Customer concerns about a fault with the antilock braking system can be best handled by performing the following:
Continued Step #1 Verify the customer concern This is very important because often a problem with the base brakes is thought by the customer to be a fault with the antilock braking system. It is helpful for the owner to drive the vehicle with a service tech if the problem is not readily apparent.
Step #2 Perform a visual inspection If both the red and the amber brake warning lights are on, look for a fault in the hydraulic system, including leaks at the following locations:
Electrohydraulic control unit
Flexible brake hoses
Brake lines and fittings
Calipers and/or wheel cylinders
Step #3 Check for stored diagnostic trouble codes ( DTCs ) Use a scan tool or other necessary methods to retrieve diagnostic trouble codes. If found, follow the specified factory procedures to isolate and determine the cause.
Step #4 Complete the repair This step may involve replacing a hydraulic component. If so, then the hydraulic system should be bled using the factory specified procedure. Clear all diagnostic trouble codes. Step #5 Verify the repair Always test-drive the vehicle under the same conditions that were needed to verify the problem to be sure that the cause has been corrected.
BRAKE WARNING LAMP OPERATION
The first step in the visual diagnosis of an antilock braking system problem is to check the status of the brake warnin lamps. Red Brake Warning Lamp A red brake warning lamp ( RBWL ) warns of a possible dangerous failure in the base brakes, such as low brake fluid level or low pressure in half of the hydraulic system. The red brake warning lamp will also light if the parking brake is applied and may light due to an ABS failure, such as low brake pressure on an integral system. See Figure 82–1.
Figure 82–1 Typical brake warning lamp operation chart. Not all vehicles use the same light sequence. The top of the chart indicates normal warning lamp operation if the system is okay. Continued See the chart on Page 1004 of your textbook.
Amber ABS Warning Lamp The amber ABS warning lamp usually comes on after a start during the initialization or startup self-test sequence. The exact time the lamp remains on after the ignition is turned on varies with the vehicle and the ABS design.
Figure 82–2 On most vehicles equipped with ABS, the ABS and the BRAKE warning lamp should come on as a bulb check when the ignition in first switched on. Continued
THOROUGH VISUAL INSPECTION
A thorough visual inspection should include the following items:
Continued See the list on Page 1005 of your textbook.
Figure 82–3 A thorough visual inspection should include carefully inspecting around the electrohydraulic unit for signs of obvious problems or the installation of aftermarket devices such as alarm systems. Continued
A fault in a wheel speed sensor (WSS) is a common ABS problem. A quick and easy test that works on most Bosch ABS (and perhaps others) involves the following steps:
Quick and Easy Wheel Speed Sensor Diagnosis Step #1 Hoist the vehicle safely. Step #2 Turn the ignition on (engine off). Step #3 Spin a tire by hand as fast as possible. Step #4 The ABS amber warning light should come on, indicating a speed was detected but not by all the wheel speed sensors. Step #5 Turn the ignition off to reset the ABS warning light. Step #6 Repeat the test on each of the remaining wheels. If any wheel fails to turn on the ABS light, carefully inspect the wheel speed sensor for proper resistance and the tone ring and wiring. If the ABS light is on all the time and does not reset when the ignition is turned off, the problem is not caused by a wheel speed sensor.
Continued NOTE: Most wheel speed sensors are magnetic and therefore can attract and hold metallic particles. Be sure to remove any metallic debris from around the magnetic wheel speed sensor.
TEST DRIVE AND VERIFY THE FAULT
A test-drive is a very important diagnostic procedure. Many ABS and diagnostic trouble codes (DTCs) will not set unless the vehicle is moving. Often, the driver has noticed something like the self-test while driving and believed it to be a fault in the system.
Continued NOTE: Some ABS units, such as the Delphi VI, will cause the brake pedal to move up and down slightly during cycling of the valves during the self-test. Each system has unique features. The tech will have to learn to avoid attempting to repair a problem that is not a fault of the system. Before driving, start the engine and observe the red and amber brake warning lamps. If the red brake warning lamp is on, the base brakes may not be functioning correctly. Do not drive the vehicle until the base brakes are restored to proper operation.
Many vehicle owners and service techs have been disturbed to hear and feel an occasional groaning noise. It is usually heard and felt through the vehicle after first being started and driven. Because it occurs when first being driven in forward or reverse, many techs have blamed the transmission or related drive line components. This is commonly heard on many ABS vehicles as part of a system check.
