Nissan Repair: A/C Compressor Shuts Down, direct correlation, gradiant

Question
QUESTION: I have a really odd A/C problem on a '96 Sentra.  It blows cold until the engine reaches running temperature and then the compressor shuts down/stops engaging.  It is definitely temperature related because I can throw the heater on to drop the temp by a few degrees and the compressor engages again for a short period.

ANSWER: Rick,

I would check the system pressures since it seems to be related to temperature.  My bet is the expansion valve is sticking if the pressures are within reason.  After the car heats up the pressure will go over maximum and the expansion valve would be the cause and then the pressure switch shuts down the system.  Warm it up and the expansion valve opens up and the pressure switch allows the compressor to apply.  You are probably going to be replacing the expansion valve and possibly the pressure switch.

---------- FOLLOW-UP ----------

QUESTION: I understand what you are pointing out but I don’t see the direct correlation to engine/water temp that I am seeing.  The shut-down of the compressor is directly related to the water temp as observed by throwing the heater on.  That does not sound like an A/C problem but an electrical/computer signal to the compressor on when to engage.

Answer
Rick,

I understand your point but refrigerant has a temperature to pressure gradiant that is basically equal.  That is, for every degree the temperature goes up the system pressure goes up a PSI or so.  You say the compressor kicks off but not the fan.  That tells me the relay for the clutch is closed so there are only the two places I mentioned that have the potential for an open circuit.  Now, if your Nissan was old enough they used a different way to make sure the compressor was applying.  There was a hall effect switch on the compressor that sensed when the unit was applied and spinning.  Get a test light and just start backprobing until you find where the power stops.