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ilI.!> *if$ STJRTUNED Information for the Independent Mercedes-Benz Service Professional June 2004 U.S. $6.00 € 12.50 Group Publisher FEATURE ARTICLES Chri stoph er M. Ayers, Jr. email@example.com Editor 4MATIC AND ETS Joe Woods Traction by Pipe and by Wire jwoods@c macomm.com The last (for now) in our traction control series MBUSA Technical Content Advisor Donald Rotolo Donald.Rotolo@mbusa.com Project Director Andrew Brzo zowsk i Brzozows ki A@mbusa.com FUEL PRESSURE AND VOLUME The simple is not easy Ar t Director Jef Sturm jsturm@cma co mm.com Production Manager Joe Cristi ano jcri stiano@c ma comm.com TEXTBOOK CASE Circulation Manager 202 Climate Control, An easy exercise Phillip A. Helon phelon@c macomm. co m List Consu ltant NFocus DEPARTMENTS StarTun ed is a quarterly publi ca- tion of Mercedes-Benz USA, LLC ("MBUSA"). No part of thi s FACTORY SERVICE BULLETINS news letter may be rep rod uced without th e express written per- These suggestions and sol utions for technica l problems are mi ss ion of MBUSA. E d it o ri a I and Circu lation Offices: 598 from service bulletins and other information published by Pi ne Po int Drive, A kron , Oh i o 44333 . Caution : Veh icle servicing Mercedes-Benz, select~d and adapted for independent repair shops. perform ed by untrained persons could r es u lt i n se ri o u s injur y t o those p er so n s o r ot h ers. Informati on co ntai ned in thi s PARTS NEWS news l etter is in te nded for use by trained , profes sional auto Whether for vehicles long out df production or for modern ones, r epa ir tech n i c i ans ONLY. Thi s inform ati on i s prov id ed your best source for Genuine Mercedes-Benz Parts is Mercedes-Benz. t o in form th ese t echni ci an s of co n dit i on s w hi ch may occur in some veh icl es or to pro v i d e i n form a ti o n w hi c h co ul d as s i st them in prope r se rv i cin g of t h ese ve hi cl es . GENUINE MERCEDES-BENZ PARTS ... NEARBY Pro pe rl y t rain ed tech ni cian s h ave th e eq ui p m e nt, too l s, Wherever you are in the United States, sa fet y in stru cti ons, an d know - holV to perform r epa i rs corr ect- theres a nearby source of genuine factory parts ly and sa fely. I f a co ndition i s for your customers Mercedes-Benz veh icles. ® d esc rib ed, DO NOT ass um e t hat a top ic cove r ed in the se pages auto mati ca ll y appli es to yo ur ve hi cle or t hat your ve h i- cle has that condition. StarTul1ed is a registered trademark of MBUSA. Mercedes-Benz
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Four-wheel-dri ve increases traction, often enough The ord in ary, fam iliar diffe rential is an open dif-to s usta in movement and co ntro l un der cond itions fe re ntia l. Torque deli vered thro ugh the drives haftthat mig ht otherwise mea n the d ri ve r wo ul d choose pinion a nd differe ntial ring gear applies equallyto stop or would get stu ck in s now, mud or sand. through the sp ider gears to each axle halfs haft,Fo ur-w hee l-drive is n ot co nsta ntly adva ntageou s, rega rdl ess of individu al halfs haft speed. Aside fromhowever, because it mecha ni ca lly coup les all th e s li ght fri cti on betwee n in te rn al parts, an open dif-whee ls a nd dr ivetrains togeth er; and so metimes, fe re ntial a pplies equal to rque to each axle - with thesurprisingly e nough, th at conn ec tion ca n work co nseq uence that th e maximum torq ue th e differen-aga in st optimi zati on of traction a nd co ntro l. There tial can a pply to its drivew hee ls is just double whathave bee n two q uite di ffe rent syste ms called th e d ri vewheel with less tracti on can trans mit alone,4MATIC, the fi rst hydrauli c, beginning about 199 0 the drivew heel that s lips first. Once that first wheeland the second electro nic, wo rk ing through th e s pin s, th e vehicle stops progress. This is exactlyservice brakes, starting in 1998. We concentrate what we see when a ca r sp in s a drivewh eel in ahe re on the earlier system sin ce yo ure more likely s nowp ile or mudh ole: Until that wheel regain s trac-to see it in an ind ependent shop. tion, a sta ti onary car does nt budge. In the extremes of lim ited traction, an open diffe rential gives youLocked and Open Differentials one-wh ee l-drive, and, worse yet, drive through wh ichever wheel has less tracti on . So we move to th e locked or limited-slip differen- "Four-wheel-drive" can be a mi sleading descrip- tial. "Locked" is an exagge rati on, as you know,ti on. In principle, of course, any driveline system because in side and outside wheels must travel dif-conducting powe r through the tra nsmiss ion to a ll fere nt speeds in a turn. Otherwise th e inside wheel,four wheels is four-wheel-drive. Th ere are, howeve r, us ually less heavily loaded as centrifugal forcesignifica nt differences ari sing from the distinction throws th e weight outward , must skid forward alongbetween locked and open differentials. Lets have t he paveme nt.tha t clear fi rst. The 4MATIC differenti als lock in a way sim ilar to the limited-slip diffe re ntia ls yo u may know fro m do mestic trucks or high-perform ance (at least fast- accelera ti on) vehi cles. In most of t hose, a co nstantly s prin g-load ed multi disk clutch lin ks the opposite- s ide axle s hafts , th rough a friction material and spe- cia l lubri cant, both requiring peri ocii c replacement.
