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FEATURE ARTICLE

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Mercedes-Benz timing chains andtensioners last and last. But, given thetremendous number of miles people puton these vehicles, they will need attentionsooner or later

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In order for a four-cycle engine to work,there must be precise coordination among theintake, compression, power and exhaust strokes.Advances in engine technology have helpedincrease performance throughout the rpm range.Variable intake manifold runners optimize airintake velocity, variable valve timing has broad-ened the power band, and variable liftfurther fine-tunes air intake and exhaust flowfor even greater performance, efficiency andeconomy. Regardless of how high-tech intakeand exhaust events have become, maintainingbasic cam/crank timing remains essential.

Keeping Time

Mercedes-Benz vehicles have alwaysexcelled in sound engineering and reliability.From Formula 1 to Indy cars, Mercedes-Benzengines have always been in the top tier. Thesame engineering principles that are used tocreate a reliable race engine are also used forproduction vehicles. One steadfast belief held bythe company's engineers is in the reliability ofthe timing chain. The valve trains found in largesix cylinder, V8 and twelve cylinder enginesnaturally have more mass to move than those oflittle fours. This puts undue stress on thecamshaft drive mechanism, making a timing belt

marginal, and making replacement at regularintervals necessary. On the other hand, in aproperly-maintained vehicle timing chains andrelated hardware can last well over 200,000miles. Eventually, however, wear will reach thepoint where diagnosis and replacement will beneeded, so you have to know how to performthe job properly. This is usually not too difficulta task providing you had the correct tools, andany shop specializing in Mercedes-Benzvehicles should have such items as a chainpin remover/installer.

First Scenario

Typically, you will need to deal with the timingchain because of noise from a stretched chainor weak tensioner, or because the cylinderhead must be removed for whatever reason.Either way, the timing chain has to be removedand put back into proper synchronization uponreassembly. In the case of noise, the job is notas daunting as you may think. Mercedes-Benzhas service procedures that make this a rela-tively simple task. Many Mercedes-Benzengines have the timing cover cast as one unitwith the block, so the cover cannot be removedto access the timing chain.

Continued on page 20

Question of Timing

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There is a straightforward procedure, however,that makes replacement possible anyway. Thefirst step is to remove the valve covers. Alsoremove the spark plugs to make it easier to turnthe crankshaft by hand. You may want to startwith the engine at TDC, it's not really necessary.In most Mercedes-Benz “Checking EngineTiming” procedures, you are asked to rotate thecrankshaft to a specific point on the pulley. Thisis usually not TDC, but a specific degree markpast TDC. If you are so inclined, you can applypaint marks on the cam gears and adjacentmarks on the timing case cover to check yourwork once you have changed the timing chain. The next step is to use the chain separating

tool to force out the two pins of one outer link.This will break the chain and leave two sides ofinner links exposed. Install a service master linkon the chain leading into the upper portion of theengine. Use one pin to attach this link to theupper side of the chain currently installed in theengine. Connect the other half of the servicemaster link to the new replacement chain. Youshould now have one long continuous chain halfinstalled in the engine. Grab the lower half of theold timing chain coming up from crankshafttoward the passenger side camshaft and pull.While doing this, rotate the crankshaft pulleyclockwise. This will draw in the chain. It is criticalnot to rotate the engine counter-clockwise at anypoint in the repair. If you do, the chain tensioner

Continued from page 17A QUESTION OF TIMING

Your paid subscription towww.startekinfo.com willallow you to access WIS documents. Here the correctstep-by-step procedures arelaid out for you. You are alsolinked to pages that let youknow what special tools youwill need. The procedure fordrawing in the new chain ispictured here.

On this 111 engine, if the camshaftsare in their home position you will beable to install a cam positioning toolin the backs of the cam sprockets.The screwdriver indicates the loca-tion of one of the holes (intakecam). Notice how the intake andexhaust cam lobes are facing oneanother. This gives you a ballparkidea that you have the timing close.

