A/C compressor not engaging

I wanted to get the a/c back for the summer holiday,
Managed to free up the blend flap motor to make them blow not only hot air

System has never been converted to r134a but is still freon. It has been empty since I bought the car in 2020. So I bought hoses and found a way to pressurise the system with air (up to 70psi) for testing purposes before filling it up with way too expensive ac gas cans.

This is what I get so far:

Clutch engages when jumped (12v on the wire going to the clutch)

At the relay:
12v between 30 and 87 (all time)
No 12v between 30 and 86 ignition on and airco on
Open line between 85 and ground , however at the pressure switch on the compressor I measure 4,5 ohm (when the system pressurised with compressed air)

0,95v between 85 and 86 engine running and a/c on ( 450mV with ignition off)

I read this could be a wiring problem

But where should I connect these two (blueyellow and black pink) wire to make it running?
Or is there anything I’m overlooking.

Many thanks to any suggestion.

Frederic …

Please tell me that you installed a new receiver/dryer and evacuated the system with a pump and it held a deep vacuum (preferably overnight) before you filled it with refrigerant … and what kind did you use?

Tanks for your reply. For now the system is pressurised with air to 60-70 psi for testing purposes. I have a new dryer and will vacuum the system once I get the system running.
It will be refilled with a duracool like gas (mixture of refrigerant propane and isobutane)

Fredric …

Normally you test the system by pulling a deep vacuum then turning off the pump The longer you now let the system sit before you continue (I prefer overnight) the better the test to see if it holds the vacuum which indicates the system is leak free. Only then install your new receiver/dryer. Now pull a deep vacuum again to boil off any residual moisture before you charge the system with the refrigerant. VERY IMPORTANT … Don’t skip putting in a new receiver/dryer, the old one will be saturated with moisture and they’re fairly inexpensive.

By pressurizing the system with air all you’re doing is introducing lots of moisture that will need to be removed. Remember moisture is the arch enemy of the inside of an A/C system. If you really feel you need to pressurize the system to check for leaks use nitrogen.


I understand your point but the system in my car is probably being empty for years now.
I wanted to test to see if I could make the compressor running therefore I pressurised with air to see if the pressure switch still works and if the clutch work.
After I made sure the system woks I will do as you suggested: vacuum and new dryer, oil, new o rings and gas with leak detector.

But for now the compressor is not engaging when there is a demand from the climate control.
Anyone have an idea what could cause this?

Frederic …

First thing to do is unplug the electrical connection to the compressor and make sure it’s getting 12v when you turn on the fan speed selector.

A gotcha on my '89 system is that if you have the climate controller “econ” button selected the compressor will never turn on. Other than that whenever you turn on the fans REGARDLESS of the temperature selected the compressor ALWAYS runs.

Have you had prior experience in charging an a/c system ? Regardless here’s an excellent video from “Eric the car guy” (I love his videos) on the subject …

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Indeed very helpful video I mainly did domestic a/c this will be the first time on a car.
Will try to see if the setting wasn’t put on econ and try to measure voltage on the clutch.
For now I’m on holiday, wanted to go with the jag but we took the daily because my dear wife insisted on the a/c.
Will dive into it when I’m back.

Looking into the future:

Mike …

Hopefully in the distant future and you have the right destination. Thanks for a great laugh and making my day :innocent:

Well, behind the facade of humor lies a great truth- you, sir, are the patron saint of the Delanair a/c systems in our XJ6 (XJ40)! And for that we are grateful!

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Thanks guys I will get there but first a need to sort a bad cold start problem.
This car runs on LPG and I noticed it was having trouble running on gas and not on petrol, now it sometimes doesn’t want to start at all.

I ordered new sparkplugs (NGK)old ones were 20000km also leads distributor and rotor. I’ll see what it gives.

Frederic, first, welcome back from your vacation. This thread started out addressing your A/C issue, and it is best to srart a new thread for an unrelated topic, but here goes…no/sluggish start problens can be thorny to sort out, as evidenced by this thread (warning: long read, but very educational):

1994 Jaguar VDP No Spark - #186 by Mike_Stone?

Spoiler alert: the problem was a worn / oxidized ignition on relay that would pass the requisite voltage but not the requisite current. If you haven’t already done so you might try the relay jumper test described towards the end of the previously cited thread to see if the relay is duff.

Thans for the the tread on the non start issue.
Have to tackle this first. So I will start a new topic as you suggested.
I will reopen this ac tread when the hot season starts next year