a/c belt slipping

[Blue Haze]

I have tightened my belt about as tight as possible yet at low speed the belt slips.  At crusing speeds it slips little to none.  I converted to R12 some time ago.  I do not believe the system is over charged if anything undercharged.  Could the compressor be going bad?  Any thing else I need to check?  This is on an 81 Caddy diesel.   

[Quantum TD]

Rusted or pitted belts will eat thru the sides of the belt, causing them to wobble, and then wear even more.

Take a look at your pulleys. If they're pitted, you'll need to find replacements.

[BigVWman]

Check to make sure its the correct size belt! If you buy it at the local parts store they round off size(example would be correct size for a certain belt being a 9.5x850mm they stock 860 but not 850 so they sell you the 860 and tell you it will fit) and just get you close sometimes half your adjustment is already gone before you even put it on.

[Quantum TD]

Yeah. That's true. And, for MK1 cars with AC, the OEM belt is too big with from the start. It calls for 10x 643. But a 10x 635 gives more adjustment room.

[Blue Haze]

Thanks guys gives me something to think about I think the belt that is on there is a 13 x 960

[JT Turbo]

Make sure you haven't a lost a pivot/mouting bolt at the bottom - large Alan head bolts.