So the other day I'm driving my car on some backroads, transmission selector in "3" to keep the torque up. Suddenly, at a certain RPM (call it 4k), there is a god-awful whining/squealing sound coming from up front. I upshifted and it went away immediately. Shifting remained smooth, and it didn't feel driveline-related, but I could reproduce the symptoms by dropping down a gear and letting the RPMs climb. I suspect I could've done so in 4th as well, but the twisty two-lane roads wouldn't allow speeds high enough, and I didn't want to keep it up for too long anyway.

For the life of me, the sound seemed pulley-related. I recalled that my steering box supposedly had a "slow leak" that I hadn't been monitoring closely, so I checked the fluid (see below - this is on a dead-cold engine):


Now, obviously the fluid is below the MIN mark, but only just. Is it low enough to make the steering pump complain at higher RPM? I don't know. Also, if I had to guess, I would say the noise was more front-right than front-left, but it's hard to really say because it happened so fast.

The only other thing that I know for sure that is going on with the car is that the front brake pads are worn (light is on, but not constantly) and I've been ignoring it because I just bought a house and moved, and need to get my garage unpacked first. Noise was not related to brake application, though.

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I'm not sure what it would be on the OM602 but in the OM617, the pulley rubs on the oil cooler hose when the driver's side motor mount is worn badly enough. I would look something similar.

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Sam

84 300SD 350K+ miles ( Blue Belle )

Thanks for the tip. Unfortunately (or fortunately, actually) the motor mounts are new.

I started the engine in the driveway and I was unable to replicate the sound in Park. Of course, I was unwilling to rev a cold engine too much.

But I'm wondering now if it's wheel or driveline related - something that requires the car to be in motion. The wheel bearings are new. Gonna crawl under the car when I'm doing the brakes tomorrow and check out the flex discs.

I just don't understand what it could be to start and stop so abruptly with RPM changes. It's like it's on a switch!

If it sounded pulley related, I'd be more inclined to think alternator or tensioner pulley. Low fluid level won't cause the PS pump to "strain". If anything, it'll do the opposite, foam is easier to pump than liquid.

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Does this happen for a few minutes while warming up then up go away?
Maybe a loose alternator belt, and a week battery.
Right after you first start the car, the alternator is trying to recharge the battery,and the resistance causes the belt to slip, especially when raising the
engine RPMs.

Well, I have no idea what's going on. I didn't drive the car for a few days; the weather has been real nice, so I took my BMW out of the garage and have been driving that. Plus, I wanted to change the front brake pads on the W124, and I was waiting for a chance to take care of that.

Well, today I finally did that, and took the car out afterwards for a spin. Drove a few blocks with no funny noises, then I drove a few more and before I knew it I was out of my neighborhood. Got the engine good and warm and then took the car to some higher speeds so that I could bed the pads - hard, controlled braking with lots of pulls back up to speed with the engine working hard - still no noises.

Very weird. Could the noises have somehow been brake related, even though they didn't occur with brake application? The wear sensor on the passenger side (the side where the noise came from) was worn down to the point of being broken - but I don't see how that would cause an intermittent noise.

Oh well. Hopefully it was all just much ado about nothing.

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You will get a groan from low power steering fluid when you turn the steering wheel extreme right or left. When you add the fluid be sure to have the engine running and turn the steering wheel all the way to the left and right a few times until the level is proper.

Well, of course the noise came back (guess changing the brake pads didn't help after all, go figure, lol).

Anyway, it's not exactly the same noise, but I think it's related. It's much less loud, and happens intermittently at lower engine speeds now. Sort of a scraping noise. I can even replicate the noise in park (so not driveline related). It seems to be at least somewhat load-dependent, I tried to capture it in the short video below.

Thoughts?

https://youtu.be/DNbpx0JGhL4

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Since you said you replaced motor mounts I am going with a transmission mount. Worthy of checking is your fan blade shroud - are all the mounting bolts in place? Grab it and shake, see if it moves any.

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The idler pulley bearing is drying out / wearing out. Noises can sound like trans whine, squeals, and or grinding. Can act up based on all kinds of conditions including humidity, heat, and so on. Probably missing the protective cap.


Try popping the belt off the tensioner system and check the idler pulley for noise / rattle. Should also have zip for fit slop.


I once thought I had a trans bearing whine, till I refreshed the belt tensioner set up.

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1986 300SDL 440,xxx

That sounds like the ticket. I did briefly check the pulley, but of course it seemed okay with the belt on. The belt shock is new, but that's it. Does the actual pulley need replacing, or can it be lubed or something.


Lube may be good for diagnostic purposes at least, but I don't know exactly what to use and don't want to gum things up.

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New shock is probably leaning harder on the bearing.

You'll pretty much know by feel. It should be rock solid smooth rotation with no lateral movement. Belt off - If it spins freely with a flip of a finger it probably on its way out. I tried getting a little motor oil in with out much luck.

Buy an INA brand unit. Fairly priced. Don't forget the dust cover cap. That's what keeps the bearing alive.

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1986 300SDL 440,xxx

Just the pulley? Already been pricing, about 25 bucks for the INA part. Some brands are a tensioner and pulley together, but INA sells them separate.


I didn't see a dust cover in the pic - is it a unique p/n?


And thanks!

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I think Peach / Pelican / MB Part #: 601-202-05-22-M22


I refreshed both the tensioner arm and the pulley. But I really like INA for stuff that plays important roles i.e. lifters, timing chains, belt pulley. So I purchased them separately.


Problem with the idler pulley is when they melt down you loose a belt there's the chance for other collateral damage due to the belt flying around. I once lost both transmission / radiator connection hoses due to belt carnage. The belt was an easy fix. Getting the connections hoses while on the road was a bear.

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1986 300SDL 440,xxx

Had a little extra time before work this morning so pulled the fan and shroud and slackened the belt. Found the tensioner arm fouled with oil (from what looks like an old leak) and the dust cap missing from the tensioner pulley. Spins freely by hand with some bearing noise (see video below). No obvious lateral play that I could detect. Think this is the culprit?

https://youtu.be/BZEMC8OUq_I

As an aside, the vacuum pump is caked with old oil as well, and it looks like the AC compressor below, too. Again, it looks like an old leak, but I'm not really sure what is going on here. I suspect the vacuum pump is original (it has a Pierburg label, but I think they may have been the OE supplier?) and probably do for a change anyway.

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