I'm trying to figure out why the AC compressor on a 91 Civic isn't
> engaging. There's plenty of freon and the compressor kicks on if I
> apply 12V directly. I've followed a diagnostic chart from an Alldata
> disc, and it appears the ECU isn't activating the relay. I'm dubious
> however that the ECU is the problem.
>
> I don't have the shop manual or wiring diagram, and the Alldata disc
> makes no mention of the AC switch on the dashboard. How do I test if
> the dash switch is telling the ECU to turn on the compressor? The blue
> light turns on when the switch is depressed. I know the kids were
> messing around in that area recently to install an aux wire onto the
> back of the aftermarket stereo.
>
> Can anyone describe how that part of the system works, and perhaps a
> pin on the ECU to check for 12V or ground with dash switch on/off?? >
> -RC
There should be a pressure switch close to your receiver/dryer that signals the ecu to power the compressor. If you don't have enough gas in the system, the ecu will not engage the compressor.
You have a leak in the system. Have it tested.