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A QUESTION OF TIMING

will collapse and probably allow the chain tojump teeth on one of the sprockets, thus throw-ing off valve timing. If you continue to rotate theengine clockwise you will eventually draw in theentire section of new double-roller chain andonce again see the service master link. At this point, you must remove the service

master link as it is only intended to be used dur-ing service. Install the permanent link with achain link pin installer. The tensioner may bepushing on the chain hard enough as it comesup from the crankshaft that the two ends won'tmeet. So, you can loosen the tensioner to helpwith the installation of the permanent link. Don'ttake the tensioner out all the way as this mayallow enough slack for the chain to jump time.The permanent link outer plates need to beinstalled and a chain rivet installer must be usedto stake down the roller pins just like those ofthe other links. Rotate the engine several timesby hand and watch for your paint marks to besure valve timing is correct. At this point it should be safe to remove the

tensioner and either install a new one or reset theold one. Resetting the tensioner is critical. As youran the new chain in and the old one out, the tensioner's spring pushed outward to take up theslack in the chain. If you were to leave it in thisposition, the chain would be far too tight.

In severe cases, this can snap off a cam sprocket! To reset the tensioner, remove the cen-ter cap with a hex key socket, and the pre-loadspring will fall out (watch out for the aluminumsealing washer). These are hydraulically-assistedtensioners that require engine oil pressure to addtension as engine rpm rises. The pre-load springpushes on a piston that in turn pushes on thechain tensioner guide. The piston ratchets outward to apply tension. Once it ratchets out, itwill not return even with the spring removed. So,remove the whole body of the tensioner, pull thepiston out and away from the pre-load spring cap.Now you can mount the body of the tensioner

into the timing case cover, making sure the aluminum sealing washer is properly installed.Put the piston into the opening of the tensionerbody followed by the pre-load spring, then installthe cap over the spring and thread the cap intothe tensioner body. Some force will be requiredto compress the spring, which is necessary tokeep the chain tight enough for start-up. Oncethe engine is running, oil pressure does the job.That's why it's important to replace both aluminum sealing washers even if you're reusingthe old tensioner. By the way, when ordering anew tensioner you should be aware that it doesnot come with the threaded cap, so you will needto use the cap from the old one.

Look at the intake camon the right and you cansee the camshaft timingadjuster. Since the enginerotates clockwise, youneed to turn the adjustercounter-clockwise all theway until it stops, thenengage the timing chain.

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access the crankshaft sprocket and you will alsohave access to the chain guides. After removingthe tensioner, valve cover(s), chain guides, camsprockets and oil pan, you will be able to removeand replace the unbroken chain. This procedurealso requires some special tooling since you haveto properly align the camshafts and crankshaft. Over the years, Mercedes-Benz has used sev-

eral different ways to assure that the camshaftsand crankshaft are properly synchronized -- toomany to be thoroughly explained in a magazinearticle. So, we will provide a simple overview.There are usually a few types of special tools tolock down the camshafts. Some enter throughthe rear of the sprockets. An alignment pin pass-es through a hole in the cam cap and a hole inthe sprocket. With variable valve timing comesthe important step of setting the intake camsprocket to the home position (retarded). Holdthe camshaft itself in the locked position with awrench and twist the camshaft adjuster in theopposite direction of timing chain rotation, whichresults in the retarded position. If there is anyquestion of a camshaft adjuster not movingfreely, now would be the time to change it. Theuse of non-approved oils in these engines cancause sludge build-up and jamming in theadjuster mechanism. This can lead to misfires,poor idle and the MIL/CEL coming on. On newerengines, special tools bolt down to the valvecover flange on the head, and tabs lock into thecam sprocket, but these will be covered in futurearticles. Generally, the crankshaft is located afew degrees past TDC and is not locked down.

Closing Up

With the timing chain and camshaft adjuster inthe proper position you can start re-installing thechain guides, oil pan, valve covers and tensioneras outlined earlier. It is always a good idea torotate the engine by hand several times to verifyall components are properly timed and that thechain meshes smoothly. The final step should beto install the spark plugs and fire up the engine,allowing time for oil pressure to reach the tensioner before accelerating.

Second Scenario

If more extensive service work such as a cylin-der head R&R is performed, it may be easier tosimply remove the timing chain as one unit. Startby removing the tensioner. You can then removeeither the camshaft timing sprockets, or thecamshafts themselves, depending on the particu-lar engine. For instance, on the M272 motor thetiming chain only drives the intake cam, whichthen drives the exhaust cam by means of gears.In this case, the cams will have to be removed toaccess the timing chain. Drop the oil pan to

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A QUESTION OF TIMING

Mercedes-Benz usually times itsengines at a position other thanTDC. In the case of this 111 engine,the timing pulley should be set to20 deg. after TDC. This puts thekeyway in the crankshaft at justabout the 12 o’clock position. Now, you should be able to line up the cams.

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Pneumatic Door Locks

Secondary Air Systems

Timing Chains

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