I have a 4.6 with 88000 miles that was squealing under the hood. I pulled the tension off the belt and found a little movement back and forth on the alternator, nothing on the other idlers and pulleys. I replaced the alternator and still had the squeal. I replaced the belt and the squeal stopped. Now when I rev the engine up, the belt jumps one notch on the crank pulley. I just replaced the tensioner/pulley and it still jumps. I had no problems with this until I replace the alternator. The belt does not jump at idle. Any ideas?

 

Your Altenator could still be loose. I have seen this happen before. Getting tension just right is not always an exact science.

 

Checked alternator, it's tight

 

The new belt could be an issue too...

 

Get a cheap laser alignment tool. Worth its weight in gold.

 

I am new to your forum! Love it! I have a 2008 V6 Mustang of course, and have a problem you guy"s or gals can help me with, I live in Iowa, my car sat for a weak due to snow storm it had a weak battery. I replaced with brand new one, driving down the street my car started to stutter and miss, no power at gas pedal it stalled! Not sure what to go from here! I have some talent working on cars but havn"t worked on them lately. I can be hands on if needed, I have had my car for 3yrs no problems. Any Advice please!!!

 

You can use string to align the pulleys too. Either the tensioner is crap, the belt is bad, alt is loose or out of alignment.

 

Are you sure you have the belt routed correctly?

 

Belt is routed correctly, new tensioner, all pullys are good and straight, alternator is good and tight. I put the old belt back on and stays on perfect, it's just that it is squeaking.
Thanks

 

When it jumps the crank pulley the belt will get damaged by the flange on the crank & it will jump again. Change the belt again.

Make sure you have the correct belt. The parts store almost gave me a 5 rib instead of 6 rib replacement.

Mine jumped from oil getting on belt from leaking SC seal.