I have a 1992 full size blazer. It has a 5.2 liter fuel injection engine in it (stock). It runs good but whenever i have the ac or the defoster on the belt pops off. As you can see in the picture it pops off number 22. What is causeing this and how can i fix it?

 

Sounds like your A/C compressor is seized up. The compressor will run in both A/C mode and in defrost.

 

let me restate what i said, the ac and defroster work, but after sometime of being onn, not long, the belt pops off. They do work though.

 

It could be getting really close to siezing up it may expand a little when heated and jam. Like swart said the compressor runs in a/c,defrost,and even like the foot and head option near a/c i believe. With the engine off can u spin the outer plate of the a/c compressor fairly easily,or is it tough?

 

The idler pulley might be going, should show wear marks on them to see.

 

Your belt tensioner may be worn out... does the belt move alot? It will especially do this when the compressor is on.
A 5.2 liter (318cid)? I'd be willing to bet there's a 5.7 liter under your hood (350cid)...

 

heyy ya sory about that it is a 5.7 liter, ok so here it is. i don't know what you mean about if i can move the ac, but what i do know is that with the truck running without the ac on, the belt is fine with no movement, when i trun the ac on, the belt moves at least half way off the idler pulley but then goes back then does it agian untill it finally falls off. that is the only spot it does move like that. so please help.

 

I would change the tensioner. Sounds like it is cocking over, probably due to being worn out.

 

X2

I didn't have this problem but on the same engine in a truck I've got everytime the A/C was on I had a rumble. Thought it had to be the compressor but I had cold air. Turned out to be a bad tensionor pulley. Only happened when the truck was in gear and the compressor clutch locked up.

Pretty easy to check out, pull the belt off and spin the tensionor pulley by hand. You'll know if it's bad.