Throwing belt

My 90 4.0 cherokee threw the serpentine belt this morning. After putting a new one on the belt only stayed on for about .02 sec. I can see were the alternator pully has a signifigant dent in the front of it and could be causing the belt to come off but, I have been driving with the pully bent for about 1000 miles with no problems.

What else could cause the belt to fly off like that?

I also noticed some play in my water pump pully, could that cause it as well?

Do a search on harmonic balancer. It's time for an R&R of the nose of the engine I think....better now than when it gets hotter out...

Would second the opinion that it is probably the harmonic balancer. It is easy enough to check out too. You can even do it with the belt off. Start your engine and look down at the crankshaft and you will probably see that the balancer is not spinning true. Replacing it should remedy the problem. Dorman makes a good aftermarket one that should be available through NAPA.

I would think that the bent pulley along with water pump pulley would be the cause, if they are both not true then that should cause it to jump just that quick. Had that issue on a 4.3 gmc once. replaced both the water pump and an idler and was cured.

The water pump is a smooth pulley, if that one's off it would be more likely to cause the belt to jump off than a grooved pulley. Check all the pulleys and harmonic balancer though, and replace anything that seems questionable, including that alternator pulley.

The alternator pully being dinged should not cause any trouble, unless it's dinged all the way across.

Seriously, take a look at your harmonic damper - the big pully down bottom. If you're not sure, lay a ruler or other straightedge across the front of the thing, it should touch both the hub and the ring. If it doesn't, replace the thing.

It's the only pully I've seen that gets "off" far enough to throw belts - especially that fast! A new one should cost you less than $60, and it's also a good time to change the front main seal (right behind the HD) while you're about it.

You'll need an HD puller and installer, or a 1/2"-20 bolt and washers can be used in place of the installer kit. Torque the main bolt to 80 pound-feet, COATED WITH CLEAN ENGINE OIL (per FSM.)

5-90

tsegeela said:

My 90 4.0 cherokee threw the serpentine belt this morning. After putting a new one on the belt only stayed on for about .02 sec. I can see were the alternator pully has a signifigant dent in the front of it and could be causing the belt to come off but, I have been driving with the pully bent for about 1000 miles with no problems.

What else could cause the belt to fly off like that?

I also noticed some play in my water pump pully, could that cause it as well?

Click to expand...

Wobble in any of the pulleys (you have seven) could cause the belt to get thrown off. Others have already told you about the crank pulley (or harmonic balancer) and that's well known, but you need to check ALL the pulleys.
If your water pump pulley is wobbling, you'll need to replace the water pump because the shaft has worn and very soon, the shaft seal will break and you'll have coolant leaking from the weep hole.
Idler pulleys sometimes wobble when the bearings wear out so check it. You CAN replace the bearings but a new pulley is cheap so it's better to replace the whole thing.
If the clutch fan pulley bearings wear out and cause it to wobble, the fan blades would hit the shroud and you'd find telltale signs of that.
A small ding in the alternator pulley won't cause the belt to slip off unless the ding is big enough to distort the pulley.
As a general rule, a wobbling grooved pulley will usually cause the belt to slip forwards or backwards along the grooves and cut into the belt without throwing it off altogether (at least not immediately anyway), whereas a badly wobbling smooth pulley (idler, water pump) WILL throw the belt off altogether much earlier.