Dear Forum

I have a loud squeal coming from my 1978 non turbo 300D. It is most apparent immediately after start-up when the engine is still cold, and it only squeals when the engine is revved or put under load.

It used to disappear just a few seconds after start-up, but in the last couple of weeks the squeal continues for at least the first few minutes of a journey (but again---only when the engine is revved or put under load, never when it is idling). It still seems to disappear when the engine is really warm, though I am not taking it on long journeys to find out right now, til I have more idea what the problem is.

I had an earlier problem with my (York) AC unit causing belt slippage (I suspect the unit is on its way out, perhaps bearings worn), but solved that by adjusting pulley location and hence tightening the AC belt---no more squeal from that belt. I don't think that tightening that is the source of the new problem, as the new squeal is now there whether AC is on or off.

So I am wondering if it is the alternator belt. With the AC belt, I could see that every few seconds the AC wheel was locking, and causing the belt to squeak. This time there is never a problem at idling, so it is hard to check this one on my own, but I looked at the belts today with someone else revving the engine. The squeal was there for sure, but nothing was obviously stopping, or locking up.

Any ideas for a diagnosis? What should I look for to see if it is the alternator? The belt is I think fairly new, though tension is not super-high. Would low tension cause a squeal like that, even if there is no really obvious slippage (like with my AC stopping completely)? Would it make sense that the noise disappears when the engine is warm, and has gradually gotten worse over the last few hundred miles? Or is there some totally different diagnosis I can check for, nothing to do with drive belts?

I can give more precise details of the noise/symptoms if there are things I can test out.

Many thanks!

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1978 300D non-turbo 383k (Claudia)

Redbull,
You already know that a properly tensioned belt should not squeal unless there is a heavy load, alternator bearing going etc. So,
1. Maybe the tension is not sufficient on one of the belts
2. I would raise the rpm's & spray belt dressing on to one of the belts. This would tell you the culprit when it quiets. Only time I use that spray.
3. Maybe it is time for all new belts.

OR, I was hearing a "belt" squeal from my wife's Town & Country A/C compressor, and pulled its clutch. The faces of the clutch were slightly oxidized, I live next to salt air & water mix. I cleaned it off and applied a slight coating of never sieze. My problem was solved, maybe temporarily. No prolems wih A/C op's.

Good Luck

Gary

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Gary
Experience is something you get, Just after you needed it....
'99 E300 295K, '83 300D 220K, '75 240D 185K sold
'80 MCI Bus Conv DD 6V92TA, w/4 cyl. Kubota diesel GenSet 12.5KW

My 82 had an alt belt squeal that was fixed initially by replacement but after an accidental oil spill , it started squealing again, I cleaned it off and that seemed to fix it. But after a stretch of wet weather it would also squeal at idle and in reverse. After an alternator adjustment though it seems to have stopped.

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1981 240D 143k 4 spd manual -SOLD

2004 VW Jetta TDI 5 speed 300k -still driven daily

Here's an easy way to determine whether it is your alternator. When it is squealing or "about" to squeal, turn on your lights and defroster. If it starts squealing than you'll know its your alternater when you put it under load. If it doesn't squeal then it probably isn't your alternator. This wouldn't diagnose a bad bearing, but it is so quick and easy to rule out you might as well give this a try.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

I have also found that a slipping alt belt will often slip more when loaded with lights etc.
Tbomachines is correct in that it doen't necessary mean bad alt bearing

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1981 240D 143k 4 spd manual -SOLD

2004 VW Jetta TDI 5 speed 300k -still driven daily

A quick test just to verify a slipping alternator belt. Take a bar of face soap. Rub it on the v portion of the belt or belts if there are a tandem pair driving the alternator. With the engine off of course.

Squeek stops then they were slipping. The belt or belts v wears narrower with time until the belt bottoms in the groove. This senario can also make it howl.

Or if a good glaze is present it lessens the friction ability. Then there are good belts that have just elongated or stretched a little with time. Adjust them.

A feeling of Deja vu here as I went through this trying to get my charging system to work. If it is slipping I suspect you aren't charging your battery well if at all. It was a difficult but ultimately rewarding experience figuring out how to remove, replace the alternator and belt and especially to figure out and make work the tensioning system as mercedes intended. That star bolt is a mother to deal with and is likely to need replacing if it is missing teeth. Through all of this I learned how to troubleshoot my charging system and the belt tensioning for the power steering pump and A/C (york). Good Luck.

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Kevin

1978 300D
1979 240D

Check belt tension. There's a spec for it in the book.

I'm almost certain you just need to tighten your alternator belt. More on that below.

Sometimes a new belt loosens a bit as it breaks in.

Yes.

