Stumped. AC belt keeps slipping off

[ThreeD]

The story starts back sometime last summer after I replaced my AC pulley and belt as preventative maintenance. The AC and AC belt prior to this never had problems.

After replacing the pulley, I noticed the AC stopped working and was surprised to find the AC belt missing. Thinking nothing of it I ordered a new belt and put a new tensioner on, only to find that after 100 miles the belt had shredded itself.

After this I figured something was wrong / seizing causing the belt to shred and looked at the system. The AC pulley spun freely with a little play but nothing seemingly out of the ordinary. No abnormalities noted with the tensioner bracket itself or how it was mounted, so I figured maybe the AC clutch was seizing and causing the belt to shred, so I replaced the AC compressor/clutch.

Throw another belt on it, and find out after another 100 miles the belt had wiggled itself halfway off, which means nothing was seizing and causing it to shred, but for some reason the belt would wiggle its way off the front of the AC and crank pulley. BTW, there are no belt sounds.

With the minimal play I noted in the "new" AC pulley, I put the old "known good" pulley back on. Drive it another couple hundred miles and again the AC belt has wiggled its way halfway off again.

I am out of ideas and am wondering WTF is going on. Is the crank pulley bad and wobbling? If so why was it OK until I changed the AC pulley/belt, and why isn't the drive belt having issues? Should I replace the tensioner again...just because?

What am I missing and what should I be looking into to diagnose the problem? I'll post a pic below.

TL;DR: AC OK until pulley/belt replaced. AC belt keeps slipping off. Belt changed like 3x, Pulley changed total x2, tensioner changed x1, AC compressor changed x1. No belt sounds.

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[Vitaliy]

I dont know if its the angle of the picture or what but the ac compressor pulley looks way out of alignment compared to the crank pulley. Notice how the crank pulley and idler pulley are in the same plane but the ac pulley looks to be way too far in. Looks like the ac pulley needs to come out A LOT to be in the same plane as the crank pulley. Take a long piece of straight edge and see if its out. Like i said, hard to tell from the angle especially with the belt in the way.

[bucs19]

Have the pullies been verified as correct size ?

[JitteryJoe]

I dont know if its the angle of the picture or what but the ac compressor pulley looks way out of alignment compared to the crank pulley. Notice how the crank pulley and idler pulley are in the same plane but the ac pulley looks to be way too far in. Looks like the ac pulley needs to come out A LOT to be in the same plane as the crank pulley. Take a long piece of straight edge and see if its out. Like i said, hard to tell from the angle especially with the belt in the way.

Ditto. Did the AC mount come loose?

[ThreeD]

Good thoughts guys. I'll check the pulley alignment this weekend as well as the AC pulley size

[pbonsalb]

The tensioner is a possibility. The AC tensioner looks similar to the main accessory drive tensioner, but is different (this assumes you have hydraulic tensioners for both drives and some cars have a mechanical main tensioner). You may not have remembered belt routing correctly; refer to a diagram. Make sure the pulley is installed correctly in terms of any spacing or washers since that affects alignment. Your issue is either tension (wrong tensioner or wrong tensioner pulley or wrong belt routing) or alignment (pulley or ac spacing, allowing belt to walk -- eventually off the compressor).

Generally when a problem shows up after you have worked on the car, you did something wrong. Here, I go back to "last summer I replaced my AC belt and pulley as preventative maintenance . . . " You screwed something up on the reinstall. No big deal. You just have to refer to a proper or stock install for guidance and redo.

[atlantisvip]

Belt looks diagonal. .

[ThreeD]

The tensioner is a possibility. The AC tensioner looks similar to the main accessory drive tensioner, but is different (this assumes you have hydraulic tensioners for both drives and some cars have a mechanical main tensioner). You may not have remembered belt routing correctly; refer to a diagram. Make sure the pulley is installed correctly in terms of any spacing or washers since that affects alignment. Your issue is either tension (wrong tensioner or wrong tensioner pulley or wrong belt routing) or alignment (pulley or ac spacing, allowing belt to walk -- eventually off the compressor).

Generally when a problem shows up after you have worked on the car, you did something wrong. Here, I go back to "last summer I replaced my AC belt and pulley as preventative maintenance . . . " You screwed something up on the reinstall. No big deal. You just have to refer to a proper or stock install for guidance and redo.

That's exactly what I am thinking too, just can't think of what was messed up. It's kind of hard to get the belt routing wrong on the AC side, but I did notice that the tensioner pulley seemed awfully close to the crank pulley. Will take it apart again and double check everything against my other E36. Thanks for the input.

[scooper]

did you get the correct belt? Did you install the belt as show in your pic above? If so, that appears to be wrong. You should not be able to see any pulley grooves.

[ThreeD]

did you get the correct belt? Did you install the belt as show in your pic above? If so, that appears to be wrong. You should not be able to see any pulley grooves.

That's a picture of how it wiggles / walks off the pulleys, not how it was installed

[RRSperry]

Tensioner.

[ThreeD]

I had a couple hours to take things apart today and compare with my other M50 E36.

The pulleys are aligned identically between the two cars, and it measures just fine with a straight edge, so it wasn't pulley misalignment.

