240 AC belt jumps off

Check the belt tension and alignment of the A/C compressor pulley. There is an adjustment to get it into plane with the main pulley. If that is not done correctly, the belt twists and quickly fails or can fall off.

Likely your accessory mount bushings are shot, allowing the pulleys to twist out of alignment. Fortunately they're cheap and fairly easy to replace with only basic hand tools.

I spent some time last night inspecting the AC compressor. The pulley is in perfect alignment with the crankshaft pulley. Even with a bar pulling on it, I could not detect any movement or deflection of the lower mounts where the bushings. This is not a surprise, though, because the unit is secured by two bolts on the top, one in the front adjustment slot, and another one in the back. I still intend to remove/ replace the bushings, but I'm doubtful that it will make a difference.

Is the harmonic balancer on the crankshaft splitting? Look at the pulleys with the engine idling and the AC on, are they both running straight and true, or is there a wobble?

Can't say I've ever had a problem with belts jumping off these.

Here's an update on what's been done lately.

Did a complete inspection of the crankshaft and AC pulleys, and they are both in excellent condition. No evidence of splitting, warping or being out of round. After installing another new belt - a cogged Continental - a visual inspection with the engine running showed the AC belt and crank pulleys in perfect alignment, with no deflection/ movement of the AC compressor. BUT - I also saw that the PS belt looked funky. The PS belt is driven by the dual groove pulley on the front of the AC. Somehow, the PS pump has shifted, so that its input pulley was out of alignment both horizontally (as seen when viewed from the side) and vertically (as viewed from above).

The quick and dirty solution was to take three washers, remove a bit of each so they could slide over the bottom mounting bolt (which acts as a pivot) of the PS pump, and insert them as shims to adjust the pump's location. This brought the PS and AC pulleys into near perfect alignmant, as viewed from the top. When viewed from side, they are close enough for now.

So far, so good. After a number of test drives, the AC can be used w/o losing the belt. My best guess is that when the AC was running and under load, once in a while the PS belt would whip a little bit when making a turn and just catch the edge of the AC belt, causing it to jump off. At least, I hope so - and that this problem is solved.

It could be that the accessory bushings are worn/loose/shrunk and need replacing. The Power steering, alternator, and A/C compressor are all mounted on the same type bushing that isolate those components from the block. They fit into the lower mounting boss on the part that is bolted to the block, and the lower pivot bolts go through them, iirc. I found the ones on my Mom's alternator so worn that when we drove it home after purchase the alternator pulley had worn the adjustment bolt and block over 50% through! __ __ Randy & \ \/ /alerie's \__/olvos '90 245 Estate - '93 965 Estate "Shelby" & "Kate"