Read the above threads again. I always suggest identifying the "On" signal before proceeding. If you cross the two low pressure terminals and there is no compressor then there are two possibilities at that connection. One is the 12v from the Klima is still open (PBC not doing job) or the voltage will be real low .2v or so and will go back to 12v if the A/C is turned off. If the voltage is pulled low when the A/C is turned on at the PBC then the problem is outside.
You can verify where the voltage comes from by pulling both wires from the low press switch. One will be 12v. Pull the Klima relay, it should then turn off the 12v. If it doesn't then the 12v is coming from the PBC and you have the other "On" signal
Bosch Master, ASE Master, L1
33 years MB technician