4MATICAND ETSThe earli er 4MATICs use hyd rauli cs to engage th e This mea ns two wheels, one on each axle, have toclutches, as well see. slip simultaneou sly for the ve hi cles motion to stop. There are al so true lock ing differentials using slid- Such four-wheel-drive wi ll , however, represent a sig-in g-dog clutches, but th ese engage with an imm ed i- nifi cant traction improvement over plain-va n i lI aate, heavy clank, a force remin iscent of excavation or two-whee l-d rive. After aU, it means at l east a dou-railroad eq uipment, an engagement force that would bling of traction, and usuall y more th an that, ass um-be quite unwelcome in a sedan. With an earthmov- in g so mew hat different traction at each wheel. Ju sting impl ement, this doesnt matter sin ce the grind- as two-wh ee l-drive through an open di fferential real-ing ea rth moves und er th e tire lugs and si nce nei- ly mea ns one-whee l-d rive, four-wheel-d rive throughther driving comfort nor speed was a priority, any- two open differentials really means two-wheel-drive.way. With a sedan, thats out of the qu estion . On a Thats more traction, but it doesnt use all th e trac-vehi cle driv ing mostl y on hard pavem ent, you dont tion ava il ab le.want a truly lockin g differential, not on ly because A four-wheel-drive syste m that locks the front andthat wou ld abrade the tires with lowe r load qui ck ly, rear ax les together still has the problem that one axlebut also because it would mea n one or t he other tire or th e other must slip in every turn, because the fronton an axl e would have to slid e in every turn , actual- wheels will invariab ly follow a track along a sma ll erly reducing traction eve n und er what should have radius and shorter path than the rears. A long tim ebeen id ea l pavement co nditi ons. ago, I had an anci ent Jeep th at never rea ll y got stuck, but took every turn with unpredictab le directionalTorque through the Transfer Case excitemen t anytim e four-wheel-drive was engaged. You co uld never tell whether the front or the rear ax le wo uld slide loose sideways first - that depended The earli est four-wheel-drive systems simpl y used on everything from the load to the quality of th e grav-a second driveshaft from a transfer case to the front el on th e road. And your co rrective maneuver had toax le differential, where anoth er open dif- wait until th e slip was clea rly under-ferential did its torque- way, because you turned thesplittin g wo rk. The steering whee l in oppositesa me l ess-trac- directions to recovertion limi tation depending on which axlest i II app l ies, slid. Exciteme nt, as I said.but with this The obvi ous manufactur-advantage: The in g fix is to in clu de anaverage speed interax le differential, whichof the fron t all modern fo ur-wheel-driveswheels WO Uld , do. You ca nt just add a centerby so lid differential and let it go at that,mechanical howeve r, or youre back withIin l<age, move at yo ur original prob l em:exactly th e same A ll the torque goes tospeed as the th e first whee laverage speed of that slips, and thet he rears ("aver- ve hi c l e stopsage" because each there. FOUf-w h ee l on each w h ee l-dri veax l e ca n st ill with three open move at a differ- differentialsent speed from beco mes, in the its opposite num- extreme, one-ber, but each axles differen- wheel-drive again. tia l ring gear turns at exactly the same speed as th e other axles). The transfer case can engage two-wheel-drive only, four-wheel-drive or four-wheel-locked. In the last case, front and rear axles turn at the same speed.
4MATIC Progressive Locks sup plementary front drivetrain and the transfer case, but also a detailed hydraulic system, from So the Mercedes-Benz 4MATIC system emp loys pump to actuator pistons, to actuate the various dif-locked four-wheel-drive when the various sensors ferentiallocks that come into play under conditionsindicate the need for the add itional traction, but not well discuss in a minute. ETS, the new 4MATlC sys-unde r normal driving conditions when ordinary two- tem emp loying 4ETS, in contrast, works mainly elec-whee l-drive through the open rear differential is suf- tronically (thus the Wire of our subtitle), with theficie nt to conduct all the forward drive moment the control unit activating specific wheel brakes to keepengine can deliver to the whee ls, and not under the wheel with the least traction from sp inning. ETSthose specia l driving conditions when - paradoxi- works specificall y with two-wheel-drive cars; 4ETScally - a locked four-wheel- drive could make a low- applies to those with a front driveax le. Somewhattracti on condition worse. confusingly, 4ETS is also called 4MATlC. Perhaps the most important such contrary condi- Here are the progressive steps 4MATICtion occurs during hard-braked, ABS-pu lsed stops, emp loys when the wheelspeed sensors and theparticularly in turns. If foul-wheel -drive were still steering wheel sensor information point to drive-engaged under those conditions, it would be possible wheel spin: Driving along an ordinary road withfor the interconnected drivelines either to mask a good traction and no whee ls pin, the car works bywhee ls slip from the sensors report and thus block ordinary two-wheel-drive with an open differen-the control units recognition and countermeasures, tial at the rear drivewheels. This provides theor to force a wheel with limited traction to turn at the best combinatio n of drive and directional tractionsame speed as the others (this last is most noticeable in the absence of a problem.during a steep turn or rapid steering changes, The old er hydraulic system provides open-dif-when whee lspeed varies at the different ferential two-whee l-drive because the multi-whee ls because each follows a path with disk clutch pack for the center differential locka different turning radius and differ- is spring loaded closed, that is,ent length (and thus, a different engaged. This couples the inputspeed ). To accommodate turns, one shaft from the transmission toof th e inputs to the 4MATIC contro l the transfer case planetary car-Llnit is a steering whee l angle sen- rier, sp lin ed directly to the dri-sor, similar but not identical to or veshaft for the rear axle. This isinterchangeable with the one used the fail-safe mode of the system,with ASR, ESP and ETS. insuring rear-axle two-wheel- The greater the steering whee l drive regardless of any electri-angle, the larger the speed difference ca lor hydraulic fault in thethe system allows mechanism.between the wheels. 4MATIC is themost mechanical (thus the Pipe of oursubtit l e) of the Mercedes-Benz trac- tion contro l systems, because it involves not only all the steelshafts and gear- works of theThe system works hydraulically, but its also internally lubricated with oil. If the sea~s on the multidisk clutch pis- •tons start to leak, the hydraulic oil can fill the transfer case and begin to run from the lent. The loss of hydraulicfluid would be obvious long before that.
