This is on my 1997 Dodge Caravan.
About 3 weeks ago, whenever my A/C kicked on I heard the drive belt (V belt that only drives the compressor) squeal. I immediately think, aha loose belt, no prob.
I check the belt and it is tight, OK, it looks a little worn.
I get out the auto stethoscope and check all the bearings and find the serpentine belt tensioner bearing is not a happy camper but the compressor sounds fine while running and freewheeling both.
I pop down to the parts store and get a serpentine belt and an A/C belt and replace the bearing in the tensioner.
All seems well… Get on the freeway and the A/C kicks on, HUGE SQUEAL for 3 seconds and the car slows down, then the A/C is running and getting cool like nothing happened. :dubious:
The compressor starts cycling and every start is accompanied with a 1 second squeal and slowing of the car, but the system still cools. I try putting it in neutral and letting the engine idle while turning on the A/C compressor, no problem, all is fine.
It just doesn’t like being engaged at high RPMS.
First double check all the obvious stuff, Belt is not upside down or loose, it is the correct belt.
At home I try turning the compressor by hand, the front part (connected to the compressor shaft) turns easily, if I turn it fast enough, I can feel a little back pressure starting to build like it should.
I ran the ventilation computer through it’s calibration, it flipped all it’s vents, got cool and hot and all was happy.
Hung my A/C gauges on the system and the charge seemed fine, I didn’t check the temp to pressure ratio but it seemed normal enough, high side pressure when way up as I revved the engine however.
It is a 134a system.
At this point I am stumped, the system cools fine and compressor sounds OK internally but it doesn’t like being engaged at high RPMS, once engaged I can rev the engine back up without problems.
Change the compressor? expansion valve? Is there a pressure bypass inside the compressor to allow engagement under load, verify the charge more carefully with a thermometer on the expansion valve?
I have already Googled it to death and found several other people with the same problem, but no answers.
BTW, this is my first new thread, be gentle, I just wish it was about something more interesting and earthshaking.
MacGyver
Since it’s behaving in a way that it didn’t before you changed the belt, I doubt it’s the compressor. I’m guessing it has something to do with the installation of the belt itself such as improper tension or alignment.
Oops, I had already thought of making that clear, then left it out. It is the same problem as before I started, but with a nice new rubber surfaced belt making a much better grip and a MUCH louder squeal.
Nope, I watched it with the engine revved, it is the A/C belt skidding on the compressor pulley, it is taking far more torque to run it at high speed than normal. It slows the engine down tremendously, say 2000 to 1000 RPM under fixed throttle.
Actually, I do find this interesting - I just wish I had an answer for you. But it does bring up the question, probably relevant to your situation, what mechanism is used to handle the situation of the A/C compressor kicking on at high engine RPMs? It seems to me that the clutch would wear out very quickly under this condition. If I started up a car with a manual transmission by revving the engine high then letting out the clutch at the normal rate, I would get a lurch, squealing tires, and a quickly worn out clutch. Why doesn’t the same thing happen to the A/C clutch all the time?
It sounds like you’ve eliminated a binding compressor and a weak clutch. What are the high side pressures under various pertinent conditions (engaging at idle, engaging at cruise [2000-2500] rpm, running at idle, running at cruise)?
You sound like a pretty smart guy to me. What do you think about the surfaces of the pulleys being glazed from the belt being bad for so long? Kind of weak I know, but it sounds like you have done all of the obvious stuff. Maybe a quick rough up with some smooth emery cloth on the clutch and and the crank pulleys will get rid of the squeel. If that doesn’t do it changing the compressor and maybe the expansion valve seems like the only option. Of course, it’s also the most expensive one as well.
Yes!! Someone who understands the problem.
I work on non-mobile refrigeration systems all the time and they have nothing to allow for variable compressor output or even try to start under load, they have a time delay or pressure sensor and the compressor waits until the pressure difference drops before starting.
This was one of my next avenues of investigation, I will make a chart tonight.
I did notice that the high side pressure got quite high when I revved the engine, like over 500PSI which to my stationary system knowledge is very very high indeed.
Yes, one of my first thoughts, but the fact that the car slows down significantly when it is squealing the belt is telling me that the belt is not just slipping.
BTW I shot blast my pulley surfaces when I have them off because I like absolutely silent running cars. No squeaky squeaks while idling here.
Yikes! Normal high pressures are in the 140-240 range. At something higher than that (couldn’t find a spec, but would guess 350-450) the system should cut power to the clutch. A mechanical pressure relief valve is set to operate in the 500-600 range.
It may or may not relate to your symptom, but I’m inclined to think the A/C pressure transducer may be faulty. I also wonder about the radiator/condenser fans. If they don’t always run with the A/C on, they should kick in at maybe 250-350.
I could tell It was more detailed than that but when I asked one of the ac techs the first thing he said was replace everything. Typical response. When I asked him how to measure pressures under a load he said “With a dyno, dumbass!”. Parts and service have that kind of tight relationship at the dealership.
I had been looking at several wrong scales on the new gauges, darn N/cm2 and deg C for 409a
Got some Pressure data.
Idle, AC off, 100 PSI High and low sides.
Turn on AC, no squeaks, high side goes up to 250 PSI, fan kicks on, pressure drops to 200 and holds. I can rev the motor with the AC running and the pressure varies only by 10 PSI.
2000 RPM, AC off, 120 PSI both sides (hot now).
Turn on AC, HUGE SQUEAL, high side goes up to 270, fan kicks on, drops pressure to 220
As a test I opened the gauge valves from high to low sides, so they would bypass any restrictions dumping the hot gas directly into the low side.
2000 RPM, AC off, Bypass open,
Turn on AC, still huge squeal, high side pressure only gets up to 150 or so.
