The downside of serpentine belts.

Dear Car Talk

Dear Car Talk | Mar 01, 2003

Dear Tom and Ray:

I have a problem with the belt on my '89 six-cylinder Jeep Cherokee. It was fine, but then the serpentine belt began to shred. I replaced it, and the new one began to shred, too. I noticed that the belt was shifting off of the pulley that is located on the crankshaft. It wasn't moving much, but it had moved exactly one groove closer to the engine. I tried another new belt and tightened it more. But halfway through a test drive, it was shredding, too. Can you give me any advice? -- Ricky

RAY: Yeah. Buy your belts by the caseload, Ricky.

TOM: Back in the old days, cars had one belt for each accessory: the alternator, water pump, fan, power steering, etc. But that was too easy.

RAY: So, now most cars have a serpentine belt, which is a long belt that runs all over the engine and runs everything. The real advantage of a serpentine belt is that it's easier to change. You don't have to reach your fat arms into little skinny openings to change multiple belts. You change one belt, and it's done.

TOM: This belt runs over and around many pulleys that power the accessories, and it sounds to me like one of those pulleys is out of alignment. That's what's pulling the belt off-line and causing it to shred.

RAY: If you've had one of the accessories replaced recently -- like the alternator or water pump -- I'd suspect that that pulley is now lined up incorrectly. It could have been manufactured incorrectly, or it could have been the wrong pulley for your engine. A pulley only has to be a few millimeters off to set a belt a-shreddin'.

TOM: Another possibility is a bad idler pulley or tensioner pulley. Those are sort of "dummy" pulleys that are used to alter the direction of the belt. If a bearing goes bad on one of those pulleys, it tends to tilt, and that could cause the belt to shred.

RAY: The worst possibility (for you -- if your mechanic has kids with tuition payments due, this will be good news for him) is the crankshaft pulley itself. It could be the harmonic balancer portion of the crankshaft pulley, or -- the worst-case scenario -- too much end play in your crankshaft.

TOM: And if you have too much crankshaft end play, even Viagra won't help. That means you need an engine rebuild.

RAY: But unless you can clearly see that one of the pulleys is not in the same plane as the others, it's going to be hard for you to figure this out yourself. So I'd recommend taking it to a mechanic. I know it'll cost you a few bucks, but think of the fortune you'll be saving on belts, Ricky.

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