The first decision to be made is whether the control is being stopped by the engine system or the AC system.

Inside the car you make a decison for AC by not hitting the EC button. Every other position has AC ON! This decision proceeds through the low/high pressure switch to the Klima relay. Electronically the decision is effected by pulling the signal from the Klima to ground. The only exception to this is when the AC gets to icing temp it is shut-off. If the problem is inside control this is the first decision to check.

So lets look at the signal. Lets do it at the low/high pressure switch, since it is just so easy. We will start with the EC position. At either terminal of the LHP switch (low/high pressure) one will see battery voltage. If the voltage is different on either side of the LHP switch the system is out of refrigerant.

Now press any other button but EC and the voltage at the LHP switch should drop to around 1v or close to zero. IF THIS DOES NOT occur, your problem is inside the car. IF THIS DOES occur then the problem is in the Klima engine control system. This system replaces dumb relays in earlier systems and allows the motor's needs to supperceed the occupants needs. To protect the engine against belt failure, overheating, low voltage, and high or low speed the request for the compressor is over ridden.

First chore is to make this decision. Inside or outside.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician