Airconditioning Failure

[Mel_Boss]

For a year, I've tried in vain to diagnose why the AC in my 1991 Elan won't blow cold air.
History:
> I refilled the refrigerant summer 2006, and a dye leak-test showed no visible problems.
> The AC worked reliably until it didn't.
> First failure summer 2008.
> A dealer in the fall of 2008 diagnosed the problem as a faulty ECU, but I stopped short of letting him order a new $750 unit. I was told that the compressor is switched on by the ECU, and that this operation was failing. I was told that a manual switch could be "rigged" on the dash to manually switch on the compressor, but I rejected this course of action.

This summer a reliable shop started looking at the problem (and the M100 still resides at his shop).
The characteristics of the failure are that the AC compressor is switched-on when the AC is first turned-on but is switched-off again within a few minutes and won't switch-back-on again. The fuse is intact. The refrigerant was a bit low again and topped-up. An alternate ECU was installed, but the problem persisted unaffected by the swap.

We suspect that there is a faulty sensor that switches off the compressor, but we are working w/o a wiring schematic.

We can see no evidence that the ECU has anything to do with turning the compressor on or off. I haven't discussed this with the dealer as yet and am reluctant to do so until I am certain that I "know what I'm talking about." This dealer has little experience with M100s (obviously).

Is there anyone who -
> has had a similar problem?
> has access to a wiring diagram and can post a portion that is pertinent to the AC controls?
> can offer some advice?

Thanks!
Mel

[Mike_H]

here is the AC wiring diagram hope it helps.
Mike

[Mel_Boss]

Thank you SO MUCH for the wiring diagram, Mike! A quick glance indicates that we were mistaken in thinking that the ECU was not a player in this loop. Still, since we already swapped out the existing ECu wo any change, I think that it is safe to say that the problem resides elsewhere. This schematic will certainly HELP!
Mel

[nitroman]

From memory a few things which will cause the A/C compressor to be shut off are refrigerant pressure too low or too high, engine temp too high, and engine vacuum too low (functionally, a high-RPM cutout).