MBMB

Help me with a diagnosis, please. For the third time my A/C belt has jumped off the compressor pulley.

The first two times a trusted mechanic had installed the belt. The third time I had.

It was a new belt, properly tensioned, within about 100 miles of installation, with all fasteners properly tightened.

The engine was fully warmed up.

The belt may have jumped when I switched the compressor on. I had been driving with A/C earlier in the day, and the A/C blows cold.

Thanks.

Mrmerlin

have you checked that the pulleys are all straight?
you may have a bad compressor

Tom. M

I vote bad compressor. Went through two clutches and many thrown belts before I cleaned up the whole system, rebuilt compressor, new lines, new exp. valve, new condenser, etc....and fresh R12 charge.... Worked great over the hot trip to the Wichita 928sRus event.. and still going strong....

69gaugeman

Also that the tension is good. Loose belt can jump.

Check if the AC has been hit by putting a straight edge on it and see if it is parallel to the belt.

the flyin' scotsman

is it the correct belt?

CraigL

If the compressor was bad (or seized), the belt would squeal and smoke.
Something is probably wrong with the alignment of the pulleys.
Maybe a photo would help.

Mrmerlin

if the compressor is bad it could be seemingly frozen when the A/C is turned on this could cause the belt to jump off

MBMB

It is the correct belt, but not the factory spec. It's 25–50mm shorter (I don't recall offhand) because the factory spec belt required the compressor to be adjusted almost to the outside of its range of travel.

The compressor is a recently rebuilt unit. I will put it up on the lift, put a straightedge on it, and snap a pic.

Tom. M

I seem to recall some discussion on the diameter of the a/c pulleys.....maybe you are spinning it too fast? causing it to seize on occasion...and then when it cools, it works again?... make sure the crank and a/c compressor pulleys are the correct size so the compressor will spin at the proper speed..

MBMB

Crank and compressor pulleys are stock.

Alan

Did you convert to R134a? do both of your cooling fans work OK?

Alan

the flyin' scotsman

thats quite a bit shorter than stock IIRC its 1080mm

I dont recall which year the a/c changed to 134a and if that change required a different pulley/belt dimension. Ill open PET and brb.

Mrmerlin

If the system was converted maybe the oils got mixed and thus could be the compressor isnt liking this maybe the system has too much oil.

IIRC Bill Ball had this same issue with the belt jumping off,
after swapping a compressor the issue was resolved

Alan

if you have R134a and if even one cooling fan is out the system can overpressure enough to stall the compressor and scorch or cause the belt to hop.

Stock R134a system have a low and high pressure switch (combined) to dissengage the compressor clutch. Converted system usually do not...

Alan

MBMB

Interesting. Both fans are running, but the RH fan is shrieking like a harpy (I think Roger has a new one winging its way to me).

I got under the car with a straightedge, and the pulleys seem to line up near as dammit.

I converted to R134a a few years ago, and thoroughly vacuumed out at the time. The compressor was rebuilt more recently for R134a (because of leaks).

I have a spare compressor that I can rebuild and install.