Pulleys are all the same size, except for the AC/crank pulley which is bigger on the S52 versus the M50. AC compressor pulley confirmed same size from old AC compressor.

Measured the belt tensioner on the M3 with my hand, seemed OK.

Swapped the belt tensioner from the 325 to the M3 (they are both "new", identical part #s), and WOW. The 325's tensioner is at least 40-50% stiffer than the M3. I think I must've swapped the tensioners around on the M3 3-4 times before I actually believed it to be true

So there you go, it is most likely the belt tensioner, given that the "new" tensioner on the M3 was nearly 40-50% looser than the known-good one from the 325. I think the problems may have started after I changed the tensioner and not the AC pulley/belt, it was just a little hard to keep track of what was done when between the two cars. Lesson learned, remember to suspect new parts, they can be bad out of the box. Thankfully FCPEuro has lifetime warranties, so back to FCPEuro the tensioner goes...hopefully my diagnosis was right and a new one will fix the ongoing AC annoyance that has been bothering me for the past year.

[MParallel]

I was a bit surprised when I changed the belts on my M3 last year. My 328i has a mechanical and hydraulic tensioner.

On the M3, the AC belt tensioner is neither hydraulic nor mechanical. You have to set the pulley with one hand against the tension of the belt itself, then tighten the pulley in place. Then there's nothing that activally pushes the pulley against the belt.

[NeilM]

I was a bit surprised when I changed the belts on my M3 last year. My 328i has a mechanical and hydraulic tensioner.

On the M3, the AC belt tensioner is neither hydraulic nor mechanical. You have to set the pulley with one hand against the tension of the belt itself, then tighten the pulley in place. Then there's nothing that activally pushes the pulley against the belt.

According to the ETK only the Euro 3.0 (S50B30 engine) is the way you describe, with no tensioner; the later 3.2 (S50B32) does have one. The US M3 engine is based on the 325/328 and does have a tensioner on the AC belt.

Neil

[pbonsalb]

The AC tensioner on my 99M3 has a hydraulic shock like my main accessory tensioner but it is actually a different part number. I remember other cars in the old days having threaded tensioners that were tightened manually but have not seen that on an E36.

[MParallel]

According to the ETK only the Euro 3.0 (S50B30 engine) is the way you describe, with no tensioner; the later 3.2 (S50B32) does have one. The US M3 engine is based on the 325/328 and does have a tensioner on the AC belt.

Neil

Uh no ETK lists it. And I have an S50B32 myself with that setup.

[NeilM]

Uh no ETK lists it. And I have an S50B32 myself with that setup.

S52US with hydraulic tensioner: http://www.realoem.com/bmw/enUS/show...diagId=11_6047

Neil

[MParallel]

S52US with hydraulic tensioner: http://www.realoem.com/bmw/enUS/show...diagId=11_6047

Neil

Who's talking S52?

You said

...the later 3.2 (S50B32) does have one.

Tensioner that is.

Then I tell you, no it does not. And I have an S50B32 that does not have a tensioner.

[NeilM]

Who's talking S52?

The OP: 99 M3, US model. It has a tensioner.

Neil

[MParallel]

The OP: 99 M3, US model. It has a tensioner.

Neil

US = S52.

But nevermind. If you don't get what you said yourself, you don't get it.

It's not that hard.

[TH3 Shifty]

wat?

[ThreeD]

wat?

Lol, agreed.

Brief update: I replaced the tensioner last week. Drove the car 600+ miles this weekend to Ithaca and back, and the AC belt is doing just fine. Glad this "mystery" is solved (after spending hundreds of $$$ trying to chase a simple problem). Good call pbonsalb and Rich...

[Braymond141]

The OP: 99 M3, US model. It has a tensioner.

Neil

Neil, you did mention the Euro motor which is what MPar has all over his sig. As unhelpful as his post was to the OP, you're a little lost there. He was following along just fine with the US motor in question.

[ThreeD]

Update: it fell off again. But this time I found out why. At high RPM the tensioner gives way and the belt wobbles off. It's particularly bad when bouncing off the rev limiter, even just a couple times.

This is a new INA tensioner, which is an OEM manufacturer, on a S52 with a rev limiter set at 7200rpm, ltw flywheel, new AC compressor, newish AC pulley without play and is tight. The only thing I haven't checked is the crank pulley bolt itself, but the AC pulley that bolts to the crank pulley is on tight. There's no pulley misalignment, measured by straightedge.

Here's a video, I think I have it at high RPM but no rev limiter bouncing, but you can see the tensioner giving way then the belt slapping around a bit before I lower the revs.
https://youtu.be/iv-plICdlJk

Do you guys think it's a simple case of the tensioner not being designed / strong enough for high RPM running, or do you think there's an underlying problem? I'm more than happy to throw in a genuine BMW tensioner if you guys think it's the only underlying issue

[scooper]

It might be due to the higher rpm and maybe ur ltw flywheel. Do u get rattling at 700 or 750rpm idle?
INA is one of THE oe brands. I trust it like I trust lemforder. The problem has got to be something else. Is ur crank tensioner old?