Snapping serpentine belts - what is causing this?

About a month ago i started to notice a squealing when starting up my xj 96 150k. It would eventually go away but took a while and would really squeal loud under acceleration.

Problem i assumed - loose belt. I tightened the belt to 1/8"- 1/4" freeplay as I do on all xj's. It helped a little, so i just assumed i needed a new belt. But just for fun, i took off the idler pulley checked and regressed the bearings and checked all the other pulleys and they were all looking okay.

I started it up today and and snap, the belt broke right in half. So, assuming that the belt was on its way out anyway I put on another one. start the jeep up and it starts squealing madly and smoking and I notice the belt is moving very slowly. I shut it down to see that the new belt is all torn up with pieces of it all over the engine bay. I've owned 3 XJs before and probably taking the belt off and on 30 times so i know it was on there properly.

I feel around and see that everything is cool to touch except for the alternator which is warm on the pulley and the main engine pulley off the crankshaft which is warm-hot.

So, what do you all think? Is it a seized alternator? Or something much more serious within the engine that I am not familiar with. . . . I know my way around most of a jeep but have not done much engine tear downs

The engine was running smoothly, so... what would make this main output pulley do this... if this is the case... i thought that was tied directly onto the end of the crankshaft? am i wrong

Harmonic balancer failing, moving back and eating the belt or as you have noted your alternator could be seizing.

Once things are cool try spinning all the pulleys, not including the crank of course.

There should be some resistance in the alternator, but not a great deal of it.

Inspect the harmonic balancer very carefully, they don't have to move much to eat the belt.

1990 FSM says belt tension, new belt, 180-200 ftlbs, used belt 140-160 ftlbs.

i would check seized alt, or a bad bearing.

did you check the idler pulley in your hand or on the block? I watched a perfectly good one be the culprit when someone forgot the spacer, or used the wrong bolt to secure it to the block. If you do that then the bearing gets canceled out as you are clamping the pulley.

joe_peters said:

Harmonic balancer failing, moving back and eating the belt or as you have noted your alternator could be seizing.

Once things are cool try spinning all the pulleys, not including the crank of course.

There should be some resistance in the alternator, but not a great deal of it.

Inspect the harmonic balancer very carefully, they don't have to move much to eat the belt.

1990 FSM says belt tension, new belt, 180-200 ftlbs, used belt 140-160 ftlbs.

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well, i would have automatically thought the alternator was seized if the alternator pulley was hot... but it was the crank shaft pulley that was hot and the alt was just warm..... UNLESS

the alt is just creating a strong resistance and the belt is just spinning on the crank... that would then make the alt warm and crank hot.... right?

wolfpackjeeper said:

did you check the idler pulley in your hand or on the block? I watched a perfectly good one be the culprit when someone forgot the spacer, or used the wrong bolt to secure it to the block. If you do that then the bearing gets canceled out as you are clamping the pulley.

Click to expand...

yeah.. spins free torqued on the block as well as off. good idea though

well, i would have automatically thought the alternator was seized if the alternator pulley was hot... but it was the crank shaft pulley that was hot and the alt was just warm..... UNLESS

the alt is just creating a strong resistance and the belt is just spinning on the crank... that would then make the alt warm and crank hot.... right?


i should be able to turn the alternator by hand right?

XJ96Classic said:

i should be able to turn the alternator by hand right?

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It should turn pretty easily by hand.

P_in_Seattle said:

It should turn pretty easily by hand.

Click to expand...

yeap.. it's not moving. . . .guess that's problem solved.

the hot crank pulley was throwing me off but i guess thats because it was slipping on the crank and holding on the alternator.. would explain why it looked like hte belt was moving slowly . . .eh. too bad i just tore up a brand new belt figuring this out. Thanks everyone.