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Here once again...

Aircon is non-functioning. Last time it was functioning for sure was spring/summer 2010. First up, gas pressure was low, but once gassed, it maintains the pressure.

Compressor is not running. The control unit indicates function. Blowers work etc.
So, it is now a case of determining where the problem lies, and what problem?
Presumably, there is not much difference between UK and EU spec cars. Is there much difference between early 3.6 engined cars, and later 3.6/4.2engined cars? Where are all of the various devices located on a 1990 car?
I have the workshop microfiche to start working through.
Will the early car provide fault codes?

Hi Mark,
if there´s enough cooling gas in the system, it´s probably the low-pressure-switch, placed near the air intake of the A/C system in the thicker tube. If the switch don´t work, the clutch of the compressor doesn´t work. Or loss of cooling gas.
Greets, Georg

there is a switch under the plastic rain shroud in front of the windscreen. part of it's markings include 20PS.
It shows infinite resistance measured with a meter, with the system gassed. But shorting the contacts or wires makes no change.
But it's a start...

It´s a start...
Normally the switch must be closed when there´s enough cooling gas in the system and the switch works. The switch brings battery voltage to the relay for the clutch. You wrote that the A/C control unit wants the compressor to work. Can you measure battery voltage on one of the pins of the switch? (ignition on)
What´s about the thermostat near the air intake? The relay doesn´t work when it´s switched of.
Excuse my very bad english

I think that means 20 psi, 1 bar, minimum pressure for the system to work. If compressor runs and gas is enough, it sinks not under the limit.