I'm doing an assessment on my AC - whether it is worth a try at resurrection. I've had the car for 9 months with no ac - the compressor is seized. It is the GM R4 version - says 'Harrison' on the label.

OK, I can deal with a compressor and risk a new receiver-drier, expansion valve, seals, and recharge gamble.

My question is whether my control system is working correctly. I monitor the voltage at the input (control relay) side of the pressure switch: it is approx. 14 volts dc at all positions of the temperature wheel and all selections with the pushbuttons but '0', where it goes to zero. I understand that the ac is supposed to be engaged for defrost as a drier function. But shouldn't this voltage (which controls the ac clutch) go off with repositioning of the temp wheel?

I'm OK with most mechanical and electrical issues, but this 1981 ACC system has me somewhat intimidated.

What should normal control voltage be to the pressure switch-clutch circuit?

Or is it 'can-of-worms' time?

__________________
Erlend
1981 300D 202K daily driver

I can't vouch for the W-123, but the clutch is normally controlled by the evap temp switch on the evaporator. The system will run at maximum potential until the evap temp gets down to a certain point............at which point it will shut the compresor down. The temperature wheel on the CCU has no effect on this loop unless the outside air temperature is colder than the cabin air temperature. The system then thinks that heat is required and the clutch will only engage on the defrost cycle (button #1).

You should see the compressor disengage if the CCU buttons #4 or #5 are depressed under all conditions.

It's not adviseable to rotate the temperature wheel significantly when the a/c is on. I've seen the system get "confused", especially if the ambient temperature is close to one of the temperature settings on the wheel.

I once had an interesting adventure with the W123. Temperature started in the high 80's and dropped all day until about 6:00. At that point, the ambient temp dropped below the setpoint on the CCU. The system went crazy and decided that the a/c was not required anymore and that it had better turn on the heat..........at maximum blower. Needless to say, I wasn't pleased. After shutting it down and restarting it the next day, it was like nothing happened.

There should be no voltage to the compressor in the "EC" or "Off" modes. If the compressor clutch is engaging in the "EC" mode, something is wrong.