I have the factory AC on my '77 F250 460 Supercab w/Auto trans. Just off idle, the belt "dances" between the compressor pulley and the crank pulley when the compresser is running. This makes a very annoying noise and vibration. I've tried tightening the belt and tighter seems a little better but I'm afraid of damaging the compressor or water pump bearings from excessive tension. Otherwise the AC works fine. Anyone else have this issue? The twin cylinder York compressor on my '64 Ford Galaxie never had this problem, I recall it had a much more massive flywheel on the compressor.

 

Make sure that the pulleys are lined up properly using a yard stick, thats where I would start. If they are, I would be inclined to think that something is wrong mechanically with the A/C compressor that is causing it to act like that way. Dont know if you might have replaced belts recently either, but make sure that it is the right size, mainly width if you did. Just a start...

 

The belt alignment is fine and the belt is new. The vibration only happens in the 900-1200 rpm range, above and below that it's acceptable. Seems like a standing wave gets set up on the belt from the compressor torque pulses. Maybe this is somewhat inherent with compressor with only two pistons. Probably partly why the newer designs are rotary or have more pistons. The high and low pressures are normal.

 

I had the same problem with my 79 351m. Double check your refrigerant charge. If it is overcharged any at all, it will pulse more. I recharged mine and it is much better now.

 

Mine moves around and makes a vibrations at around 550 rpm's, like when the A/C is on and it's in gear idling at a red light. I've gotten to where I just flick it to vent when at a redlight and then back to A/C when I take off. Cause I haven't gotten around to installing the idle control solenoid to bump up the idle when the A/C is on.

 

Mines the same as jsutton. I think I need to adjust my idle solenoid.