Alternator belt keeps loosening up.

[Bmachine]

Hello there.
I have a 1990 325is auto with 105Kmiles. All stock. The previous owner left it parked for 12 years so I had most of the "soft" components replaced. Ie, all belts (including timing), all brake components, all rubber hoses, water pump etc... It now runs really well... except for one problem:

The alternator belt wears out within one or two months (ie 1,000 or 2,000 miles). The first time, it totally chewed it up until it flipped inside out and ended up about half of its original width. I brought it to South Bay Independent in Gardena, CA who replaced it with a new Conti and checked everything. He said the alternator was properly aligned and that everything looked good. There are no soft bushings in this alternator so that is not it either.

Three weeks later, it started screeching again so I tightened the alternator adjustment bracket and it is now fully extended. But yesterday it started whining again and as I checked it, there is probably 1 inch of travel in each direction when I push or pull on the belt between two pulleys. I can rotate the alternator by hand under the belt.

Logic dictates that only two things can be happening: either the belts keep stretching out or the alternator keep creeping closer to the water pump. But the belts I put on are the correct PN and either Conti or BMW brand. And the alternator adjustment bracket is still at its fullest extension.

I have done a search on this forum but to no avail. Would anyone have any ideas? I've already had to tow it twice because of that and it is a daily driver so it is a big pain.

Thank you for any help.
Bo

[fronton]

Carry jumper cables.

I don't think your belts are stretching out. Are you missing any teeth in the bracket or bolt? When you adjust the bracket out, are you loosening the pivot bolt as well then tightening it back down once tensioned correctly? For the belt itself, you need it taught, not tight with maybe 1/2 inch of travel in the middle sections.

You may want to try getting a 10mm shorter belt so you don't have some room to adjust it on the tight side if you are currently all the way extended out.

[jlevie]

It sounds like the belt is stretching. That can be the result of over tightening the belt, but it also can be the result of excessive load on the belt. Check the water pump and alternator to make sure that they aren't binding or have bad bearings.

[Bmachine]

Thanks for the reply. No the teeth are l in good shape so the bracket works the way it is supposed.

I think installing a shorter belt would be a temp fix but would not solve the main issue.

Carry jumper cables.

I don't think your belts are stretching out. Are you missing any teeth in the bracket or bolt? When you adjust the bracket out, are you loosening the pivot bolt as well then tightening it back down once tensioned correctly? For the belt itself, you need it taught, not tight with maybe 1/2 inch of travel in the middle sections.

You may want to try getting a 10mm shorter belt so you don't have some room to adjust it on the tight side if you are currently all the way extended out.

[Bmachine]

Thank you. But this I checked for over tightening and ther was 1/4 in h deflection both times so I think we're ok there.

I an move the alternator relatively freely under the belt. And the water pump is new. I should probably take the belt off or at least loosen it completely and make sure the alternator is fully freely rotating.

I keep thinking it has something to do with the fact that the car sat unused for so long. Because it ran perfectly before.

It sounds like the belt is stretching. That can be the result of over tightening the belt, but it also can be the result of excessive load on the belt. Check the water pump and alternator to make sure that they aren't binding or have bad bearings.

[jlevie]

Get the belt off the pulleys and make sure that both turn freely. You can do this check w/o removing the belt.

[paperplane94]

One more thing. Check that the alternator mounting bolts on the bottom of the alternator are not loose or missing.

If the alternator belt keeps loosening with the adjuster at full this could be it. Otherwise, the belt.

[Bmachine]

OK, here is the latest, in case someone else runs into a similar problem. As I thought, this seems to be related to the fact that this car sat unused for many years.

It appears that, over the years of inactivity, rust started coating the inside of the pulleys. So although they were turning mostly freely, everytime the car was started, there would be some friction that would grind the side of the belt. After a few hundred miles, the belt was becoming too thin and did not grab in the V of the pulley properly. This in turn, accelerated the sand paper effect to the point where the belt could not grab anymore.

We sanded the pulleys back to normal and installed a new belt. Fingers crossed.

Bo

PS: Thank you for all the replies and good suggestions!