Yes. Right after you start the car, the battery has just been slightly drained and the alternator puts out some amps to recharge it. After a while the battery is charged and the load on the alternator goes down. If the belt has been getting loose over time, either from break-in, wear, or some other reason, the noise would get worse.

Yes, but I don't think that will be needed. You can eliminate belts and belt driven accessories in a search for a mystery noise by removing the appropriate belt and then (briefly) running the engine to see if the noise is still there.

Tighten your alternator belt, have a brew, and get back to us.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

Thanks guys, I appreciate it. Busy week and it took me til today to get to this. Hot in Oregon today though....

Barry, I tried the soap trick, and the howl stopped. So I guess that pretty definitively picks out the alt belt tension as the problem. Skippy, thanks also for your comments---that totally makes sense now that the howl was disappearing soon after start-up.

So I'm now gonna tighten the belt. However, I am a bit clueless---I'm looking at your how-to guide here at peachparts:
http://www.peachparts.com/Wikka/OM617AccessoryBelts

I know from when I tightened the AC belt that the pics here are for a newer Benz, as mine (a 1978 w123) has the (older) York compressor. Is the alternator set-up different, too? Here is the picture I am looking at from that how-to page.



And here is the view of my own alternator from underneath the car.



I can't see the tensioner. Am I blind? I am assuming that the how-to view is from above, but looking from above, my AC compressor is directly above the alternator:



How can I get to the tensioner? Thanks again guys, and sorry if I am being dumb. You got me through the AC belt before, I hope the same thing will happen here!
-Jim

Attached Thumbnails

     

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1978 300D non-turbo 383k (Claudia)

you have different style bracket

loosent he bottom bolt and the pinch bolt, and then turn the adjustment bolt ont he front. Its like gear that rolls on teeth to tighten the belt.

Its very hard to get to.

I usually use a 17mm open ended wrench and loosen the pinch bolt from the top. and then use my offset 11/16 to turn the adjustment bolt from underneath.

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1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg

OK guys thanks for the help

@79Mercy-- The mechanism on the older-type bracket is pretty simple, thanks for the explanation. Didn't have the offset wrench but picked one up today and did this. You were right that adjustment bolt is `hard to get to'! Actually, once I got the pinch bolt and the back end of the adjustment bolt loosened, I found it easiest to reach in with the wrench from the side, under the AC compressor, with the wrench almost horizontal. So that's a bit different from how you got to it.

Anyway, the belt is tighter now, and no howl so far.

(That could still be the effect of the soap I put on following Barry's advice to check if it was the belt. I will drive it around a while and then see at next start-up what happens.

@barry is the soap like cheap belt dressing to increase grip, or is the idea just that the soap just makes it slip quietly?)

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1978 300D non-turbo 383k (Claudia)

PS

two things I noted while doing this

(1) maybe a picture (like mine above) of the older bracket could go on the how-to page, with arrows pointed at the pinch bolt and adjustment bolt? Might help people like me who are (a) clueless and (b) have older w123s.

(2) I think I could get the belt tighter if I have a second person to hold the adjustment bolt tight with a wrench while I tighten things up underneath. Is that how y'all would do it?

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1978 300D non-turbo 383k (Claudia)

The soap is just an approach used if I develop a slipping alternator belt far from home. It appears to act as a belt dressing rather than increasing slippage in my opinion. Never really considered exactly how it works. Yet work it does for some time.

When this problem raises its head the belt should be inspected. You do not want to see the belt riding at the bottom of a pulley because it has worn thinner in width and now rides too low in the pulley vee. Lessens the side traction that was designed to transfer the power.

Also if the vee portion of the belt has developed cracks it is time to change the belt as well. Extreme glazing of the belt surface is probably an issue as well. I do not know enough about that to comment though. Other than if the glaze can be broken without material removal to any degree. Not a bad ideal. Perhaps that is one function of a belt dressing yet again not sure.

What this is all about besides reliability is you do not want to apply too much belt tension to overcome a belt problem. It adds uneeded additional load to the alternator bearings.

I have also had belt problems with sitting cars if there is developed light surface rust buildup on the pulley surfaces. It did seem in some fashion to shorten the lifespan of belts to me. No conclusive evidence but just what I thought I was seeing. So on a car that has sat around enough time to develop the rust I am describing in the pulley vees I clean it off.

Also remember on a new alternator vee belt you usually tension it twice. Once when installing it and a few days later. Or at least check the tension as some if not all belts will stretch a little in initial belt breakin.

I'm very glad I found this thread. Thank you forum search!!!

I did not mention it earlier. The one test I find conclusive squeeling or otherwise. If you can turn the alternator pulley by hand using the alternator cooling fins the belt or belts are too loose.