4MATIC AND ETS When the sensors indicate the rear drivewheels are turning faster than the front wheels, at a thresh- old determined by the control unit as wheelslip, the system switches over to its first shift stage, compen- sated four-wheel-drive. Compensated four-wheel- drive is neither an open differential nor one thats locked. Instead, the geometry of the transfer case gears routes 65 percent of the torque to the rear axle and 35 percent to the front. This allows all four whee ls to turn at different speeds, but within plau- sib le constrai nts of how much traction they might have and how much drive torque they can absorb . The system se lects this stage by applying hydraulic pressure to both the clutch packs in the transfer case, unloading th e spring-loaded center differential lock and engaging the front axle drive clutch. The third stage, the second shift stage, locks the interaxle differential by releasing hydraulic pres- sure to its clutch (allowing the spring to lock it) whil e maintaining engagement pressure to the front ax le clutch. Now we have traction sim ilar to that old Jeep - it wont get stuck easily, but a steep turn could be a problem, were it not for the steering angle sen- sor, which would signal time to return to the previ- ous shift stage. To prevent engine surge, the system briefly engages this stage every time, releasing the center lock shortly thereafter. The fourth stage, the third shift stage, adds the lock for the rear differential, preventing those drive- whee ls from turning at significantly different speeds. There is no differential lock on the front axle differential because that could cause a steering problem. Better that a driver gets temporarily stuck than temporarily loses steering. Since the disks dont mechanically lock, there is sti ll eno ugh difference of speed between the half- shafts to accommodate the difference of speeds in ordinary maneuvers and turns. The system locks with a substantial but not excessive resistance to turning at different speeds. There is, in fact, a test yo u perform with a torque wrench to check the fric- tional force of the hydraulic lock. Specifications vary by model, largely reflecting the weight of the car and its consequent traction . Like any test i nvolv- ing a live drivetrai n on a hoist with the engine run- ning, th is requires considerable attention to the cor- rect procedures to prevent an acc ident. There are three differentials involved in 4MATIC: the familiar one in the pumpkin case at the center of the rear axle, a similar one in front sharing a hous- ing with the oilpan, and a sun-and-planetary design in the transfer case connecting the front and rear dri- veshafts. There is no lock ing clutch in the front axl e• differential, because locking the steering wheels together wo uld destabilize the car during turns. The lock ing clutch in the rear axle differential is essen-
sions. The reason for using a Ravineaux gearset is th at the geometry allows th e sun and ring gears to turn in the same direction. The relative sizes of the sun and planetary gears determine the torque split front to rear when the center lock is disengaged. Notice this may mean there is still no usable trac- tion. It is much less likely that traction con ditions will be so poor that none of the wheels will have suf- ficient traction to move the car, but th at is possible. We can all imagine (a nd some of us who live in the North have expe rienced!) conditions under which thi s is just what happens - all the wheels spin, but th e ca r still cant move. Like all the traction-control systems, 4MATIC is limited to working in the real world with real-world physical limitations. It works very well, but not magically. Working with 4MATICtially the same mechanism familiar from ASD, whichwe discussed in an earlier issue. Notice each of these Here are some service points when working on adifferentials is necessary for all but the most primi- 4MATIC car. As with any hydrauli c system, air bub-tive of four-wh eel-drive systems. And the Mercedes- bles and leaks sometimes occur. You ca n often flushBenz system is far from the most primitive. air through the system by merely toggling th e serv- The interaxle differential does not look much like a ice va lve for ward to the test position and running thedifferen tial, but heres how it works. The transmis- e ngine. This opens the circuits and lets the oil flow.s ion for a 4MATIC car is the same as for the others, If there is sti ll a ir in the system, you can vent it frombut the tail s haft ends in a ring gear sh ell rather than the caps on the rear differenti al pistons and by loos-the output shaft and three-lobed driveflange th at enin g the hydra uli c lines at the transfer case untilco uples to the driveshaft through the fl ex ible rubber the oil flows clea r - usually only a few seconds. Byjoint (th e Gobbo joint). The inte rn al teeth of that anecdote, th e most co mmon part requ iring replace-output ring gear shell engage what initi all y appear to me nt is the pump, but this is probably because thebe th e planetary gears on a carri er. Closer inspection, pump is th e high point in the system a nd loses lubri-however, reveals a second set of planetary idlers driv- cation first if th eres a leak. Thi s tandem pumpen by the first set, a nd these seco nd gea rs engage the forms a unit with th e power steering system and alsos un gear at the center. provides hydra ulic press ure for a ny vehicle with This arrangement of sets of pl a netary idlers on th e hydropne umatic suspe nsion.carri e r is ca lled a "Ravin ea ux gearset," and gri zzled Th e accumulator, a compressed nitrogen chamberStarTul1ed readers long on th e job may recall that was atop th e hydra uli c control unit, in cludes a diaphragmthe central gearset of the two-speed automati c tran s- to separate gas from oi. If that diaphragm eve ntuallymissions long used in domestic autom atic trans mis- Continued on Page 14
4MATICAND ETS Shift Modes The earlier, hydraulic 4MATIC works through the axle; and there are three multidisk clutches: two in transfer case and rear differential, so it is worth- the transfer case and one in the rear axle. Heres while seeing exactly what happens when the system how they sequentially engage as traction conditions functions normally. There are three differentials: in require (reflected in the sensor input information): the rear axle, in the transfer case and in the front 12 Most of the 4MATIC engagement work occurs in the transfer case, the gearbox that delivers 11 torque from the transmis- sion output shafts ring gear to the front and rear differentials. Initial drive torque is 100 percent to the rear axle, switching to 35/65 front/rear or 50/50 front/rear, as calculated by the electronic control unit from the wheel- speed and steering- wheel-angle sensors. The final shift mode engages the interaxle (transfer case) differential at 50/50 and, as needed, locks the rear differential. First Stage Normally, the 4MATIC car runs under the same two-wheel-drive .. as every other Mercedes-Benz. 100% 100% Engine output ~ torque through .......... the transmis- L1fIr~~~1i~n;;;~~~~;=~;;r"l:~~F~~ sion twists through the transfer case to the rear-axle output shaft. The front multi- disk clutch engages through its preload springs. The rear multi- disk clutch (to the front axle drive) remains dis- engaged. Keep a 4MATIC car on good, dry pave- ment and dont nail the pedal to the floor, and this could be the constant shift mode of the sys- tem for many thousands of miles.
