shredder110

I have an 1987 944S. My A/C clutch is cycling on and off when cruising along. I put a pressure gauge on the high side and at idle I was showing 220psi but when I revved the engine up to 2000 rpm the a/c pressure went up over 400psi. Any ideas...too much freon (R-12)....clogged expansion valve...any other ideas?

Bri Bro

If the expansion valve is plugged, the low side will be way low, even at vacuum. Too much freon will also cause high preasure, but 400lbs worth...sounds strange.

shredder110

Thanks. I did not check the low side. I will do that today.

951Tom

Insufficient condensor cooling will skyrocket the high-side pressure. Check that both fans are running ok & your condensor isn't clogged with leaves or something.

shredder110

the condensor looks clean/no clogs. I am starting to run out of ideas

Bri Bro

And the low side preasure is at xxx when the high side is at 400PSI?

shredder110

Ok I went uder the car. Both fans work and the condensor is clear. I hooked up a gauge to the low side. Starting pressure was at 50psi. Turned on a/c pressure dropped 10psi then the clutch kicks off. Pressure increased back to 50psi clutch back on,..etc. On the high side the pressure increased up to 400 psi +. Help, Any ideas out there?

Charlotte944

First, the clutch is cycling because the suction side pressure is dropping below the set point for the low pressure protection switch. If memory serves, this is around 20 psi.

Second, the high side pressure of 400 psi is WAY to much. Either the high side is restricted or the thermal expansion valve has failed.

Your high side pressures should be more in the 200 to 300 psi range, depending on outside (ambient) air temperature and heat loading from the passenger compartment.

You low side pressures should be in the 20 to 30 psi range and will be a function of the flow rate through the thermal expansion valve and the temperature of the evaporator.

As the passenger compartment cools off, the TXV will close down until the system reaches an equilibrium with the existing heat load.

Once the cabin temperature matches the climate control setting, the compressor clutch is disengaged. Once the cabin temperature reaches an arbirtary upper limit, the climate control engages the compressor clutch, and cooling resumes.

During this process, NONE of the protection devices (low pressure switch, icing switch, high pressure switch), have activated.

In your case, the low pressure swtich is controlling the compressor, which is an obvious indication that there is a problem.

shredder110

I guess I need to make myself more clear. On the low side the pressure dropped from 50 psi to 40 psi a total of a 10 psi drop then the clutch kicked off (temp control switch inside car full cold 80 degrees outside temp)then pressure built up to 50 psi clutch back on etc.... High side pressure above 400psi... From what you said it sounds like my thermal expansion valve is bad. Is that right?