MK 3 C5 airconditioning not working ...help

[crazyfach]

Hi all, I am in need of some help, I have a MK3 58 plate C5 2.0 HDI, my air conditioning stopped working so thinking it may require a top up I had it done and the garage confirmed there was no leaks evident, however even though the light on the switch is on, the compressor is not kicking in.... any ideas out there.... please help its getting Hot.

[Superaquarama]

Hmm, as far as I know this doesn't have to be reset via an ECU, when ours was regassed after a new condenser it just got cold straight away. Can you hotwire the compressor and see if it gets cold ? Only don't leave it permanently on or you may blow it ! Otherwise maybe a faulty thermostat ? Try bypassing it but no idea where it's located.

[RichardW]

No point in doing anything except getting it on a diag computer - it will all be ECU controlled, so if the ECU can't see the pressure or it's too high then it will not run it. If nothing shows up on the computer, then mechanical intervention may be required....

[Mandrake]

I don't know if the C5 is the same, but on the Xantia the pressure switch connector at the top of the dryer has four wires, two thin ones and two thick ones. One pair (I forget if its the thin pair or the thick pair) is a pressure switch which closes when there is sufficient pressure in the system for the compressor to run. If the system is too low in pressure these contacts will open and the compressor clutch will never be engaged regardless of what the dashboard button and light may indicate. This is to prevent the compressor being operated with insufficient pressure.

The other pair of wires is another pressure switch which closes when the pressure is too HIGH, which could occur on a really hot day if the system was being over worked, these contacts switch the radiator fans to high speed mode.

To test if its the pressure switch preventing it working you can temporarily bridge the pair of wires for the low pressure switch (I think its the thin wires but someone will have to confirm that from a wiring diagram) and the compressor clutch should engage after a couple of seconds if you have the air con turned on. If it does you either have low pressure, or the pressure switch is faulty. If it doesn't come on the problem is elsewhere.

Faulty pressure switches can happen - the air con in my previous Xantia very rarely if ever worked when I first got it, I found that when I bridged the two wires it seemed to run and cool very efficiently, on a hunch rather than getting the system re-gassed I obtained a second hand replacement pressure switch and fitted it (which can be done without losing gas pressure) and presto the problem was fixed, continuing to work perfectly for the 4 years I kept the car.

Of course the C5 might have a completely different pressure switch control system so you'd want to double check first. (I'd look it up but my computer with Sedre on it has died)

[RichardW]

Think it's probably a bit more sophiscated on the later cars Simon.... On the C4 Picasso, the rad fans doesn't run all the time, so the ECU must be monitoring the refrigerant pressure or temperature, and only puts the fan on when it goes high. Shaves a few mg off the CO2 figure on the test I guess

[Mandrake]

You could be right Richard, somehow I missed where it said Mk 3 C5, in my mind I had the idea that it was an early C5 which I think does have the same system as a Xantia, but a Mk 3 is highly unlikely