65%Second Stage of the gear wheels in the transfer cases Ravineaux If the wheels peed sensors report a difference gearset. Except for the momentary pause to confirmbetween rear drivewheels and fronts beyond a cer- engagement of the rear multidisk clutch before thetain threshold (a somewhat flexible threshold front releases, all three of the cars differentials aredepending on the steering angle sensor input as well open at this point, and you have drive to both axles.as the vehicle speed), the first traction-controlmeasu re is to engage the front axle drive. The sys- Third Stagetem does this by engaging the multidisk clutch at If there is a difference in wheelspeed from frontthe rear of the transfer case. Once this clutch to back even with the compensated four-wheel-driveengages securely, the system then immediately dis- engaged, the system locks the center differential.engages the spring-loaded front transfer-case clutch. The mechanism for doing so is to vent the pressureA check valve and fixed orifice preclude disengage- at the front multidisk clutch, allowing the spring toment of the front should the rear engagement fail - fe-engage it. Once both transfer case multidiskto insure the vehicle can still drive with at least rear- clutches engage - the rear by pressure applied, theaxle two-wheel-drive under all circumstances, as front by pressure released - both output shaftswell as to prevent engine overs peed. rotate exactly together. As explained, this improves This shift mode is called "compensated four- traction as long as the vehicle moves in a straightwheel-drive" because the torque to the wheels falls line, but can be a problem in a turn. However,into a 35/65 front/rear proportion as a geometric 4MATIC includes the steering wheel sensor, so thatconsequence of the relative sizes and configurations problem should not occur.Fourth Stage modes, the system locks the rear axle to make its wheels turn at the same speed. This final state of The front differential is a permanently open dif- the 4MATIC system has both rear wheels turningferential because otherwise there might be a prob- exactly together and the front wheels turning anlem with steering. The rear differential is usually average speed exactly equal to the rears. Only if aUopen, but if there is still a difference in drivewheel these measures are insufficient will the driver havespeed, after aU the corrections of the previous shift to call for help.
4MATICAND ETSContinued from Page 11leaks, the accumulator can cease priming hydraulic Traction by Wirecomponents and can allow the system to make peri-odic water-hammer clunks as the pump cycles. If ETS and its multi-axle analog, 4ETS, are simpler,the noise goes away with the service valve in the test electronic variants of ASD and the earlier hydraulicposition, suspect the accumulator. 4MATIC. The simplification comes from the absence A mechanical problem that may set you to scratch- of the hydraulic distributor-lockup componentsing yo ur head comes if you remove the front drive- involved in ASD and 4MATIC. Dont be misled, how-shaft. With the rear driveshaft, you can bend it in ever, there is still plenty of hydraulic work afoot -the middle at the center bearing to shorten it and but through the brake system. Instead of controllingremove it from the car. But there is no joint in th e wheelslip by locking one or more differentials , ETSshort front drives haft. Instead, you pull back the se lectively actuates the brake on whichever drive-rubber boot at the front, loosen the locking cap and wheel s lips. Not all ETS cars have four-wheel-drive,slide back the splined coupling. Pay close attention but those that do employ plain open differentials allto centering the shaft in th e flexible joints on around, counting the unclutched sun-and-planetaryreassembly, or you cou ld have a running noise that in the ETS transfer case as plain and open.will be very hard to diagnose. Match-marking the Such traction increase must obviously toggle onparts with chalk or paint is a reasonable precaution. and off very rapidly, as fa st as the driving conditions change. This is possible on ly with a digitally con- tro lled automatic system, one that works directly from se nsor inputs rath er than just from driver controls. The driver, after all, has many other tasks to perform. ~Ob Like ASD, this could mean there would still be dri- vewhee l slip if there were more delivered eng in e torque than the available traction could employ. In ~®~t W- 70e such a case, both drivewhee ls would s lip, not just th e one; but a driver with a ny level of aware nes s of 70d driving dynamics shou ld have noticed the problem and lightened his foot. To encourage development of th is awa reness , the in strum ent panel includes a warning lamp that the syste m is at work retaining traction, and that the driver should accommodate hi s expectations to the physics of the availab le wh eelpatch-to-pavement friction. ETS works in ways quite familiar to people w ho know ABS, ASR and ESP. Preventing drivewh eel sp in by app lying the whee lbrake, in fact, could be ma rg inally more effective than locking a differential, s ince the system could control all four wheels at dif- ferent speeds effectively. With the open front differ- ential, the older, hydraulic 4MATIC cannot do that.
OH road, More OHroad and WayOffroadl Four-wheel-drive for vehicles that mostly remain on pavedroads serves a different purpose fro m four-w heel-dri ve for trac-tor-lug wilderness explorers. Ju st as with th e other tracti on-con-tro.1 systems, this on e increases the effective use of th e limitedtraction to help maintain optimi zed co ntrol for the driver, eve nunder circum stances of un expectedly slippery surfaces or oth e rsuch control risks. Cars dri ving on pavement have diffe rent, butnot less serious need for traction controls. Th ey may not becalled upon to climb ove r a fa ll en log or obstructing bould er, butthey may have to deal with a poth ole, a patch of black ice and agrave led surface, all within a fraction of a second a nd at differ-ent whee ls. The dri ve traction enha nce ment is simil a r, but fo rquite diffe rent purposes. 4MATIC and 4ETS cars a re the su bject of our major featurethis iss ue, but four-wh eel-drive is nothing n ew to Mercedes-Ben z. While thes e cars can drive on unimprovedroads and even briefly on relatively smooth fields, the company build s oth er four-wh eel-drive vehicles moresuited to greater distan ces from th e pavement. No doubt, you re famili ar with th e M-class Mercedes-Benz,Stuttgarts entry into the SUV market. It actually ha ils fromTuscaloosa, Alabama, but its still a Benz. We pl an to carry fea-tures covering the M-class in upcoming issu es of StarTuned. A bit more rugged and a b it more suited to offroad travel is the GeWndewagen. Whil e thi s is an esta bli shed des ign (th e bulletp ro of Popemobile h as bee n around for yea rs ), the ca rm a ke r has bee n importing th em to the USA s in ce 2000 so you should be seein g th em in yo ur wor kbay. Fin ally, and most gn arly of all , is th e Unimog. Proba bly th e most capa bl e offroad vehi- cle on wh eels, if you can t ge t so mewhe re in a Unimog, you s hould probably recon sid er wh eth e r yo u rea lly want to go th ere. When som ebody with a Mo nster-Mudder truck gets buri ed in t he mu ck, he h as to find a h elper to co me with a Unimog. When so me body with a Unim og gets mi red, he has to fin d someo ne who ca n com e help with a ve hi cle th at cla nks along on its steel tracks. Th e re are va ri- ou s s i zes a nd t y p es of Unim og, fro m abo ut th e size of a picku p to a match for a qua r- ry t r uck. Mo gs h ave bee nim po rted in small numbers fo r ma ny years, ofte n used forheavy con stru ction , railroad sw itch e ngin es, ex pediti on moth-er vehi cles or, equipped with so me of th e hu ge va riety of spe-cial-purpose imple me nts (even s norke ls - really!), doingeverything from digging ditch es, plowing s now a nd blastingou t rain sewers to cutting weeds a nd pl a ntin g te lep hone po les. [f yo u want to learn more abo ut Unim ogs, letus know. We would, too!
