaged. Do not use substitute brakelines here. Use the flexible braided lines only. Rubber brake hoses should be replaced if cut, cracked, swollen, or leaking. Rubber hoses must only be replaced. They are not repairable parts. The steel brakelines should be checked every time the vehicle is in for normal maintenance. This is im- portant on high mileage vehicles. It is especially im- portant when the vehicle is operated in areas where Fig. 2 Bleed Hose Immersed In Fluid salt is used on road surfaces during winter. (6) Bleed wheel brakes in recommended sequencewhich is: right rear wheel; left rear wheel; right frontwheel; left front wheel. Procedure is as follows: (a) Attach bleed hose to caliper bleed screw. Im- merse end of hose in glass container partially filled with brake fluid. Be sure hose end is submerged in fluid (Fig. 2). (b) Open bleed screw 1/2 turn. Then have helper depress and hold brake pedal. (c) Close bleed screw when brake pedal contacts floorpan. Do not pump brake pedal at any time while bleeding. This compresses air into small bubbles which are distributed throughout system. Additional bleeding will be necessary to remove trapped air. (d) Repeat bleeding operation at each wheel brake unit until fluid entering glass container is free of air bubbles. Check reservoir fluid level fre- quently and add fluid if necessary. (7) Perform HCU bleed procedure with DRB scantool as follows: Fig. 3 Master Cylinder/Combination Valve/HCU Brakeline Connections
  17. 17. ZJ BRAKE FLUID—BRAKE BLEEDING—BRAKELINES/HOSES 5 - 17 Fig. 4 Front Brakeline Routing And Connections Heavily rusted/corroded brakelines should be care- component damage. Severely rusted parts should befully inspected. Heavy rust buildup can hide severe replaced if doubt exists about their condition.
  18. 18. 5 - 18 MASTER CYLINDER—BRAKE BOOSTER—COMBINATION VALVE—HCU ZJ MASTER CYLINDER—BRAKE BOOSTER—COMBINATION VALVE—HCU INDEX page pageCombination Valve Installation . . . . . . . . . . . . . . . . 20 Master Cylinder Installation . . . . . . . . . ... . . . . . . 23Combination Valve Removal . . . . . . . . . . . . . . . . . 20 Master Cylinder Removal . . . . . . . . . . ... . . . . . . 21General Service Information . . . . . . . . . . . . . . . . . 18 Master Cylinder Reservoir Replacement .. . . . . . . 21HCU Installation . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Power Brake Booster Installation . . . . . ... . . . . . . 27HCU Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Power Brake Booster Operation . . . . . ... . . . . . . 19Master Cylinder Bench Bleeding . . . . . . . . . . . . . . 23 Power Brake Booster Removal . . . . . . ... . . . . . . 27GENERAL SERVICE INFORMATION booster is different as well. The new parts on 1995 A two-piece master cylinder with a 25.4 mm bore models are not interchangeable with prior models.and a 205 mm (8.07 in.) dual diaphragm power brakebooster are used for all applications. Component Service A combination valve is used on all models. The The power brake booster, HCU, and combinationvalve consists of a pressure differential switch and valve are not repairable. These components must bevalve, and a rear proportioning valve. replaced as an assembly whenever diagnosis indi- cates this is necessary.Changes For 1995 The nylon reservoir and grommets are the only ser- A different master cylinder, brake booster, and viceable parts on the master cylinder (Fig. 1). TheHCU are used in 1995 Grand Cherokee models. combination valve bracket is an integral part of the The master cylinder reservoir has a single filler cap valve; it is not removable.and is no longer interconnected with the HCU. The The only serviceable parts on the power brakenew HCU has built-in accumulators to store fluid re- booster (Fig. 1) are the check valve, and vacuumleased for antilock braking. The pedal travel sensor hose. The booster itself is not serviceable. Replacehas been eliminated which means the power brake the booster as an assembly whenever diagnosis indi- cates a malfunction has occurred. Fig. 1 Master Cylinder/Brake Booster Assembly
  19. 19. ZJ MASTER CYLINDER—BRAKE BOOSTER—COMBINATION VALVE—HCU 5 - 19POWER BRAKE BOOSTER OPERATION How Brake Boost Is Generated Power assist is generated by utilizing the pressureBooster Components differential between normal atmospheric pressure The booster assembly consists of a housing divided and a vacuum. The vacuum needed for booster oper-into separate chambers by two internal diaphragms ation is taken directly from the engine intake mani-(Fig. 2). The outer edge of each diaphragm is at- fold. The entry point for atmospheric pressure istached to the booster housing. The diaphragms are through a filter and inlet valve at the rear of theconnected to the booster primary push rod. housing (Fig. 2). Two push rods are used to operate the booster. The The chamber areas forward of the booster dia-primary push rod connects the booster to the brake phragms are exposed to manifold vacuum. The cham-pedal. The secondary push rod, which connects the ber areas to the rear of the diaphragms, are exposedbooster to the master cylinder, strokes the master to normal atmospheric pressure of 101.3 kilopascalscylinder pistons. (14.7 pounds/square in.). The atmospheric inlet valve is opened and closed Depressing the brake pedal causes the primaryby the primary push rod. The booster vacuum supply push rod to open the atmospheric inlet valve. Thisis through a hose attached to a fitting on the intake action exposes the area behind the diaphragms to at-manifold. The hose is connected to a vacuum check mospheric pressure. The resulting pressure differen-valve in the booster housing. The check valve is a tial is what provides the extra apply pressure forone-way device that prevents vacuum leak back. power assist. Fig. 2 Power Brake Booster Internal Components
