Aircon issue - Astra J

AstraJ · Jul 12, 2023

The air conditioning on my Vauxhall Astra J has stopped working and I'm running out of ideas on how to fix it.
Steps so far:
The compressor clutch is not engaging.
I had the AC system re-gassed and it's not leaking.
The compressor seems free to spin, so not jammed.
There are two cables to the compressor. One has ~0.4V. Other has 0V (power to compressor clutch).
I replaced the pressure switch on the condenser, but no change. And my local garage tells me the pressure switch is reading the correct pressure (around 7 bar), apparently they can see that in diagnostics, but they said I'd need to take it to Vauxhall for a full diagnosis.
I've checked fuses and relays - all seem OK.
I then took the car in to a Vauxhall garage for them to take a look at. They did their diagnostics checks and agreed with it being an issue with power supply to the compressor clutch, but they couldn't determine what exactly the issue is without doing full wiring checks. They wanted ~£100 per hour to do the wiring checks, but couldn't give an estimate of time (they said likely more than 3 hours, but probably a full day's work, or more). I didn't want to throw an unpredictable (but likely very large) amount of money at them, so took the car home.
Someone suggested that there is a relay for the compressor clutch on the printed circuit board of the UEC, so I replaced that (£25 second hand). Unfortunately that still didn't solve it.
Any advice on what next, and how best to trace the electrical issue?

plugsandsparks · Jul 12, 2023

Ideally you need a wiring diagram for that model of vehicle, this will at least indicate what components are in play. There are safety controls to only bring in the clutch when certain conditions are met, pressure being in the correct zone is just one. If you are confident that the a/c has gas , then, if it were me... lol - I would switch on the a/c , disconnect the plug to the clutch and put 12 V directly on the clutch. Hopefully the clutch will engage, gas pressurised and cold air starts to come out of vents. As soon as i detected cold air i would remove the 12 V to the clutch as you dont want to seize the a/c pump (in case there actually is an issue).
If that works, then you need to work back using the wiring diagram to the next component, i.e. a relay and check that the signal to the relay is actually arriving at the relay, if it is and the relay has not switched, its the relay, if no signal reaches the relay you need to go to the next component following the diagram, if you end up at the controller, it becomes more difficult as the controller will be taking multiple inputs which all must be satisfied before it will engage the signal to the clutch (the controller is handling the safety issues).
Good Luck

brianmoooore · Jul 13, 2023

plugsandsparks's answer above is the way to approach it, but "the controller will be taking multiple inputs which all must be satisfied before it will engage the signal to the clutch " will be where the diagnosis gets difficult. Modern cars are so 'integrated', that a problem in something, at first sight, totally irrelevant can cause problems in a completely unrelated area.
Reminds me of a Merc that I was asked to look at some years ago some years ago. Several had tried and several had failed, so I declined to get involved, but the cause of the problem - very poor throttle response, turned out to be a slightly poor connection on a brake lamp, which for some reason, the car's electronics interpreted as trying to accelerate while braking at the same time.
Don't discount anything. Just guessing, but it could even be something like a door switch or window position sensor, with the aircon being programmed to work only when the passenger compartment is closed.