4.6is Throwing A/C Belt
 

The air conditioning belt on my 4.6 keeps skipping forward a rib (or two) on the crank pulley. This causes it to rub against the other belt, one time pushing it into the fan and destroying it (making for a fun night on the side of the highway). I replaced the tensioner, thinking that perhaps the roller had tapered, and was working the belt forward, but a few days later it had moved forward on the crank pulley again. So I'm guessing it's got to be a problem with the A/C compressor or the crank pulley being out of alignment. The odd thing is that the first 4 months that I had the car, there was no issue, so something's changed recently.
Has anybody had this issue before?

seems that something mechanical is out of whack... was the belt OE or an aftermarket?

The first belt was OE. The current one is Continental.

continental is OEM, if I am not mistaken... hopefully should be as good as OE...

I do not have an answer from past experience but based on your symptoms, i would suspect that one of the pulleys is out of alignment, just as you suspected. the rotating axle could be tilted just a bit, and as a result, letting the belt slide off...

Lets see if anybody else will chime in to this thread...

I wanted to update this thread with a little new info.
The A/C belt jumped forward two more times shortly after my last post, but hasn't done it since then. Possibly because it's been above 30º out. I believe there is a problem with the crank pulley, but the belt doesn't seem to jump in warmer weather.

I'm having the same problem. Did it to me on a road trip 500 miles from home. Think I've narrowed it down to a bad harmonic dampener. I can see the crank pulley wobble a bit.

Was the water pump replaced? If so, the crank pulley comes off so first check to see if all of the bolts are torqued properly.