A/C Compressor Cycling

[AUS-172]

Looking for some info on the AC compressor functions of a Ph2 172. For context, I've recently replaced the TX, dryer, pressure switch, condenser and compressor on a car I'm recommissioning after ~15 years off the road. System is gassed and lubed, no leaks, and blowing icey cold in the cabin. Functions on the climate control unit inside the car seem to be operating as per factory, too.

First question - Am I right in saying the compressor should cycle on and off in response to evap temps/pressures?

Second question - If my compressor is not cycling when set to cold (compressor clutch is engaged and running constantly), what are the likely culprits? Have tested the relay, cabin temp sensor and evaporator probe, and all seem to be functioning normally. Relay is definitely okay, and the other two are showing varied resistance when bench tested (assuming they combine to provide a signal for the ECU to shut off the compressor when cold enough?)


If anyone has any concrete info on how the AC/climate control system actually works, or any knowledge of known faults etc, I would be very grateful!

 

[AUS-172]

More info on this if anyone happens to be having the same issues.

- Yes, the compressor should cycle (clutch engage/disengage as a self-regulation).

- The climate control unit itself is what sends the signal to the ECU to cycle the compressor.

- The climate control unit receives two signals to help it make this decision. One from the interior temp sensor (in the dome light), and one from the thermistor which sits in the evaporator/mixer behind the dash. It can be accessed pretty easily from the passenger footwell. Part number 7701048391. Still available online - didn't bother checking with Renault Australia.

- From what I have been able to work out, the climate control unit uses the reading from both signals to determine when to shut the compressor off (i.e when the evaporator is too cold OR the temp in the car has reached the desired window). It's the same process when set to auto, and the climate control will raise the temp/run the heater etc to control what the climate control ECU is seeing from both signals.

- You can test both sensors for resistance change to temperatures. This is from the workshop diagnosis manual:

The good news is that in 99% of cases, your issue is likely going to be either one of these sensors providing a bad reading. The climate control modules themselves have been known to go bad, but it's far less likely.

Now the bad news? Both of my sensors tested to spec. I have used 2x spare climate control modules, and a thermistor from a known working car, and I'm still getting the same issue.

Will keep this thread updated when I find the problem!