87 300TD belt popped off... - PeachParts Mercedes-Benz Forum

zu! · #1 09-25-2016, 03:10 PM

The question is...why would it do that? Bad tensioner? Bad belt? Something worse?

I stopped the car as soon as I could, temps were climbing fast (almost to 120!!) and of course battery light and power steering gone. I shut off the engine as soon as I was safely parked.

Is it safe to simply thread the belt back on and drive until I get a new belt and tensioner? Anyone have a picture handy of how the belt serpentines it's way around the various pulleys?

Sorry for all the abrupt questions...but I really need the car this week especially!! Thanks!

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zu! · #2 09-25-2016, 03:21 PM

Just saw this...http://w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Engine/602_603/13-1202hw.pdf

Do I really need to remove the radiator and fan to do this? That's massive pain ...

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Phillytwotank · #3 09-25-2016, 04:14 PM

It's a lot easier to get the belt on with the fan out of the way but it is entirely possible to get it on there with everything in place

sixto · #4 09-25-2016, 04:15 PM

The belt can get thrown off if the tensioner arm bearing or tensioner pulley bearing gets loose and doesn't stay aligned with the crankshaft axis. You'll need a pulley holder and 12mm hed bit. Such tools aren't found in the typical toolbox unless you work on these cars. The fan is t difficult to remove with a pulley holder. I use an 8mm hex key in a deep 10mm socket and breaker bar to r&r the fan. A 2-piece shroud makes things easier still and was ~$70 from the dealer many years ago. If you have an old style vacuum pump, now's a good time to update (insert shameless plug for the used one I have for sale). Don't forget a new damper/belt shock if yours feel loose or the end bushings rattle in their holders.

Sixto
83 300SD

zu! · #5 09-25-2016, 04:37 PM

Ok guys. Thanks for the quick replies. Looks like it's not a simple 5-minute job like I hoped it would be. I'm at the shop car park and didn't bring my 12mm hex with me. Rats.

Btw sixto...what does the old style vacuum pump look like? Always thought the issues with VC pumps was more with the older 123s and not 124s.

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Mxfrank · #6 09-25-2016, 04:44 PM

If the belt popped off, then you have a bit of a chore ahead of you. The most likely cause is either a bad tensioner or a worn pulley. The tensioner is the moe serious issue. If the bearing wears to the point that it can be shaken forward and back, it can shatter the timing cover. The pulleys wear conical over time. I like to replace them both every 100K miles or so.

sixto · #7 09-25-2016, 04:51 PM

You don't need a 12mm hex bit to pull the fan, just for the tensioner arm. You can get the belt on without pulling the fan so you can get home but it's a frustrating process. Keep AC off so the shock from the compressor engaging doesn't throw the belt again.

I'm not aware of 615/6/7 vacuum pump issues, only 601/2/3 vacuum pump issues. The original style vacuum pump has open roller bearings with plastic cages. Over time the plastic gets brittle and breaks. The balls can then shift to one end allowing the races to misalign and dislodge the balls which drop into the timing chain vault. Later style vacuum pumps have encased bearings so the balls can't escape. The pump can still break and cause engine damage but I've only read of that happening on a modified engine on a dyno. Later style pumps are identified externally by scews holding down the front pump cover but that revision predates the bearing style change so there are a few pumps out there with cover screws and old style bearings.

Sixto
83 300SD

zu! · #8 09-25-2016, 05:20 PM

Ok. Had a closer look just now. The belt is indeed cut in a number of places, mostly on the inside. The tensioner cannot move even with my best efforts so at least I think it's bearings are still intact.

This was a good scare for me. I've ordered the tensioner as well as a new belt from my parts guy. Hopefully can pick it up tomorrow and get it wrapped up by Tuesday.

Sixto I couldn't quite see the vacuum pump just now. I'll have a closer look tomorrow. That's the one on the right of the alternator right?

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sixto · #9 09-25-2016, 05:24 PM

You won't be able to move the tensioner against the spring. Question is, is the tensioner pulley aligned with the rest of the pulleys? If not, is it because just the tensioner pulley is skewed or is the tensioner arm skewed?

The vacuum pump is inboard and below the power steering pump.

Sixto
83 300SD

babymog · #10 09-25-2016, 08:01 PM

The belt can be done with everything in place if you have long arms and don't mind a little pain, remove the bolt at the front of the head to release the spring rocker, tension the spring again when finished with a couple of socket extensions or equivalent in the hole on the plastic rocker as leverage. The fan can also be removed, you need a short allen wrench to clear the radiator but without the belt to hold the pully it's unlikely you'll break it loose.
As Dave mentioned, a 2-piece shroud is a good investment if you're keeping the car, you can find them used on an E 320 W124.

Phillytwotank · #11 09-25-2016, 08:16 PM

For the general knowledge... The handle end of the MB tire wrench is THE tool to relive the spring tension to get the belt on and off.

babymog · #12 09-25-2016, 08:37 PM

Good to know it fits, although my toolbox is usually closer than my trunk when I work on my engine.

sixto · #13 09-25-2016, 09:21 PM

OT - I was helping a friend remove the tensioner spring. We were wondering what fancy sort of spring perch we were dealing with with no hole to lever it. Turns out the monkey who installed it installed it upside-down.

Sixto
83 300SD

Mxfrank · #14 09-25-2016, 09:35 PM

If you replace the tensioner, replace the idler pulley. You have to remove it to replace the tensioner anyway. If you get it apart and find it's NG, you'll be a day at least waiting for the part.

Jeremy5848 · #15 09-25-2016, 10:09 PM

The "lever arm" portion of the belt tensioner uses a simple plain bearing: a steel screw that screws into the chain case cover, the aluminum lever arm, and a plastic liner separating the two (plus a little grease). This so-called "bearing" has a plastic cap on the outside and a plastic seal on the inside. Both are needed to keep water and dirt out of the "bearing." These are often missed when making repairs because they don't seem necessary, but they are.

Another point to consider, especially for those upgrading to a larger alternator, is an over-running or "clutched" pulley for the alternator. This allows the alternator to spin freely as the engine decelerates rather than fight the changing speed with its rotational inertia (worse with larger alternators). All of this is hard on the belt tensioner--it's one of the main causes of worn-out belt tensioner shock absorbers. The special pulleys are available from several manufacturers for less than $50. I vaguely remember seeing a thread here on that subject.

Jeremy