240 AC belt jumps off

I have a 1993 240 that can't keep the AC belt in the pulleys when
using the AC. I installed a new belt a month ago when I thought the
old one had broken (it was just gone!). It worked well with no AC on,
but it popped off on the way home on a hot day. I reinstalled it,
making extra sure it had the proper tension, but it jumped off again.
The AC compressor sounds good and works fine - no indication it is
seizing up. Has anyone else had this issue, or have any suggestions?
Thanks.

 

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"