  20. 20. 5 - 20 MASTER CYLINDER—BRAKE BOOSTER—COMBINATION VALVE—HCU ZJCOMBINATION VALVE REMOVAL (1) Disconnect clean air and PCV hoses. Then un-snap and remove engine air cleaner top cover (Fig.3). Cover intake manifold inlet with shop towel. Fig. 5 Combination Valve Bracket Attachment (8) Remove combination valve and bracket assem- bly (Fig. 6). Fig. 3 Air Cleaner Top Cover And Hoses (2) Remove brakelines that connect master cylin-der to combination valve (Fig. 4). (3) Disconnect wire from combination valve switchterminal (Fig. 4). Be careful when separating wireconnector as lock tabs are easily damaged if not fullydisengaged. (4) Disconnect brakelines that connect combinationvalve to HCU (Fig. 4). Fig. 6 Combination Valve And Bracket Assembly COMBINATION VALVE INSTALLATION (1) Position valve bracket on booster studs (Fig. 7). (2) Install but do not tighten nuts that secure valve bracket to booster studs. (3) Align and start all four brakeline fittings in combination valve by hand to avoid cross threading. Then tighten fittings just enough to prevent leakage. (4) Tighten combination valve bracket attaching nuts to 25 N⅐m (220 in. lbs.) torque. (5) Connect wire to differential pressure switch in combination valve (Fig. 4). Fig. 4 Combination Valve Connections (6) Tighten brakeline fittings at master cylinder (5) Slide HCU solenoid harness connectors off com- just enough to prevent leakage.bination valve bracket. Then move harness aside for (7) Bleed brakes. Refer to procedure in Brakeworking clearance. Bleeding-Fluid Level-Brakelines section. (6) Remove nuts attaching combination valve (8) Attach HCU solenoid harness connectors tobracket to booster studs (Fig. 5). combination valve bracket. (7) Slide combination valve bracket off booster (9) Install air cleaner top cover and hoses.studs (Fig. 5).
  21. 21. ZJ MASTER CYLINDER—BRAKE BOOSTER—COMBINATION VALVE—HCU 5 - 21 (8) Remove nuts that attach master cylinder to booster studs. a 6-point deep socket is needed to reach nuts (Fig. 9). Retain nuts as they are spe- cial locking types.Fig. 7 Positioning Valve Bracket On Booster Studs Fig. 9 Master Cylinder Attaching Nut LocationMASTER CYLINDER REMOVAL (9) Remove master cylinder from booster (Fig. 10). (1) Disconnect Loosen clamp that secures clean airhose to intake manifold. Then disconnect PCV hoseat valve. (2) Unsnap air cleaner top cover and remove coverand hoses (Fig. 3). Air filter can either be removed orcovered with shop towel at this time. (3) Cover intake manifold air inlet with shoptowel. (4) Remove windshield washer reservoir attachingscrews. Then lift reservoir upward and position it ontop of fender (Fig. 8). It is not necessary to dis-connect reservoir hoses or wires to positionreservoir on fender. Fig. 10 Master Cylinder Removal MASTER CYLINDER RESERVOIR REPLACEMENT The only serviceable master cylinder parts are the reservoir, grommets, and seal (Fig. 11). The cylinder body is not a repairable item. The body and pistons are only serviced as an assembly. RESERVOIR AND GROMMET REPLACEMENT (1) Remove reservoir cap and empty fluid into drain container. (2) Remove pins that retain reservoir to master cylinder. Use hammer and pin punch to remove pins (Fig. 12).Fig. 8 Windshield Washer Reservoir Positioned On (3) Clamp cylinder body in vise with brass protec- Fender tive jaws. (5) Remove brakelines that connect master cylin- (4) Loosen reservoir from grommets with pry toolder to combination valve (Fig. 4). (Fig. 13). (6) Remove nuts that secure combination valvebracket to booster mounting studs. (7) Slide combination valve bracket off boosterstuds (Fig. 5).
  22. 22. 5 - 22 MASTER CYLINDER—BRAKE BOOSTER—COMBINATION VALVE—HCU ZJ Fig. 14 Reservoir Removal Fig. 11 Master Cylinder And Reservoir Assembly Fig. 15 Grommet Removal (7) Lubricate new grommets with clean brake fluid. (8) Install new grommets in cylinder body (Fig. 16). Use finger pressure only to install and seat grommets. CAUTION: Do not use any type of tool to install the grommets. Tools will cut, or tear the grommets cre- Fig. 12 Removing/Installing Reservoir Retaining Pins ating a leak problem after installation. Install the grommets using finger pressure only. Fig. 13 Loosening Reservoir From Grommets Fig. 16 Grommet Installation (5) Remove reservoir by rocking it to one side andpulling free of grommets (Fig. 14). (9) Start reservoir in grommets. Then rock reser- (6) Remove old grommets from cylinder body (Fig. voir back and forth while pressing downward to seat15). it in grommets.
