After further evaluation, here is what I've found. When the car is off, the relay input from the battery has power and nothing else.

With the car on, and the air conditioner completely shut down, the battery line is live, the blue/white line is live and the red/black line is live. The only line that is not live is the one that goes to the compressor

With the ignition on and the air conditioner completely on (ac on, manual on, and blowers on, all the lines are live except the one going to the compressor.

So, basically nothing changes at the relay from when the air conditioner is on or off.

Any ideas?

 

Don't assume that the new relay is good.
My new relay for the auxiliary fan lasted 4 switches before it failed.
I either bought a dodgy relay (in Jaguar packaging) or there is an underlying fault that is causing an amperage spike to cause the relay to fail.
Or maybe I just bought a dog and it should be taken out and shot.

 

To summarise: Low current side:
The relay low current feed from the aircon switch is always live whether the aircon is being called for or not.

One thing at a time: The system may, repeat may, be triggered by an inverted relay system. That is by the relay stopping receiving current from the aircon switch, rather than by starting to receive it. Only a guess mind, someone who has the wiring diagrams or actual experience, should be able to say for sure. To test for this, start the car, call for aircon, and remove the wire that comes from the aircon controls to the relay. See if the clutch kicks in. If the compressor does start, then duff switch/something similar in the aircon controls.

Report back, but if this test does not result in the aircons compressor starting, please do the continuity test of the wire from the relay to the compressor I mentioned in the previous post.

Greg

 

One further thought: It might also be that the relay operates in the normal manner (ie earthing the switch circuit triggers the high current) BUT that the aircon control switch just earths the circuit when switched on (ie the system is live though to the aircon control switch which when turned on merely makes the path to earth. In other words, the problem is that the aircon switch's path to earth has failed.

Worth testing for this too: just take the relay terminal that is connected to the aircon switch, and earth it while engine running, and see if the compressor starts.

 

The fault was in the line going to the compressor from the relay. Traced wire, ran a new one, problem solved. Thanks for your help.

 

That's great, wish they all turned out to be that easy :-)

 

Not to hijack this thread but I have the same issue with my '95. What compressor is in it so I know what to google for a new compressor clutch? It's not the A6 compressor still is it?