1984 Mercedes 300D. I recently discovered a screeching belt noise sounds last night as I was on the road, it would come and go at first, but then it just got louder and louder as I was driving. I pulled over and turned the headlights off the screeching noise went away on its own when I turned them back on and accelerated it came back. So it seems to me it only happens when the car is under load.

The car has less than 10 months new Alternator/Battery/Water Pump and 2 new belts.


Any ideas?

Sounds like loose alternator belt since it goes away when you switch the lights off. Check your tension. It's not uncommon to have to re-tighten new V-belts a couple times after replacement.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

I just went through this last Fall ~ I looked at the belts and saw a tiny crack and bought new ones from the host, they have M-B logos on them but are the cheaper same brand part .

Installed and now, several months later are beginning to squeal again, time to re adjust .

FWIW, you need 10MM belts, they sent me 9.5MM belts, not O.K., I was in a pinch and used them, it looks like I shouldn't have .

Remember to loosen ALL the bolts and re tighten them ALL again after ~ I once forgot to tighter the bottom alternator pivot bolt and it loosened and fell out on the road, I didn't notice it until the next time I was working underneath the car .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

I had the exact same issue. Started by tightening all of the belts and now it's a million times quieter. When tightened properly, each belt should have about a 1/2 inch of give in the middle of its longest stretch. Worked for me anyway so best of luck!

Do yourself a favor and get the tool MB recommends (its like someone planned this out), a Gates Krikit. $15.

For the tip on the Gates tool, here it is : https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=gates+krikit&_sacat=0

My brother called, he's driving my '84 300CD and says the belts squeal constantly under 40 MPH so I guess I'll be re tensioning them in the morning....

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Old trick I learned as a teen, is to use an emery board radially to scuff the metal surface in the V channels of the pulleys (also on the old glazed belts) to get less slippage.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Yes but, this also wears the belts faster.....

In the used car trade, once the oil was all cleaned away some cheap hair spray (back them it contained laquer) did the trick .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Another thing to consider which could cause this problem, is that something you have has a bolt that is not totally fastened - power steering pump, alternator, or A/C compressor.

Dkr.

The narrower belts will never stay tight enough. The narrower belts go on the two pulley setup, the singles take the larger belt. They are pretty hard to tell apart without checking the numbers on them.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Just cut it off and get a new battery every week

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Can't do that, they run the water pump too.... .

The catalog says 10MM on the dual belt setup and that's what came off....

I tightened then up yesterday and they were quiet until we went up the freeway ramp, normal acceleration (not my usual flying up as fast as possible) and they began squealing at 30 MPH and didn't quit until 40 MPH so I'm going to order in the correct 10MM ones, I also need a spare air filter and the front calipers are dragging so I guess I'll be ordering in a pair of rebuilt calipers .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Temporary fix: brush on some Gasgacinch Belt dressing.

__________________
78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Yes, I considered that but thought I should first ensure the entire engine and motor bay is 100 % oil free.....

I ordered the new correct (I hope) 10MM belts and some other bits & bobs so one way or t'other I'll get it sorted out soon .

When I was a used car lot Mechanic long long ago I learned to use cheap hair spray as belt dressing ~ back then it contained lacquer and worked very well .

I prefer to do things as correctly as possible whenever I can .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better