Chevrolet Repair: 93 Chevy K Blazer belt problem, proper tension, ac compressor

Question
Van: The serpentine belt on my 1993 K Blazer (350,4X4)slips off when I turn the AC on. I have replaced the belt and the automatic belt tensioner, but the problem persists. The belt seems to track fine, with proper tension, until the AC compressor switches on. Then it immediately starts to run loose over the guide pully (the one that does not drive any equipment and has no channel on the rim) just below the compressor. The belt usually then jumps off that pulley and I have to stop the engine and reset the belt. My AC compressor is old and makes some noise but seems to work OK. Would a new compressor fix this? Any ideas?   Ron

Answer
Hi Ron,
I think I would attach some a/c gauges, and see what kind of pressure it is pumping. It must be very high to strech the belt more than the tensioner can take care of.
Check the diagram, and make sure the belt is routed correctly.
If it is, and the pressure is normal, a compressor may be in order.
With the engine off, try to turn the shaft of the compressor by hand. It should turn rather easily.
Van