90 SWB Montero. All the mechanical parts of the A/C work well. When the compressor is jumpered from the battery it blows a 35'F at the center vent and system held pressure for the two years of the vehicle rebuild.

It's a no-go trying to use the "A/C" button or defrost mode from the panel. The condensor fan does not engage either. I checked the 20A fuse behind the glove box (good), and the other indicated fuses are also good. I checked the wiring to the A/C button and it is solid (it was apart during the stereo install but seems good).

Has anyone run into a common failure that keeps the A/C from firing off the compressor?

Thanks, Pete

Russell - do you have a good dash light rheostat and horn pad?

Try bypassing the freon low pressure switch and see what happens. If you're a bit low on freon, the switch will open and you get no compressor action. I think it's located on or near the radiator support, just behind the right headlight assembly. I don't have my FSM handy.

Thanks Jim - the pressures are right. I had a service come out and evaluate the system, evac and recharge with R12 adding oil. The plumbing/mech part is fine when the compressor is jumpered.

No compressor rattle or restriction in the system - just no elec to the system from the controls.

Try jumpering that switch anyway, as it may be bad. It's a combo low/high pressure cutoff IIRC.

I'd download the 89 factory service manual and start wading thru the wiring diagram. It's hairy, really complicated. I can't remember for sure, but there may be a high coolant temp switch in the circuit, too, somewhere on the t'stat housing. There's a heater relay and an a/c relay in the circuit, too, and iirc, more than one fuse. I'd start at the a/c switch with a voltmeter or test light and start looking for where the power stops.

Or just wire in a new compressor switch from ign on to the a/c compressor, but you lose all the safeties that way...

The pressure switch is indeed a dual switch. There are 2 A/C power relays, the first (relay B) providing power to the second (relay A), but the second (relay A) is triggered by a third relay contained within the ECU. Simple, eh? Nope.

The condensor relay gets power from Relay A, but is triggered by the dual pressure switch.

Ditto on downloading the FSM to get the schematic. It is indeed a complicated set of circuits, but understandable if you take your time and trace it all the way through.

Once you grasp it all, it's clear as a bell, but right up to that point, it can be quite muddy.

The thing that's easy to miss is the ECU relay control. When I replaced my OE ECU with Megasquirt, I installed a separate relay, triggered by the (+) signal from the ECT switch to provide a (-) trigger for the A/C Relay A coil.

Frank