What’s That Noise and Vibration? As soon as the ABS controller senses speed from the wheel speed sensors after an ignition cycles on, the controller will run the pump either every time or whenever the accumulator pressure is below a certain level. This can occur while the vehicle is being backed out of a driveway or being driven forward because wheel sensors can only detect speed—not direction. Before serious and major repairs are attempted to “cure” a noise, make sure it is not the normal ABS self-test activation sequence of events.
See the diagnostics charts on Page 1006 of your textbook. NOTE: Some systems are diagnosed by “antilock” and “brake” warning lamps, vehicle symptoms, and the use of a breakout box.
RETRIEVING DIAGNOSTIC TROUBLE CODES
After performing a visual inspection and verifying the customer’s complaint, retrieve any stored ABS-related diagnostic trouble codes (DTCs). The exact procedure varies with the type of ABS and with the make, model, and year of the vehicle.
NOTE: With some ABS systems, the diagnostic trouble code is lost if the ignition is turned “off” before grounding the diagnostic connector. Always consult factory service information for the vehicle being diagnosed. Some systems can only display flash codes (flashing ABS or brake lamp in sequence), whereas other systems can perform self-diagnosis and give all information to the technician through a scan tool.
KELSEY-HAYES ANTILOCK (NONINTEGRAL)
The Kelsey-Hayes rear-wheel antilock uses two solenoids and valves to control the rear-wheel brakes. KH four-wheel antilock uses the computer to pulse the valves. The pulsing is called pulse-width modulated (PWM) and the valve is called a PWM valve.
Retrieving Diagnostic Trouble Codes GM trucks’ (RWAL) DTCs are retrieved by flash codes or scan data through the use of a scan tool or connect H to A at the data link connector (DLC). Figure 82–4 General Motors diagnostic connector. Flash codes are available by using a jumper wire to ground (terminal A) to terminal H. This connector is located under the dash near the steering column on most General Motors vehicles. Continued
Ford trucks’ (RABS) DTCs are retrieved by jumper lead flash codes only. Chrysler light trucks’ DTCs are retrieved by ground diagnostic connections. See Figure 82–6.
NOTE: Be sure that the brake warning lamp is on before trying to retrieve DTCs. If the lamp is not on, a false code 9 could be set. Figure 82–5 Connecting a jumper wire from the diagnostic connector to ground. The exact location of this diagnostic connector varies with the exact vehicle model and year. Continued
Figure 82–6 Chrysler diagnostic connector location varies with the model and year. Continued NOTE: If the ignition is turned off, the failure code will be lost unless it is a hard code that will be present when the ignition is turned back on.
Kelsey-Hayes Diagnostic Trouble Codes
See these charts on Page 1007 of your textbook. Continued
The owner of an S-10 pickup truck complained that the red brake warning lamp on the dash remained on even when the parking brake was released. The problem could be one of the following:
RWAL Diagnostics 1. A serious hydraulic problem 2. Low brake fluid 3. A stuck or defective parking brake switch 4. If the brake lamp is dim, RWAL trouble is indicated. The tech found that the brake lamp was on dimly, indicating that an antilock braking problem was detected. The first step in diagnosing an antilock braking problem with a dash lamp on is to check for stored trouble codes. The tech used a jumper between terminals A and H on the DLC (ALCL), and four flashes of the brake lamp indicated a code 4. Checking a service manual, code 4 was found to be a grounded switch inside the hydraulic control unit. The hardest part was getting access to, and replacement of, the defective (switch. After bleeding the system and a thorough test-drive, the lamp sequence and RWAL functioned correctly.
BOSCH 2 ABS (NONINTEGRAL)
The Bosch 2U/2S ABS is used on many domestic and imported brands of vehicles.
Retrieving Diagnostic Trouble Codes On GM vehicles, DTCs can be retrieved by connecting A to H at the data link connector (DLC). On most Bosch 2 systems, a scan tool can and should be used if available to retrieve DTC.
The Teves Mark IV is a nonintegral (remote) ABS.
Continued Figure 82–7 A scan tool is the recommended method to use to access General Motors Teves Mark IV systems. Retrieving Diagnostic Trouble Codes Trouble codes are accessed only by a bidirectional scan tool connected to the data link connector (DLC).
See these charts on Page 1008 of your textbook. Continued
Clearing Diagnostic Trouble Codes A scan tool is required to clear DTCs on some vehicles. Driving the vehicle over 20 mph (32 km/h) will clear the codes on some vehicles. Disconnecting the battery will also clear the codes but will cause other “keep-alive” functions of the vehicle to be lost.
DELPHI (DELCO) ABS VI (NONINTEGRAL)
The Delphi (Delco) ABS VI is unique from all other antilock systems because it uses motor-driven ball screws and pistons for brake pressure to reduce, hold, and apply.
Continued Retrieving Diagnostic Codes The Delphi (Delco) VI antilock braking system has extensive self-diagnostic capability. Access to this vast amount of information requires the use of a scan tool designed to interface (work) with the Delphi VI system. See Figures 82–8 and 82–9 and DTC code chart.
Figure 82–8 Delphi (Delco) VI system components. Notice that each front brake is controlled by a separate piston, whereas the rear brakes are controlled by the same piston. Continued
Figure 82–9 The Delphi (Delco) VI attaches to the side of the master cylinder and connects hydraulically through transfer tube assemblies. Continued
See the chart on Page 1009 of your textbook.
A customer complained that, sometimes during normal braking, the ABS would be activated just before coming to a stop. However, the ABS light would not come on. The service tech was able to duplicate the condition and there were no DTCs stored. Using a scan tool to monitor the wheel speed sensors, the tech discovered that the left front wheel speed was slightly different than the others.
The Nervous Taurus A thorough visual inspection revealed that the tone wheel (sensor ring) was cracked. This crack created a different wheel speed signal to the ABS controller than the other wheels and the controller activated the ABS as it would normally—that was why there were no DTCs. Other things that could have caused this problem, which is often called “false modulation,” include a bent wheel, mismatched tire sizes, or metal debris around the sensor.
Strange electrical problems can occur including false DTCs or intermittent operation of electrical sensors, ABS, accessories, or gauges. Sometimes these problems are due to rust and corrosion after a vehicle is involved in a flood. Here are some telltale signs that a vehicle may have been in a flood or deep water:
Look for Flood Damage If Electrical Problems
Mud, silt, or caked dust under the dash and inside the doors
Corroded electrical connectors at the computer, fuse box, or ABS controller (computer)
Visible water line in the doors or behind panels
Rust in abnormal places, as seat springs or brackets behind the dash
Moisture in lenses, musty smell and/or strong air freshener smell
Powdery corrosion on aluminum parts such as intake manifold and inside the throttle bore
Rust or moisture inside electrical switches or relays
Areas normally dusty such as an ashtray or glove box are very clean
The owner of an Acura Legend complained to a service tech that the ABS warning light would come on but only while driving down from a parking garage. When the driver turned off the ignition and restarted the engine, the ABS amber light was not on and did not come on again until the vehicle was again driven down the spiral parking garage ramp. The service tech used a scan tool and found that no DTCs had been stored.
The Mystery ABS Amber Warning Light - Part 1 Some ABS will not retain a DTC unless the problem is currently present and the ABS amber warning light is on. All the brakes were in excellent condition, but brake fluid level was down a little. After topping off the master cylinder with clean DOT 3 brake fluid, the vehicle was returned to the customer with the following information:
The ABS amber warning light may have been triggered by the brake fluid level switch. While driving down the steep parking garage ramp, the brake fluid moved away from the fluid level sensor.
The Mystery ABS Amber Warning Light - Part 2 While the brake fluid level sensor normally would turn on the red brake warning light, in some systems it turns on the amber ABS light if the brake fluid falls below a certain level in the ABS reservoir.
The difference in wheel speed between the outboard and the inboard wheels could have triggered a fault code for a wheel speed sensor during the drive down the spiral parking garage ramp.
WHEEL SPEED SENSOR DIAGNOSIS
Wheel speed sensor (WSS) circuits are often the cause of ABS problems. These components may suffer from physical damage, build-up of metallic debris on the sensor tip, corrosion, poor electrical connections, and damaged wiring.
See Figure 82-10.
