Suddenly no ground to AC
 

Hi,

1992 300D.
Out of the blue my AC turned off at a stop light, then came on a minute later.
The next day it worked, but cut off after a few minutes of driving. Now it works for a minute then dies. Comes on for a few seconds while driving.
I performed the diagnostic from FSM.
There is no ground from the CCU (pushbutton unit) to pigtail on high pressure switch. I tested that with 3 different CCUs.
I replaced the evaporator sensor, no change.
What should I do next?
AC charge is good, checked. If I provide ground to high pressure switch, everything is cold.
Aux fans are operating good. Belt is not slipping.
Thanks

Does your tach work? OVP relay good? Next stop is the Klima relay.

Yes, it works well. OVP is new with two good fuses.

I don't understand how Klima relay would be affected, since the signal is interrupted before reaching that relay

Do you actually have 12V coming from the Klima relay? The CCU switches via a transistor, it requires current to flow. You'll get a better idea what's going on by measuring voltage with the wires still connected to the pressure switch. Measure to ground. If you have 12V present, the CCU isn't commanding the compressor on or you have a wire broken somewhere.

Another thing is that aux fans are on high setting (bypassing pre-resistor) when the compressor cuts out. Even when I have jumped the switch for fans, it runs on low setting, then compressor cuts out and fans go on high. So it seems CCU is reading high temperature out of sudden and decides to cut out the AC and cool down the system?
I took off the connector from temperature sensor and nothing happened. Maybe it is bad, giving incorrect signal? Engine temp about 80C, ambient 110F.

Yes 12V on the pigtail from Klima.

The fans on high is a big clue. The CCU is in charge of the high fan speed on these later cars. If the temperature goes high, the compressor is locked out and the fans are forced on high. The sensor in the head may be bad, or you could have a bad wire in the wiring harness. Has your harness been replaced, or is it still the biodegradable junk?

Old harness AFAIK.
Yes, I will measure sensor resistance on cold engine then place a resistor to the connector once engine is hot and AC is dead. That should kill the fans and turn on the AC if the sensor is faulty.
Update: replaced the coolant temp sensor and everything is fine. Looks like $6 chinese sensor lasts about a year.