Air Con advice, AC Clutch not engaging - Alfa Romeo Forum
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(Post Link) post #1 of 8 Old 11-07-11 Thread Starter
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Help Air Con advice, AC Clutch not engaging


My AC isn't working, I've narrowed the fault down to the AC compressor clutch not engaging. And I think it might be the multilevel pressure switch, but thought I'd ping here to see if there is other things I should be looking at,

I've verified that the compressor / clutch isn't engaging by removing the wheel arch lining undertray and bumper to get full access to the compressor. When running you can see the belt free running round the comp, when you request AC from the centre console the clutch does not click and engage, or even attempt to.

Things I've checked:
  • R134a pressure ok ( Low Side 88 - 100 psi depending on temp, same as the Avensis sat beside it on the drive when not running, If the comp was running that should drop to 20-30ish) So it has gas and is comparable pressure to a working car.
  • Relays removed and tested on the bench all working.
  • Fuses visually inspected
  • Compressor turns by hand with expected resistance
  • Clutch works (Jumped 12v to it and it engages)

So it should engage unless the system thinks it shouldn't start, which points me towards the Pressure switch. All other faults i can think of the ac system would start and then switch off when it detected a fault which isn't happening.

Or a break in the wiring loom :-(

Before I fork out on a new switch, anybody have any idea's on what else I should check?

2004 Alfa GT 3.2 V6

Edit : My poor English... back to school for me...
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You could try firing the compressor manually, by running 12V from the battery to the connection on the pump.

Sometimes a bit of forced running makes the pressure in the switch pipe behave...
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(Post Link) post #3 of 8 Old 11-07-11 Thread Starter
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Update :

I jumped the compressor clutch whilst the car was running and I get nice cold air through the system, so fundamentally the system is ok and the fault is electrical.

As per Iain's message (Thanks you for the tip Iain), I'd hoped that this would 'kick' the switch as it is on the High side, but no joy. I removed and re seated the switch but no joy.

The pressure switch has a closed circuit between the middle two pins. according to
that is pin 1 & 2 request for Aircon and Enable, suggesting that it is in the right pressure range and the aircon should work. but the diagram on the link disagrees his post and with the pins I buzzed out. eLearn differs again saying request and enable are pins 5 and 2 (which counting the pins left to right would be the pins I would say are closed circuit)... but nothing on the connector is labelled.
If I remove the switch so pressure is ambient (low) the connection from the middle two pins goes open circuit so I think the switch is ok... :-/

I tried to buzz out the feed from the Relay to the connector to the Comp but it isn't connected directly. following the loom the line to the compressor goes via the ECU on the back of the pellum green wire with red stripe.. but there are two green with red stripe in the loom at the other end..... ugh...

So I go to check the manual to ensure I have the right relay & fuse.... and the manual is completely different! I presume it is for the other GT's not the v6.

the label under the cover shows the relay as T05 in slot 2, if the label is upside down that would be slot 5... I'm now completely baffled.

eLearn's javascript appears not to work so does not show any wiring diagrams so I can't even check the diagrams.

So i'm stuck and frustrated.
Any other tips?

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If I were you I would first do a regas on th system.

Your readings may not be correct and if your pressure is too high or too low the compressor will not engage.
You cannot compare the reading on two different systems, because they can use different pressures.

You should first rule out the Regas question before thinking there is a problem with the system.

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I'd go with the advise above, a static pressure test is not an accurate diagnostic method.

The refrigerant needs to be weighed in as capillary metering systems have a critical charge amount, once running a typical suction (low side) pressure should be around 40 Psig.
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(Post Link) post #6 of 8 Old 18-07-11 Thread Starter
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Thanks for your responses Alfa Seltzer and Alecto,

I've got my pressure low side gauge back so I'll measure the pressure whilst the compressor is 'jumped' tonight. When I jumped it last time the vents kicked out plenty of cold air, so I'd concluded that mechanically and gas wise the system was fine, particular as the Multi pressure switch had the return to the ECU as enable*, so the pressure was above 'low' and below 'high'. That should at least start the compressor when the a/c is requested which it doesn't.

* The multi pressure switch moves to the disable position when it is remove from the system so at ambient pressure.

I'll double check the pressure whilst running 'jumped', as you advise to confirm that the system is genuinely ok. but I'm fairly confident the problem is electrical (ughh)

Thank you for your suggestions,

Best regards,
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(Post Link) post #7 of 8 Old 28-07-11 Thread Starter
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So it was a break in the cable from the ECU to the Compressor.

Rather annoyingly the reason the cable out of the bottom of the Relay is different colour to the cable to the compressor is that the high current line from the relay goes back to the ECU then changes colour from The ECU to the Compressor... ugh.

I fitted a new run of cable from the relay to the compressor, now when I request AC from the cab, temp is low enough and the outside temp is high enough, the AC kicks in correctly. Yay.

I had to splice in the cable to the ECU so it did not fault.

The reason it goes to the ECU before the Compressor is so the ECU can fault if the AC fuse / relay fail, same it does not monitor the current as well as that would of been able to detect the break in the cable as well.

Any whoo.. all sorted and the wife is happy to in the car now it has AC.... ugh.

Cheers for your help guys,
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Crazed I have the same problem.

After i changed my body computer with ecu and AC not works, the clutch not enganging. Can you help me which colour of cable i have to change. And which and where must i to splace the cable from ecu to relay.

Last edited by Alfa147_105hps; 20-04-14 at 13:28. Reason: because of quote
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