Why is my serpentine belt continuously shredding?

[ntippet]

Does the center idler have an inside lip, or it it flat all the way across where the belt rides?

[wogmail]

I believe that both the aftermarket one and the dealer one are completely flat. Going to check now.

[wogmail]

Looks like all of them are completely flat, including the old parts the dealer gave me back.

Looks pretty much like this, smooth and flat:
http://www.ebay.com/itm/Gates-38489-...d0bd06&vxp=mtr

[1 Lucky Texan]

idler looks suspicious but, someone may need to physically compare some positioning with a known good vehicle. Some measurements from some common points.

I once had a Ford Windstar with a bad tensioner that would throw its belt off. Is there any motion to the tensioner when the belt is off?

[wogmail]

Originally Posted by 1 Lucky Texan

idler looks suspicious but, someone may need to physically compare some positioning with a known good vehicle. Some measurements from some common points.

I once had a Ford Windstar with a bad tensioner that would throw its belt off. Is there any motion to the tensioner when the belt is off?

The idler has been replaced twice and the tensioner once. The idler locked up when the belt broke the first time, with an aftermarket part NAPA idler. Dealer replaced it with a dealer part, saying it wasn't holding the belt correctly.

The first shop (Merchant's) said the tensioner was fine, my normal mechanic said it had some play but should be fine, and the dealer said it had too much play and had to be replaced.

I am hoping at this point it has to be one of the pulleys that has not been replaced, because if it is one of the ones that has already been replaced (perhaps twice) that is really going to throw off my troubleshooting process.

[plain OM]

There's two "idlers", one is just that, the other is the tensioner. Then there's four "working" pulleys; crank, AC compressor, alternator, and ps pump. (see attached)

The belt is clearly shifting toward the engine on the AC compressor pulley, and it appears that is where the shredding is happening.

Also, the AC compressor is between the crank pulley and the alternator pulley in the belt's path. So those are critical to the belt going over the AC compressor pulley straight.

The two idlers are between other working pulleys where the belt is guided by grooves and where it appears the belt is centered in the photos. So the idlers would not likely be causing the shifting at the AC compressor.

The pulley feeding the belt to the AC compressor is more likely to cause the belt to move across the grooves than the pulley on the back side. I believe the belt travels from the alternator pulley to the AC compressor and is being pulled by the crank pulley below. As the belt appears to be centered on the crank pulley, and there's no reason to believe the pulley has shifted inward toward the engine, the crank would be a correcting factor, rather than a cause of the shift at the AC compressor. (I don't think the H6 crank is a two piece, with an outer ring mounted by a rubber insulator to the inner part. If, however, it is, then it is possible that the belt contact part could have shifted inward -- this has been seen in the H4.)

[correction: It appears that the crank pulley is a two piece, based on the second attachment photo attached below. It's hard to see in wogmail's photos, but check that the inner and outer parts are in line with each other and that the outer ring isn't wobbling as the pulley turns.]

Following this, it would appear that either the alternator pulley is too far back, or the compressor pulley is too far forward. (Or, the compressor is not mounted flat -- the pulley is not vertical and this can cause the belt to move across it.)

Has either the alternator or compressor been moved, replaced, worked on?

Is there enough room to place a ruler across the alternator pulley, toward the compressor, to see how they are aligned?

[wogmail]

Originally Posted by plain OM

There's two "idlers", one is just that, the other is the tensioner. Then there's four "working" pulleys; crank, AC compressor, alternator, and ps pump. (see attached)
Understood.

The belt is clearly shifting toward the engine on the AC compressor pulley, and it appears that is where the shredding is happening.
Agreed.
Also, the AC compressor is between the crank pulley and the alternator pulley in the belt's path. So those are critical to the belt going over the AC compressor pulley straight.

The two idlers are between other working pulleys where the belt is guided by grooves and where it appears the belt is centered in the photos. So the idlers would not likely be causing the shifting at the AC compressor.
Hmm.
The pulley feeding the belt to the AC compressor is more likely to cause the belt to move across the grooves than the pulley on the back side. I believe the belt travels from the alternator pulley to the AC compressor and is being pulled by the crank pulley below. As the belt appears to be centered on the crank pulley, and there's no reason to believe the pulley has shifted inward toward the engine, the crank would be a correcting factor, rather than a cause of the shift at the AC compressor. (I don't think the H6 crank is a two piece, with an outer ring mounted by a rubber insulator to the inner part. If, however, it is, then it is possible that the belt contact part could have shifted inward -- this has been seen in the H4.)

[correction: It appears that the crank pulley is a two piece, based on the second attachment photo attached below. It's hard to see in wogmail's photos, but check that the inner and outer parts are in line with each other and that the outer ring isn't wobbling as the pulley turns.]

Following this, it would appear that either the alternator pulley is too far back, or the compressor pulley is too far forward. (Or, the compressor is not mounted flat -- the pulley is not vertical and this can cause the belt to move across it.)

Has either the alternator or compressor been moved, replaced, worked on?
Neither that I am aware of since I bought the car over the summer.
Is there enough room to place a ruler across the alternator pulley, toward the compressor, to see how they are aligned?
I will try this tomorrow and see.

Thanks

[signage]

It appears to me that the AC compressor pulley is the culprit!

[1 Lucky Texan]

because of the ribs, it would seem to me, one of the smooth pulleys could more easiy cause this probleem and I suspect it is the idler. If it isn't the pulley, then ther is a problem with its mountiing. Wrong bolt or cracked/bent alt. bracket. The belt is walking back on it, as belts do - they walk towards the highest tension. (its why a crowned smooth pulley can keep a flat belt centered - well, sorta, it's a complex issue)

I bet you could mount a new belt, and use a wrench to slowly crank the engine around and watch the belt creep backwards on the idler , THEN jump some ribs.

hey! maybe we just need some slo-mo video cameras!

[plain OM]

Originally Posted by 1 Lucky Texan

because of the ribs, it would seem to me, one of the smooth pulleys could more easiy cause this probleem and I suspect it is the idler. If it isn't the pulley, then ther is a problem with its mountiing. Wrong bolt or cracked/bent alt. bracket. The belt is walking back on it, as belts do - they walk towards the highest tension. (its why a crowned smooth pulley can keep a flat belt centered - well, sorta, it's a complex issue)

I bet you could mount a new belt, and use a wrench to slowly crank the engine around and watch the belt creep backwards on the idler , THEN jump some ribs.

hey! maybe we just need some slo-mo video cameras!

I had thought about the "smooth" pulleys, but discounted it when I looked at the path of the belt. (see below) The two flat pulleys are on the tensioner and the idler. The other pulleys, on the crank, AC compressor, alternator and p.s. pump are grooved in some way. If one of the idlers is not "square", the belt, as it passes over the idler will indeed tend to move off center. But looking at the path, in both cases of smooth pulleys, the next pulley is one of the grooved ones. If the smooth pulley is causing the belt to go off center, one would expect the belt to jump grooves on the following pulley, but that's not the case. It's jumping at the compressor, and both the pulley before the compressor, and the pulley after the compressor, are also grooved, and the belt isn't crossing the grooves on those.