The ac clutch won't engage, I have tested the clutch with 12v battery and it is operating. I have power to the relay base and the fuse in the dash is good. Even when I select manual and ac on the dash control panel, the clutch does not come on. I have tried to add freon with my gauges and it accepted some, but the high side reading stays at zero. I have tried following the electric schematics but I lack full knowledge in this area. Any ideas of what I can do? It seems to me that whatever closes the circuit in the relay to send power to the clutch is not working.
Thanks, John

 

Would be helpful to know your vehicle year and engine. Most V12 have the Harrison A6 compressor; earlier versions have a fusible connection at the compressor clutch, and later versions have the clutch grounded through the HSLP (high side low pressure) switch on the rear of the compressor- this prevents clutch engagement if pressure in system is below about 25 psi.
Common that if the system has been converted to R134a that the Schrader valves are so deep that your gauge connectors cannot push the stem to allow pressure readings or charging. Usually you can charge by weight through the low side port only, stable 25-30 psi with the compressor engaged at 75f would be close to a full charge. High side usually will be 150-200.

 

it is a 94 xjs 2+2 with the 4.0 straight 6 and it is 134a. The schrader valves are fine, as I have had the gauges on and off them before. This problem abruptly occured with it going from blowing really cold ac to just blowing hot air. So I changed the expansion valve, which brings it to where it is now
John

 

I also did a vacuum pump down and it held the vacuum and when starting to refill the freon it took about 20 oz. Could it be the pressure sensor switch is not letting the relay for the ac clutch engage?

 

To "trigger" the compressor relay needs "+" voltage from the climate control ECU and a ground. The ground comes via the pressure switch.

The diagram shows a "dual pressure cutout switch". My guess is that it breaks the ground path if pressure goes higher than xxx-value or lower than yyy-value

Cheers
DD

 

great info Doug.Would the switch you mentioned be the dbc 11279 ac pressure / switch? I have yet to locate it's location in the engine bay, but it's down there somewhere.
John

 

Should be on the right side of the engine bay, on a metal A/C pipe, down low along the rail, about a third of the way back to the firewall. Screwed into the pipe from the top.
Jon

 

Yes

Item #3 in this illustration, located as mentioned on the AC pipe

https://parts.jaguarlandroverclassic...uMTkwNTk4NzAz/

Cheers
DD

 

thank you Doug and RGK, i will attack the problem this weekend and I will post progress and results.
John

 

I removed the ac clutch relay and with the engine running I put a jumper wire from pin 30 slot to pin 87 slot in the relay base. The ac clutch engaged so the compressor was turning, but still no pressure or freon flow. So I can only conclude the compressor is bad and that the ac pressure sensor/ switch is working as intended.
John

 

No change at all on your gauges?
I hope the compressor didn't grenade and throw junk thru your entire system.
As long as there is about 27-30 psi of refrigerant in the system, car not running, the switch should close and operate the clutch relay. The compressor does not need to be running for the switch to close. Maybe you have two issues.

Jon

 

There is 0 psi, so I will change the compressor and access from there. So the zero psi is probably from when I did a vacuum on the system, but now with the ac clutch hot wired it should try to take in some freon, but it is not.
John

 

Revacuum the system for 30 minutes. Turn off the vacuum pump and see if it holds the vacuum for several hours. If so, dump one can of refrigerant into the system without the car running. You can turn the can upside down so it goes in faster. Let it go into both the high and low sides. Once that can is empty, close the high side at the gauge set, and start the car. Turn the a/c on and there should be enough gas (about 27 psi) in the system to allow the clutch to operate. The high side should show an increase and the low a decrease on your gauge set. Let it suck in more refrigerant until you have put in the required weight. You ALWAYS need to start to put gas in when the system is in a vacuum.
Jon

 

Thank you Jon, that is good advice which I did not know

 

One further note of clarification, never open the high side valve when the engine is running, and you have a can of refrigerant hooked up. Only open the low side.
Jon