'93 500 SEL air conditioning woes

I'm still having trouble with my air condtioning.

I started off with leaks and have replaced the evaporator core. The car does not lose coolant now.

I am having problems with the compressor turning off and on intermittently. I have replaced the sensors in the evaporator core and the one behind the insturment cluster. This did not help. I have not replaced the pressure sensor on the drier (I think it was there I'm going on memory.) I get a code for a bad sensor, but when you run the sensors on a scope they do not show anything wrong. One thing I did find is that if you disconnect the mono valve (or more correctly on this one the duo valve) the compressor starts working within 30 seconds. Leaving that disconected will not keep the compressor running.

Can anyone help with this?