  23. 23. ZJ MASTER CYLINDER—BRAKE BOOSTER—COMBINATION VALVE—HCU 5 - 23 (10) Install pins that retain reservoir to cylinder MASTER CYLINDER INSTALLATIONbody (Fig. 12). (1) If new master cylinder is being installed, re- (11) Fill and bleed master cylinder on bench before move protective cover from end of primary piston.installation in vehicle. Then bleed cylinder on bench as described in this section.MASTER CYLINDER BENCH BLEEDING (2) Slide master cylinder onto booster studs. Align A new master cylinder should always be bled be- booster push rod in cylinder primary piston and seatfore installation in the vehicle. This practice saves cylinder against booster.time during brake bleeding because air in the cylin- (3) Install master cylinder attaching nuts (Fig. 18).der will not be pumped into the lines. Tighten nuts to 25 N⅐m (220 in. lbs.) torque. Cylin- The only tools needed for bench bleeding are a vise, der attaching nuts are special locking type. Doa pair of bleed tubes and a wood dowel the same di- not use substitute fasteners.ameter as the cylinder push rod. Bleed tubes can ei-ther be purchased, or fabricated from sparebrakelines and fittings. The bench bleeding procedure is as follows: (1) Mount master cylinder in vise. Clamp vise jawson one of the cylinder mounting ears. (2) Install bleed tubes in cylinder outlet ports anddirect tube ends into appropriate reservoir chambers(Fig. 17). Fig. 18 Master Cylinder Attaching Nut Removal/ Installation (4) Mount combination valve bracket on booster mounting studs. (5) Install brakelines that connect master cylinder to combination valve. Start brakeline fittings by hand to avoid cross threading. (6) Install nuts that attach combination valve bracket to booster mounting studs. Tighten nuts to Fig. 17 Typical Method Of Bench Bleeding Master 25 N⅐m (220 in. lbs.) torque. Cylinder (7) Fill and bleed brake system. (8) Install windshield washer reservoir. (3) Fill reservoir chambers about 3/4 full with (9) Install engine air cleaner top cover and hoses.fresh, clean brake fluid. (4) Bleed cylinder by stroking cylinder pistons in- HCU REMOVALward then allowing them to return under spring (1) Loosen clamp that secures clean air hose to in-pressure. Use a wood dowel, or similar tool to stroke take manifold. Then disconnect PCV hose at valve.pistons (Fig. 17). (2) Unsnap engine air cleaner top cover and re- (5) Continue stroking pistons until bubbles no move cover and hoses (Fig. 19).longer appear in fluid entering reservoir. (3) Remove filter from air cleaner housing (Fig. (6) Remove bleed tubes and install plastic plugs in 19).cylinder outlet ports. Plugs will prevent fluid loss (4) Remove bolts/nuts attaching air cleaner hous-and keep dirt out until cylinder assembly is ready for ing to body panel. Then work housing off ambient airinstallation. duct and remove housing from engine compartment. (7) Top off reservoir fluid level and install cap. (5) Remove windshield washer reservoir attaching screws. Then position on top of fender (Fig. 8). It is not necessary to disconnect reservoir hoses or wires. (6) Slide HCU solenoid harness connector off re- taining tab on combination valve. Then unplug con-
  24. 24. 5 - 24 MASTER CYLINDER—BRAKE BOOSTER—COMBINATION VALVE—HCU ZJ Fig. 21 Combination Valve Assembly Removal (14) Remove three nuts that attach HCU bracket to suspension support panel (Fig. 22). Retain bracket nuts. Fig. 19 Air Cleaner Componentsnector from engine compartment harness and move itaside (Fig. 20). Fig. 20 HCU Solenoid Harness Connector Fig. 22 Removing HCU Bracket Attaching Nuts (7) Disconnect wire from combination valve switch (15) Disconnect HCU pump motor harness (Fig. 23).(Fig. 4). Be careful when separating wire connectoras lock tabs are easily damaged if not fully disen-gaged. (8) Remove brakelines that connect master cylin-der to combination valve (Fig. 4). (9) Disconnect remaining two brakelines (fromHCU) at combination valve. (10) Remove nuts attaching combination valvebracket to booster mounting studs. (11) Slide combination valve bracket off boostermounting studs and remove valve (Fig. 21). Ifbracket is tight fit, use pry tool to work bracket offstuds. (12) Remove nuts attaching master cylinder tobooster mounting studs (Fig. 22). Retain cylinderattaching nuts as they have a special interfer-ence fit thread. Fig. 23 HCU Pump Motor Harness Connector (13) Remove master cylinder (Fig. 10).