just_drive

89 S4. Assuming factory parts, does the AC clutch cycle on/off, or does it stay engaged as long as the HVAC/AC system is on? Preliminary speaking it seems that mine is turning as long as the AC button is "On" and the temp lever is "on". I should mention it was converted to R134 at some point by one of the many PO's. Thanks.

j.kenzie@sbcglobal.net

It stays on unless certain undesirable conditions occur, such as: your evaporator freezes up, which can occur because of freeze switch malfunction, high humidity, low evaporator air flow, or low refrigerant. There is a coolant sensor that shuts off a/c with over temp. Some models have a pressure switch that would turn off the compressor clutch for under or over pressures. Even modern cars often use this arrangement for humidity control.
Good luck,
Dave

worf928

There are - on S4+ - exactly two devices in between the ‘button’ in the HVAC head and the compressor clutch that are designed to interrupt the circuit:
- low pressure switch (the device on the bottom of the tube from the condenser.)
- anti-freeze switch

If your A/C functions as it should with sub-30°F vent temperatures then a functional anti-freeze switch will periodically interrupt the clutch circuit.

If your A/C functions poorly then the anti-freeze switch may never open.

If your A/C functions but ices the evaporator then the freeze switch is dead.

New freeze switches need to be check/calibrated. And they last 10 years +/- 5 years (it seems...)

just_drive

Thanks for the quick and clear answers. Fumbling towards diagnosing the AC issues....TBC

Michael Benno

FYI, I was able to remove and test my freeze switch on ice water and properly calibrate it. It was slightly out of adjustment.

new freeze switches are fairly inexpensive as well.