Figure 82–10 Typical wheel speed sensor. When a tooth on the sensor ring is close to the sensor, the strength of the magnetic field is stronger because the metal of the tooth conducts magnetic lines of force better than air. When the tooth moves away, the magnetic field strength is reduced. It is this changing magnetic field strength that produces the changing voltage. Frequency of the signal is determined by the speed of the rotating sensor. Continued
Test a WSS by measuring its output voltage and circuit continuity. A breakout box ( BOB ) cable connects to the ABS harness near the ABS module. All WSS resistance checks, including the wiring to the sensors, can be measured at one location.
Figure 82–11 (a) A breakout box is being used to diagnose an ABS problem. The controller (computer) is located in the trunk of this vehicle, and a digital multimeter is being used to measure resistance and voltage at various points in the system, following the service manual procedure. Continued Follow manufacturer’s instructions for connecting the breakout box and probing appropriate pins. See Figure 82-11b
Figure 82–11 (b) Another vehicle being tested for an ABS fault. In this vehicle, the computer is located under the passenger seat, which has been removed to gain better access to the wiring and terminals. Continued
Figure 82–12 Measuring the resistance of a wheel speed sensor.
Resistance Measurement The resistors of most WSS range from 800Ω to 1,400Ω. Therefore, a reading of about 1,000 ohms or 1 KΩ would indicate proper sensor coil resistance.
Checking for Short-to-Ground Connect either lead of the ohmmeter to one of the WSS wires and the other to a good, clean chassis ground. The resistance should be infinity (OL). If a low resistance reading is obtained, the sensor or sensor wiring is shorted-to-ground and must be replaced.
AC Voltage Check Connect a digital meter to the WSS terminals or input to the controller in the breakout box and set the meter to read AC volts. Rotate the wheel by hand at a rate of one revolution per second. A good WSS should produce voltage of at least 0.1 volt (100 mV). A sensor voltage of lower than 0.1 volt (100 mV) may be caused by three things:
Excessive clearance between the sensor and the tone ring.
Buildup of debris on the end of the sensor. Most WSSs are magnetic and can attract metallic particles, which can affect the operation of the sensor.
Excessive resistance in the sensor or sensor wiring, which can also cause a weak signal to be produced by the WSS.
Scope Testing Attach the scope leads to the sensor terminals or to the input connector on the breakout box. With all four wheels off the ground, rotate the wheel by hand or by engine power . A good WSS should produce an alternating current (AC) sine wave that increases in frequency and amplitude with increasing wheel speed.
Figure 82–13 A scope can be used to check for proper operation of a wheel speed sensor. Continued
Damaged or missing teeth on the tone ring will cause flat spots or gaps in the sine wave pattern.A bent axle or hub will produce a wavelike pattern that fluctuates as the strength of the sensor signal changes with each revolution.
Figure 82–14 A broken tooth on a wheel speed sensor tone ring shows on the scope trace as a missing wave. Continued
Scan Tool Testing A scan tool can be used to check for the proper operation of the WSS. As an assistant drives the vehicle, connect the scan tool and monitor the speed of all of the sensors. All of the sensors should indicate the same speed. If a sensor shows a slower or faster speed than the others, carefully check the tone ring for damage such as a crack.
WHEEL SPEED SENSOR ADJUSTMENT
Some ABS applications use adjustable WSSs. Most sensors adjust by first loosening a set screw, then inserting a nonmagnetic brass or plastic feeler gauge between the tip of the sensor and a high point on the tone ring.
Continued Figure 82–15 Use a nonmagnetic brass or plastic feeler gauge to check wheel speed sensor gap. A steel gauge would be attracted by the magnet in the sensor and would produce a drag on the gauge as it is moved between the sensor and the tone ring. This drag could be interpreted as a correct clearance reading. Adjust the sensor position so there is slight drag on the feeler gauge, and tighten the setscrew to lock the sensor in place.
Figure 82–16 (a) Always use a nonferrous (brass or plastic) feeler (thickness) gauge when measuring the gap between the toothed ring and the wheel speed sensor. (b) Sometimes a sensor is equipped with a paper spacer that is the exact thickness of the spacing required between the toothed ring and the sensor. If equipped, the sensor is simply installed with the paper touching the toothed wheel. A typical gap ranges from 0.020 to 0.050 in. (0.5 to 1.3 mm).
Adjust the sensor so the tip just touches the tone ring and you can slip the paper or plastic out without ripping it. Tighten the setscrew and the air gap is properly set.
