I replaced the belt on my 87 300e and after a few days, it started squealing. I thought I had adjusted it correctly, so I popped the hood. I pressed down on the belt and it was very slack. I then loosened the 19mm nut on the tensioner, and started to turn the 13mm adjusting rod. The belt stayed loose. I looked down at the pointer and it was moving in tune with the adjusting rod. The tensioner appears to be shot. My question is, is this the fault of the tensioner mechanism, the "shock absorber" part of the tensioner, or both? I believe the adjustment mechanism is fine (not stripped) because the pointer moves when I turn the adjuster. Thanks!

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1987 300e "200,000+ and going strong"

It sounds like the rubber bushing in the tensioner has separated and the tensioner is shot...

It is common for them to fail when being re-tensioned after a belt replacement.


I read a test procedure on ALLDATADIY:

If you secure the shaft that the 19 mm bolt goes through in a vise, you should be able to 'deflect' the tensioner arm by up to 30 degrees and a good tensioner will return to the original position. If it does not, the arm is slipping on the shaft and the tensioner must be replaced.


Over-tensioning a tensioner will ruin it.

Here is how I, a DIY person, set my tensioners. If a pro has anything to add, or correct, please chime in. This method has worked well for me for several years, and in several cars!


Locate the series of marks on the tensioner, near the pointer. One mark is away from the others. Install the pointer directed at that mark. Install the tensioner and tighten the tensioning link until the pointer reaches the first or second mark. I usually set it to the first mark and have had no problem.

Good luck!

UPDATE:
Leave the large thru-bolt a bit loose until the tension is set, then tighten it to lock things down.

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02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)