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Old 03-17-2003, 12:48 PM
bhatt's Avatar
Registered User
Join Date: Apr 2000
Location: Oakville, Ontario, Canada
Posts: 421
300SDL belt squealing

My 300SDL has been making a "chirping" noise at idle for the last few months.. it makes a quick "chirp" about once or twice a second, and only when at idle.

Last weekend, the chirp turned into a constant squeal. It now happens at idle and at low RPM ranges, and sounds exactly like a belt slipping. Belt is 6 months old or so. Problem starts when everything is warmed up (after driving about a mile).

I could be wrong, but I looked at the tensioner pulley and the belt is only 80% covering it. What I mean is that I can see about 20% of the pulley(wheel) which I don't think was visible. Looks to me like the belt has slowly moved off of the tensioner wheel...

I've also noticed small rubber chunks on the coolant reservoir. This is in line with the belt, so it seems the belt is self destructing.

Is this the classic OM603 belt tensioner "arm" problem? I'm assuming I need to replace the tensioner arm and shock, but not the pulley if it's in good shape.. is this correct??

How do I verify this is a tensioner arm problem and not my p/s pump or A/C compressor or some other accessory seizing up? The squeal doesn't change if I turn the steering wheel while stopped or if I turn the A/C on or off so I suspect it isn't one of those items causing the problem.. how do I know it isn't the alternator??

Thanks to anyone who can help out!

'83 300SD 335,000km (207k) mi SOLD
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'07 S550 4MATIC 170,000km (105k mi)
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Old 03-17-2003, 03:15 PM
sixto's Avatar
smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 18,745
Actually the pulley is probably sliding away from the belt.

The rubber chunks on the reservoir could be from the alternator pulley chewing up the ribs of the belt because the tensioner pulley is feeding the belt at an angle.

Get that thing changed ASAP or you might be in for a new fan, clutch and radiator as well.

This doesn't mean that there isn't a problem with one of the other pulleys in the belt path.

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