Hello All,
So I'm new to the forum and to Mercedes ownership. I've been doing some tuneups on an '87 300dt sedan I just bought and thought I would replace the serpentine belt which was cracking across the ribs and had about a 5" split runing along it. Ordered a new 2145 mm contitech belt from "auto anything." It was pretty tight going on but I got it on. At first startup there was some squeaking and I thought no problem, just wearing in. Well, after about 700 miles the belt had worn down so that I could see the cord through the back side. It seems that it was rubbing on itself where it is doubled over between the fan pulley and crank pulley. I put the old one back on and it works fine, but there is a considerable difference between the lengths of the old and new belts when layed on top of each other. I also roughly inspected the tensioner and tensioner shock. Both seem fine; if anything the tensioner shock as some wiggle room in the bushing and vibrates a little bit but I don't think this could be the cause.

I'm wondering if anyone else has had such an experience or any wisdom to offer. To me it looks like the belt is so tight that it is pulling the tensioner all the way down and causing the belt to rub on itself. Should I order another belt, try a different brand, replace the whole tensioner assembly? Any advice is appreciated.

With the serpentine belt off, does the tensioner pulley spin freely? The only place the back/flat section of the serpentine belt comes into contact with is this pulley, so it would stand to reason that if there is wear on the back side this is the only place it could be coming from.

2145 mm is the correct length and ContiTech/Continental is a very high quality belt.

The tensioner pulley spins freely and is straight up and down but has a very slight amount of wiggle in it.

Replace the pully, you have some serious alighnment issues if your burning out a belt in 700 miles.

If the shock is lose replace that as well.

Make sure all the accessory pully's spin freely and are in good shape.

Contitech are the belts you want to run, but I don't know if thats the right size off the top of my head.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Model OM603 belt length is 2145mm so you would seem to have the correct length. The fan/water pump pulley and the crank pulley should not be able to be close enough (there is no adjustment -- both are integral parts of the block) to cause the belt to rub on itself. The only thing I can think of is that somehow you got a belt that is thicker than what is required?

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

I think you have to be careful about the words "spin freely" on belted devices on the front of the car.

The belt tensioner pulley should actually have some resistance to its spin so it will stop within a rev or two, and not spin freely. Spinning several revolutions is a shot bearing, or about to be shot, so replace that.

And the alternator should not spin too freely either, for the same reason, about its bearings. While the belt is off next time give that a spin to see if whizzes around for several revolutions. If so, bank on an alternator rebuild soon.

If there is any grease coming out of the belt pulley, replace it.

The pulley tends to sit askew when worn, this could explain bad belt wear. It doesn't take much tilt to farg it up.

If there is any side-side play in the belt tensioner shock replace it.

Good thing you are catching these belts before they blow, they can wrap any which way around devices and stop them up in harmful ways, and it can whack your A/C devices behind the headlight, and not to mention making mincemeat out of your hood liner. How do I know all this stuff?? Seen it.

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Cheers!
Scott McPhee

1987 300D

From what I'm hearing it sounds like that small amount of play in the tensioner pulley is probably an indicator of worn bearings. The increased stress of a new tighter belt must be causing the pulley to drag on the belt and wear away at the back side. So, I'll replace the tensioner pulley and probably the shock while I'm there. The bearings for the pulley are located in the pulley itself, not the tensioner arm, right?

Thanks for the advice everyone.

Bearings in the pulley.

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Cheers!
Scott McPhee

1987 300D

i would also check the alignment of the pulleys. i would visually see if the pulley is straight. replacing the tensioner pulley assembly is a semi big deal because the radiator fan has to come off (at least on my w126)

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Ben
1987 190d 2.5Turbo

Me 3... Upper rad hose, lower rad hose, vacuum line tee, a/c wiring...fun lesson to learn...

SOME play is normal in the tensioner pulley, just make sure it cant wiggle fwd/aft MUCH. Its not a bad idea at all to just replace it, as that is the main wear item in the otherwise perfect serp belt system on the 124.

Also make sure to check pulley alignment on the tensioner - there are several versions of that tensioner, some had spacers which positioned the pulley just right. Not using the spacers will result in lessons already mentioned

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Paul
Benz-less
I need an SDL !

The tensioner arm has its own bearings and its own propensity to sit crooked when worn so you can still have a brand new pulley sit tilted. My rule of thumb test is to see if the tensioner arm clicks as you move it about the crankshaft axis. It shouldn't click.

I can replace the tensioner system in an 87 300D without pulling the radiator but I don't know that I save any time keeping it in place. While you're shopping, consider a two-piece fan shroud so you can work on the fan without having the one-piece shroud in the way.

Sixto
87 300D

I'm just and old mechanic , and I know this might sound silly but did you check both belts side by side to see if you had the right belt, I can't tell you how many times I was given the wrong belt, I always stretch the belts together to make sure you have the right belt, sounds to me like you have a belt to small and it is to tight, just a thought, good luck

Multi-V belts usually have the length printed on the outside flat of the belt, so it should be easy to verify the correct belt. Conti/Continental belts definitely do.

You didn't by chance get one of those Mopar belts that's ribbed on both sides, did you?

Sixto
87 300D

To follow up on this post. I spent a fair amount of time cursing out German Engineering for various reasons and then found out the whole issue was my fault. When the car starting running hot and the battery went dead it finally hit me that something was going on with the belt system. Rechecked the belt diagram and sure enough, I had it routed wrong. Didn't think it was possible to to do this but it was, and I did it. This is why the belt was running so close to itself and rubbing on the back side. Should have figured this out far sooner, but oh well. Everything works fine now and I can start obsessing about the next thing (probably the little shimmy I get in the rear end at highway speeds).