- Welcome to StarTuned, the magazine for independent service technicians working on Mercedes-Benz vehicles. Mercedes-Benz sponsors StarTuned and provides the information coming your way in each issue._ The worldwide carmaker wants to present what you need to know to diagnose and repair Mercedes-Benz cars accurately, quickly and the first time. Text, graphic, on-line and other technical sources combine to make this possible._ Feature articles, derived from approved company sources, focus on being useful and interesting. Our digest of technical information can help you solve unanticipated problems quickly and expertly. Our list of Mercedes-Benz dealers can help you find origjnal, Genuine Mercedes-Benz Parts._ We want StarTttned to be both helpful and informative, so please let us know just what kinds of features and other diagnostic services youd like to see in it. Well continue to bring you selected service bulletins from Mercedes-Benz and articles covering the different systems on these vehicles._ Send your suggestions, questions or comments to us at: StarTuned One Mercedes Drive Montvale, New Jersey 07645 Phone: 1 800 225 6262, ext. 2647 e-mail: StarTuned@mbusa.com
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FUEL PRESSURE AND VOLUMEIts not always easy to checl< something simple In days of techno-yore, when ca rburetors mi xed Fuel injection brought many combustion and the fuel and air for a n engine s intake, you could emissions advantages, but it did not bring di agnos- usually check for fuel with a glance at the dash tic simplicity. You still glance at the dash gauge first gauge followed by a look down the venturi as yo u (and if a functional Benz gauge says th e tank is cycled the linkage. If yo u saw a squirt from th e empty, its e mpty), but yo u need more than your accelerati on-enrichment pump nozzle, th ere was eyes a nd fin gers to check fuel delivery now. If you fuel; otherwise, th e bowl was empty. Then the ques- dont find the right pressure and volum e, of cou rse, tion was why. th e question is still why. The tool s necessary are a fuel pressure gauge a nd 11) Electric fuel pump. suitable adapter fittin gs, a graduated two-liter flask 1 Suction side, 2 Pressure limiter, 3 Roller-ce ll or bea ke r, a switchab le electric jumper (like a rem ote pump,4 Motor armature, 5 Check valve, starter button) , a clock with a second hand, and (for 6 Pressure side . many mode ls ) a h a nd-actuated vacuum pump a nd gauge. You also need to know what to do with them, 2 3 4 5 which is our business here. Finally, though we cer- tainly hope its perm a nently unnecessary, have a fire extinguisher within reach. For safety, disable spark and any other source of flame. Dont smoke or let anyone else s moke nea rby. Theres a lot of heat e ne r- gy in sprayed gasolin e, a nd it catches fire very easi ly. Thats why we suggest the pushbutton jumper - if yo u sudde nly jump back, the pump stops. 111e electric fuel pump used for fuel injection is Genera lly, we think of pressure a nd vo lume tests an electric motor and a meshed lobe or roller as the procedure to check th e fuel pump. Thats true, forced-displacement pump running in and cooled of co urse, but th at s not all. Those people who think by the fuel. Wear points include the brushes and working on cars is easy now because the computer commutator at the pressured end of the arma- detects the probl em and tells yo u what to rep lace, ture, the bearings at either end, the check valve should cons ide r what yo u can find with pressure and the pressure relief With enough grit in the and volum e tests, n eith er one of th em from th e com- fuel, there could even be wear of the lobes or puter. Ten potential problems imm ed iately sprung to rollers themselves. mind, though pinpoint tests rem ain on the agenda to determine which. You ca n probably think of more:
12) Operation of roller-cell pump. 6. The fuel sock and pickup. Most problems with 1 Suction side , 2 Rotor disk, 3 Roller, 4 Roll er these co mponents arise after a ta nkful of dubious race plate , 5 Pressure side . gaso lin e, leaving a coating on the initial filter and blocking suction to the pump. Sometimes you may discover problem s resulting from earlier work in th e tank. 7. The fuel tank. Some cracks or broken welds at th e top of a fuel tank can go unnoticed since the res no visib le leak, but th ey can allow moisture into th e unprotected stee l interior, producing rust that abrades th e pumps working surfaces and plugs th e filter. As on an oil pump, the armature is slightly eccen- 8. The vaporjevap system. If a car has good pres- tric to the pump housing centerline, so the rollers sure a nd volum e to begin with but gradually loses move in and out as the armature turns, forcing both as it travels a hundred miles or so, the fue l fuel by positive displacement through the fuel fil- deli very probl em may not a ri se from the pump but ter and through the supply lines to the fuel rail from a plugged vaporjevap system vent. That ca n and injectors. Wear or scoring on the lobes, produce a vacuum in the tank working against th e rollers or races allows fuel to bleed back toward pump, e nough of a vacuum sometim es to let the fuel the low-pressure side. vaporize at the pump intake. 9. The pressure retention valves. The regul ator a nd the pump contain check valves and pressure 1. The fuel pump. An electri c motor co upl ed with limi ting va lves that can allow pressure loss if thea roll er pump, the fuel pump lasts a long tim e. But sea l between their mating surfaces becomes pittednot, us ually, as long as a car. Bea rings, brush es and or a piece of grit lodges in between.the commutator all wea r; th e check valve may deve l- 10. The electrical circuit. If there is notableop leak back after tim e. How lon g the pump lasts res ista nce on the power lin e to the pump or on th edepe nds largely on the quality of fue l, but yo u can ground line returning the current, that resistancebegin to wonder abo ut a pump wh e n a ca r has effectively throttl es the electri c motor down, reduc-between 100 and 200 K on th e odo-c lock. ing bot h speed and torqu e. It doesnt take much loss 2. The fuel filter. Current fuel filters can contain of electri cal power to drop the pump below mini-an a mazing amount of dirt and grit before th ey clog mum delivery thresholds.up, a nd modern fuels are cleaner than eve r. But some Keep in mind the delivery specifications are notpeople still manage to collect enough debris into approx imation s and rough guesses. If th e pressuretheir tank to block passages . A motori st who regular- doesnt reach the minimum, th e inj ectors may deliv-ly uses offbrand fuel or drove with th e tank fill er cap er no fuel at all. If the volume isnt enough, the caropen for a time is a good candidate for a new fi lter. will run out of power before the pedal runs out of 3. The fuel pressure regulator. If it spills fu el travel. Be sure the pressure reaches the specifiedback through the return lin e before th eres enough leve l and there is the s pecified amount of fuel in thepressure, a bad regulator ca n prevent starting or beaker within the tim e limit.ca use reduced power at every throttl e setting. Like a The ge neral procedure is thi s: Connect the pres-defective check valve in a pump, a defective pressure sure gauge to the test port following the diagrams forregulator can dump system pressure right after th e that model; clamp off the return lin e with a suitable,engine shuts down, requiring a long crank to restart. non-dam aging clamp; route th e line into a suitable 4. The fuel lines. Th e most obviou s loss of pres- graduated container.slIre a nd volum e comes with a n ac tive leak, but When you compl ete a fuel pressure and volumeth ats rarely hard to diagnose. A more s ubtle and dif- test, make perfectly certain a ll the fuel lines youfi cult problem comes with an air leak upstream of opened are securely sealed, without the slightestthe pump, drawing in bubbl es to displace fu el and seepage of fuel. As da ngerous as a fue l fire is in aabsorb pressure. shop with knowledgea ble professionals availabl e to 5. The fuel injectors. If you clamp off th e return put it out, its much more so as a surprise on theline to eliminate the regulator from question , know road with no equipment or people to extinguish a nythe pump holds press ure but observe loss of pressure flame.in the fue l rail beginning with s hutoff, you have good In followin g is sues well cover fuel pressure andreaso n to suspect on e or more injectors of leaking. vo lume tests model by model.
