air condition

My air condition does not work all the time. Last summer I noticed that it would not come on. When i turn it on it operates for about 30 seconds, and then I quess the thermostat cut it out and then it stays out. I have tried to measure the voltage at the compressor magnet and it was around 9 volt, obviously too low to engage the compressor magnet. Is there a relay I could check? Or could it be something else? I learned a trick when driving and It wouldn`t come on, rev the engine to 3-4000 rpm and then it engages.
The charging is ok, around 14 Volts at the battery. I also noticed last summer that the voltmeter in the dash also had a low reading 11-12 volts. That is also higher now in the winther.

89 limited
Torfinn

On the Right inner fender of your '89 XJ (I'm assuming gas not diesel) there is a group of relays under a plastic cover - in the US there'd be qty=4 relays; not sure how it goes over there -(if you need the O2 sensor heater relay - my guess would be that you do as well)...

... anyway - the A/C relay is the relay closest to the firewall (and the qty=2 diagnostic connectors) - just for reference counting from the front to the back (headlamp-to-firewall) the relays are: O2 heater, Fuel Pump Relay, Power Latch Relay, and then your A/C Relay...

The relays are all the same part number, so feel free to "swap" em around a bit.

They are fed power from the fuseable links connected to the stud terminal on the start relay (just in front of the relays we've been looking at there's a silver can-style relay that has a large stud-terminal -- that one!) if you're only seeing like 9V at the clutch you may wanna clean all of the contacts there on that stud (it acts as a primary distribution point for your battery power!)

Also, look for 12volts at the socket for the A/C relay while you have it out - if you don't have 12V there it'll be more than difficult to get 12V from there to the A/C Compressor clutch.

Any help?

Could be an electrical problem, but it sounds like low refrigerant. There is a low pessure switch in the system, to shut it down, when there is not enough refrigerant. It shuts it down in case of a small (or large) leak to save as much freon as possible and to keep the system from going into a vacume, on the low side, to prevent sucking air into the system. Reving up the motor, could generate enough pressure, for a short period, to let the system run for a short time. But as soon as the low side of the system gets near 0 pressure it will shut off the clutch again.

It is definitely an elektrical fault. I bypassed the thermostat and then it stayed on until the system froze and would not let any air through the ducts of the ac heating system.

I will check the relay and the voltage. Yes it is th 4.0 litre and i have 4 relays.

Thanks
Torfinn
89 Limited XJ
42 Ford GPW

It sounds like two problems...
Electrical and low/high freon. Low freon will cause the safety switch to shut down the system, also low may cause the freezing of the evap., coil.

I would start with getting the unit properly charged.
That may solve all you problems, but if not the thermostate on the plenum should be check.

You mentioned that high rpm will engage the system and make it work for a while....That smells like low freon...or my name is not Turd Furgler.