Volvo Repair: AC compressor dont start, volvo 940, engine cold

Question
Hi!
Volvo 940 -94. In hot weather condition =over 26 centigrades the AC compressor does not activate if engine is hot. Works OK if I start with engine cold, but if I leave the car for half an hour and then restarts it, the AC is not cooling. The control unit is perfectly OK. The problem is inside the compressor. There seems to be a clutch or valve operated by a 12 signal from the controler (red/green wire). Normally you hear a little noise from compressor when the 12V is applyed to compressor, and you also see/hear that it starts working. However, at the above mentioned case there is no sign of any reaction in the compressor as the 12V comes on. I have checked with an instrument that the 12V is OK. Also checked for bad/broken wire, but as there is a curren going throug the wire i assumes that the elecrical parts inside is OK. How is the compressor activated electromechanical clutch? Valve? Or do you have ay other ideas?
/LarsOlof, Sweden

Answer
the compressor has an elctromagnetic clutch.when it get 12 volts it creates a magnetic field that pulls the outer part of the clutch inward to contact the pulley.I would check for 12 volts at the compressor wire right at the clutch if it has 12 volts the clutch is bad.if no power it could be a problem from the cotrol panel they have a problem with bad solder joint inside.