There are many similarities from one Mercedes- Lil<e other Mercedes-Benz climate control sys-Benz climate control system to anoth er, but it ca n tems, the 202s is fully autom atic. The driver setsbe useful to look in detail at one of the more co m- the desired temperature, and the car engages eithermon , as thats one youre more likely to see. The the heater or the air condition er to realize that tem-Model 202 C-Class was built between 1994 and perature in side the car. Given normal comfort set-2000 and, by Mercedes-Benz sta ndard s, in relative- tings, this mea ns the air co ndition er starts as soonly large numbers. as the engine is runnin g smoothly or the heater Ne ither the car nor the climate control system is begins blowing warm air as soon as the enginethe most complex of the co mpanys products. The coolant temperature ris es to a range at which it can202 HVAC, howeve r, ca n keep its owners as comfort- co ntribute so mething. If youre familiar with howab le as almost any automoti ve heating an d cooling automatic climate controls worked in older veh icles,system. About the only climate-system luxury it like the 190, th e 124 or the 126, there aren t manylacks is a separate temperature selector for eac h surpri ses waiting for yo u in the 202.~on~s eatpasse n ge~ Thi s tim e of year, air co nditioning is likely to be more on yo ur mind than heating or defrosting, but th e system is not so distinct. Defrost, for instance, toggles both 11eat and the air co nditioning on, to in sure th e dri est and hottest air possible blown across the windshield. The system will do this rega rdl ess of climate co ntrol setting when the defrost button is push ed. This arrange ment is not uniqu e to th e 202, nor is it for that matter to Mercedes-Benz. Given that safety considerations come first, the defrost mode must use every possible meas ure to dry and clear th e windshield. Both air co nditionin g and hea t are humidity-redu cing
TEXTBOOK CASE fun ctions, the first by condensing moisture out Clearing the Air and th e second by raisin g th e ca pac ity of th e air to abso rb moisture in solution. Thu s both com e on Almost the fi rst element of the climate co ntrol sys- simultaneously. tem th e incoming air encounters, wheth er the sys- tem is heating or air conditioning, is th e dust filter. This is a relatively new component in ca rs (not just in the 202) and one that many car own ers are not aware of in th e car. Not being aware of it, they also are often not aware th at it requires periodi c replace- ment, like any filter. There is no set in terval since th ere is no set amount of du st and poll en a driver will drive through, but checkin g at least once a year is a reasonabl e precaution , more often wh ere theres lots of airborne materi al. At th e beginning of air con- ditioning season is a good opportunity, since the hea t and humidity of summ er are th e conditions most likely to encourage growth of any mold on the element. A suffi ciently clogged du st filter, naturally, co uld restri ct th e ventilati on quantity in a noti ceable way, eve n without any adverse odor. 3 2 The filter comes out through the passenger side footwell. Remove the access panel and pull out the old one. This cabin filter does not use an evacuated bead to seal the edges of the fittel; as was true on some other models. That means its easy to remove and replace. Some cars have a charcoal-element filte/~ some do not. The two types are not interchangeable.24
Just upwind of the filter is the hood inlet drawingventi lation air in. One aspect of this requiring occa-sional in spection is the water separator and drainsystem ju st below the windshield wipers. Ordinarily,water and relatively small grit will just wash downthrough the trough and pipe to the pavement, but ifthere are many leaves, insect parts or other suchdebris in the air locally, it is possible to clog thedrain and all ow water to remain. This could possiblyallow moisture into the venti lation intake. ~. ~~~~~~ -:::::::--------- - ... - r::-,--::;"---
TEXTBOOK CASE A9 Its fairly easy to remove the lower panel under the passenger-side instrument panel and replace the heater blower. Be careful doing this, however. In particular, dont replace the blower merely because there is audible noise coming from it. Possibly the bearings have worn out after many years service, but these are very long-Jived components - its more likely the blower has gotten louder because of debris reducing airflow, just as a vacuum cleaner gets louder if you cap off the hose with the palm of your hand. A slight piece of debris in the squir- rel cage could also produce noise by unbalanc- in g the cylindrica l fan and introducing a slight wobb le. By the way, dont test the blower by jumpering it with a straigh t 12 volts; the system ..... never uses that much to run the blower. It usu- ally runs between 4 and 6 volts, with system -- Me --!!! voltage reduced through the series-resistor block. The air conditioning compressor includes an rpm sensor to enab le the MAS control unit to toggle the compressor off should the drive belt slip, as indicated by a change in the ratio of com- pressor to crankshaft speed. Because the system makes very efficient use of the modest amount of refrigerant, it is critical to measure the amount of oi l in the system precisely jf the com- pressor is replaced. Neither an excess nor a shortage of air cond itioning oil is compatible with proper functioning of the climate-control810/1 system. Sometimes problems with the entire climate contro l system arise from the simplest of causes. Like most Mercedes-Benz heating and coo lin g systems, the mechanism in the 202 employs a cabi n temperature sensor in the overhead com- partment. A very sma ll fan draws air through Continued on Page 28
Testing and Design Perhaps some engi neersand designers chose theirwork so they could travel toexotic and beautiful locationsand enjoy the weather. Thatsprobably not true of climatecontrol gurus, though. Theymust test their prototypes inplaces that will challenge thecapacity of the heat exchang-ers to shift enough BTUs,whether into or out of the car. If they go to Houston, its inAugust, to see whether theirnew, low-profile AI C con-denser can cope with 130degrees in the shade. If theygo to Bemidji, Minnesota, itsin January, to see how long ittakes to clear a windshield ofthe Arctic hoarfrost. Often, of course, they dontgo anywhere special. Instead,they build a little cube ofGreenland or the Sahara atthe plant near Stuttgart,using insulated chambersand overwhelming heating orrefrigeration equipment. Is it overdone? Of course. Ifit werent, there would be noway to insure the climatecontrol systems could workin ordinary conditions. Afterall, ordinary conditions arenot always so ordinary. Anyidea how hot it can get installed traffic on the Cross-Bronx Expressway on theFourth of July? Or how coldwhen the Alberta Clipperswoops through Minneapolisin February? And this is not just for pas-senger comfort. Absent adriver who can pay attentionto what he or she is doingwith the wheel and the ped-als, you cant have automotivesafety. A climate control sys-tem can and does save lives.
TEXTBOOK CASE Continued from Page 26 th e louvers at that sensor anytime th e ca r is run- Second, it s almost always a good practice to nin g. The test for that fan is amaz ingly low-tech: take replace th e rece iver/ drier w hen you open the refrig- a sma ll shred of paper and hold it against th e lou- erant side, just to be safe. Besides its function as a ve rs while the engin e runs. If th e fan is working, the moisture abso rb er, the receiver/drier is also a filter paper w ill stay against the grate. If not, the paper th at can catch most bits of debri s that may work will fall away. loose in th e syste m. Oth erwis e, th ey could just cycle Anyti me you open up the refri ge rant sid e of th e through th e co mpressor, expa nsion valve and heat air cond itioning system, there are two preca uti ons exchangers over and over until they found the most to obse rve. Oil for th e system, like air conditi oning in co nve ni ent and expe nsive place to lodge. oils for previous refrigerants, is hygroscop ic, so it Fin aUy, it so metimes happens th at the windows wi ll draw moisture from the air, even from air that fog and stay partly fogged, even when you push th e seems entirely dry to you in th e shop. Once in th e defro ster button. Th e first pl ace to check with such a refrigerant passages, of course, th at could qui ckl y probl em (after yo ure sure th eres no sodden mass of produce rust, and theres hardl y a worse place for wet leaves at th e base of the wind shi eld) is the rust in the car. Be sure to cap any ope n pipe sec ure- sw itchover va lve and linkage working the ly right away, or flu sh the system if it remains open fres h/rec irculate door, all of it in the same passen- for any length of time . When in doubt, evac uate th e ger-side area wher e the blower motor resides . There system again . is a small hook working the door that may have bro- ken or may have disengaged from the vacu um motor. N65 Keeping Cool, Keeping Warm The now-d isco ntinu ed Mode l 202 Benz can nonetheless keep its driver and passengers at a reasonable temperature in the passenger compartment, regardless of the outside climate. Follow standard diagnostic and mainte- nance techniques, and you can keep your customers happy and their cars cornfoltable. Whenever yo u find th e temperature co ntrol sys- tem doin g so methin g unfamili ar, it s worth your whil e to rev iew what th e system is supposed to do with the controls in a given position . Don t be embarrassed to look in th e owners manual to deter- min e that. That littl e book let ca n be a surpri sin g source of good diagnosti c inform ati on.
_ Th ese suggestions and solu- meas ure to confirm th at it i s close tions for technical problems enough to th e teeth to generate th e come from service bulletins signal (check th e speci fication s for and other technical informa- th e ca r to confirm the minimum tion published by Mercedes- altern atin g voltage Benz, selected and rewritten for independent repair shops. Cleaning the Cleaners Sparked, Unsparked and C lanked of th e crank shaft se nso r. There All Models with Engines cou Id be oth er sorts of probl ems. 102 or 103 Tf you find an altern ating cur- rent signal, but th ere is still a Th er es not mu ch you hea r in a startin g or runnin g probl em, check ca r more alarmin g than a knockin g th e signal not onl y with a volt- metallic sound coming from the meter but with an osc ill oscope. engin e compartm ent. Sometim es, Som e se nsors should have pul ses happil y, the cau se is not mech ani- of sequenti ally vari abl e amplitude ca l within th e engine but som e- wh en th e fl ywh ee l in clud es seg- thing else. Check with your stetho- ments and mag nets. On ca rs sco pe and pay attention to the hi gh wh ere th e se nsor follows th e fly- voltages, but on e more thing to wheel teeth directl y, th e si gnal check is th e ignition coil itself. If should be even. th e iron core in a coil com es loose intern ally, the electrom agneti c pul ses in th e windings ca n bang L5 th e core to and fro rapidly, cau sin g You dont have to replace every thi s suspi ciou s noi se. Coils known filter anytim e you di scover du st on to have this problem have produ c- it. Certain air filter s, like th e ti on codes between FD 845 and cycloni c version or the stee l-m es h 847. Ther e could be others. backed flat filter, allow remova l of large amounts of dirt by com- Crankshaft Position Sensors pressed air. In areas with ver y large amounts of airborne du st, it is frequ entl y good practice to clean out th e filter hou sing and the sur- face of th e filter itself periodical1y , , before you repl ace th e filter ele- ment itself. Many air filters actu al- ly beco me more effecti ve with use, as parti cles fill th e lar ge r If th e crankshaft positi on se nso r mi crop assages. Eventu all y, of doesnt work, the engin e won t course, all filters will present sta rt. If it fail s whil e th e engin e is enough obstru ction to airfl ow to runnin g, it stops imm edi ately. A Wh en repl ac in g th e se nsor, impede th e engines breathing, loss of crankshaft position signal, however, does not mean a failure make sure it does not contac t any part of th e flywh ee l, but also - and th en it is tim e to r eplace them.