(a) (b) Continued
Digital Wheel Speed Sensor Diagnosis Test a digital WSS by first checking that battery voltage is available at the sensor with the key on, engine off. If the sensor does not have 12 volts, the problem is the ABS controller or wiring between controller and sensor. If there are 12 volts at the sensor, measure the signal voltage. The voltage should switch from about 0.8V to about 1.9V as the wheel is rotated by hand.
HYDRAULIC ABS SERVICE
Before doing any brake work on a vehicle equipped with antilock brakes, always consult the appropriate service information for the exact vehicle being serviced. Some manufacturers recommend discharging the hydraulic accumulator by depressing the brake pedal many times before opening bleeder valves. Many service checks require that a pressure gauge be installed in the system. Air can easily get trapped in the ABS electronic-hydraulic (E-H) assembly whenever the hydraulic system is opened. Some E-H units can be bled through use of a scan tool where the valves are pulsed in sequence by the electronic brake controller (computer).
Some units are equipped with bleeder valves while others must be bled by loosening the brake lines. Bleeding the E-H unit also purges out the older brake fluid, which can cause rust and corrosion damage. Only DOT 3 brake fluid is specified for use in an antilock braking system. Always check the label on the brake fluid reservoir and/or service manual or owner’s manual.
Continued CAUTION: Some ABS units require that the brake pedal be depressed as many as 40 times to discharge brake fluid fully from the accumulator. Failure to discharge the accumulator fully can show that the brake fluid level is too low. If additional brake fluid is added, the fluid could overflow the reservoir during an ABS stop when the accumulator discharges brake fluid back into the reservoir.
Bleeding ABS During routine brake service, attempt to keep the air from entering the hydraulic system by doing the following:
Do not allow the brake system to run dry. Use a brake pedal depressor or plug any open brake line to keep brake fluid from flowing out of the brake master cylinder reservoir.
Do not allow the master cylinder to run dry during the bleeding operation. Check the master cylinder reservoir and keep it filled with fresh brake fluid from a sealed container.
Always bench bleed a replacement master cylinder to help prevent against introducing air into the hydraulic system.
NOTE: To avoid bleeding the hydraulic unit, use a brake pedal depressor during service to avoid losing brake fluid. This precaution keeps air from getting into the hard-to-bleed passages of the hydraulic unit.
Figure 82–17 Special bleed valve tools are often required when bleeding some ABS units such as the Kelsey-Hayes 4WAL system.
After depressing the unit as per manufacturer’s recommended procedures, the brakes can be bled using the same procedure as for a vehicle without ABS. Air trapped in the ABS hydraulic unit may require that a scan tool be used to cycle the valves.
Figure 82–18 Two bleed valve tools are needed to bleed the Kelsey-Hayes 4WAL system, which attaches to the bleed valves on the accumulator.
The bleeding procedure for vehicles equipped with antilock brakes is often different than vehicles without ABS.
Consult service information for the specified bleeding procedure and sequence. Continued
ABS SAFETY PRECAUTIONS
Avoid mounting the antenna for the transmitting device near the ABS control unit. Transmitting devices include cellular (cell) telephones, citizen-band radios, and so on.
Avoid mounting tires of different diameter than the original tires. Different size tires generate different wheel speed sensor frequencies which may not be usable by the ABS controller.
Never open a bleeder valve or loosen a hydraulic line while the ABS is pressurized. The accumulator must be depressurized according to the manufacturer’s recommended procedures.
If arc welding on a vehicle, disconnect computers (electronic control modules) to avoid damage due to voltage spikes.
Do not pry against or hit the wheel speed sensor ring.
ABS diagnosis starts with checking the status of both the red brake warning lamp and the amber ABS warning lamp.
The second step in diagnosis of an ABS problem is to perform a thorough visual inspection.
The third step in diagnosis of an ABS problem is to test-drive the vehicle and verify the fault.
Always consult the factory service information for the specific vehicle being serviced for the proper procedure to use to retrieve and clear diagnostic trouble codes (DTCs).
breakout box is used with a digital multimeter to diagnose electrical ABS components.
Hydraulic service on most integral ABS units requires that the brake pedal be depressed as many as 40 times with the ignition key “off” to depressurize the hydraulic system.