CABIN FILTERS We dont always think of the quality of the air as Besides what the cabin filter does for the dri verpart of a climate contro l system, but one of the most and passengers in terms of the comfort level fro mimporta nt improvements in the internal atmosphere a cleaner air to breathe, it also has a safety fu nc-of vehicles in recent years comes from the dust fil- tion. Most people are relatively careful about keep-ters that clean the air passed through the heating, ing the outside of the windshield clean, so they cana ir-co nditioning and ventilation system. The most see what is coming down the road. Its much lessobvious benefits are a reduction in road dust in the common, however, for car owners to clean thepasse nger compartment, making life more comfort- inside of the windshield. Besides being less con-able for almost everyone, but particularly for those venient to clean, the buildup of dust and grime iswho have allergies or who drive in very dusty places. much more gradual, so there isnt the kind of per- The filters do more than that, however. Youve ception of the vision-obstructing layer there is fromprobably tried to clean air conditioner evaporators road spray or bugs.and heater cores by spraying various chemicals The cabin filter cant eliminate that problem, butthrough the ductwork If there is much growth of it can reduce it substantially. The amount of dustmold at all, its a pretty thankless task, unlikely to and small particulates that used to collect on thework for long. With a dust filter, however, it is possi- inside of the windshield is much lower, so the need -ble to prevent that problem from arising at all. If the to clean the glass from the inside is proportionatelymold spores stop on the accord ion-fo ld paper, you reduced.can remove them with the old insert.
Alabama Belmont Oakland Van Nuys Autobahn Motors Mercedes-Benz of Oakland Keyes Europea n 650-637-2333 510-832-6030 818-461 -3900DothanMike Schm itz Automotive Beverly Hills Palm Springs Walnut Creek334-794-6716 Beverly Hills Mercedes-Benz of Palm Springs Stead Molors of Walnut Creek 310-659-2980 760-328-6525 925-937-1655HooverCrown Automobile Buena Park Palo Alto West Covina205-985-4200 1 ·louse of Imports Park Ave nue Motors Penske Motorcars 714-562-11 00 650-494-0311 626-859-1200HuntsvilleRegal Auto Plaza Calabasas Pasadena Calabasas Motorca rs Colorado256-837-5752 Rusnak Pasadena 818-59 1-2377 626-792-0226 Colorado SpringsMobile Carlsbad Mercedes-Benz of Colorado SpringsMcConn ell Automotive Pleasanton Hoehn Motors Me rcedes-Benz of Pleasanton 719-575-7950251-472-3187 760-438-4454 925-463-2525 DenverMontgomery Chico MUITay Motor Importsjack Ingram Motors Courtesy Motors Auto Ce nter Riverside Walters 303-759-3400334-277-5700 530-893-1300 909-688-3332 Claremont LittletonTuscaloosa Rocklin Mercedes-Benz of LittletonLeigh Auto moti ve Penske Motorcars 909-568-2600 Von Housen Motors 303-738-7700205-556-11 11 916-924-8000 El Dorado Hills ConnecticutAlaska Sacramento Me rcedes-Benz of Eldorado Hills Mercedes-Benz of Sac ramentoAnchorage 916-567-5100 Danbury 96-92 4-8000 Mercedes-Benz of DanburyMercedes-B of An chorage enz Encino907-277-3383 San Diego 203-778-6333 Auto Stiegler 818-788-0234 M ercedes-Benz 01 San DiegoFairbanks 050-279-7202 FairfieldCook s Import Mercedes-Benz of Fa irfi eld Escondido 203-360-6725907-459-7070 Mercedes-Benz of Escondido San Francisco 760-745-5000 M ercedes-Benz of San FranciscoArizona 415-673-2000 Greenwich Fremont Mercedes-Benz of GreenwichChandler Claridges San Jose 203-869-2050Mercedes-B of Chandler enz 510-623-1111 Beshoff480-403-3444 408-239-2300 Hartford Fresno New Cou ntry Motor CarsPhoenix San Jose 866-346-2369Phoenix Motor Mercedes-Benz of Fresno 559-438-0300 Smy the Europea n602-264-47 91 408-903-5200 New LondonScottJ;dale Glendale Carriage House of New London Ca lstar Motors San Luis Obispo 860-447-3361Schumacher European Kimball Motor480-99 1-11 55 818-246-1800 005-543-5752 North HavenTucson Laguna Niguel Mercedes-Benz of North Haven Mercedes-Be nz of Laguna Niguel San RafaelMercedes-B of Tucso n enz HAB. Motors 203-239-1313520-886-131 949-347-3700 415-454-0582 DelawareArkansas La Jolla Santa Barbara Hein z Gietz Autohaus Sa nta Barbara Auto Group MilfordFayetteville 858-454-7137 805-682-2000 I.G. Burtonjones Motorcars 302-424-3042 Long Beach Santa Monica479-521 -72 81 Mercedes-Benz of Long Beach 562-988-8300 w. 1. Simonson WilmingtonLittle Rock 310-829-45 11 Mercedes-Benz of WilmingtonRiverside Motors Los Angeles 800-800-1949501-666-9457 Santa Rosa Downtown L.A. Motors Smothers Europea n 213-748-895 1 FloridaCalifornia 707-542-480Anaheim Modesto Stockton ClearwaterCa li be r Motors Modesto European Berberian European Motors Lokey Motor714-777-1900 209-522-8100 209-944-55 11 727-530-1661Arcadia Monterey Thousand Oaks Coral GablesRusnak/Arcadia Mercedes-Benzof Monterey Silve r Star A.G. Bill Ussery Motors626-447-111 7 83 1-375-2456 805-37 1-5400 305-445-8593Bakersfield Newport Beach Torrance Daytona BeachMercedes-Benz of Bakersfield Fletcher jones Motor Cars Mercedes-Benz of South Bay Mercedes-Benz of Daytona Beach661-836-3737 949-718-3000 30-303-3500 386-274-4775