My AC has been intermittent since I got the truck. It works, it doesn't work. It's been annoying, especially when it's truly hot and humid out, like when we were in Houston (it played toad behind the motorhome after disconnecting the rear driveshaft).
Not long after the new engine, I had the AC converted to R134a.
Lately, the AC hasn't worked at all. I used to be able to get it to come on about every 2nd-10th try. Guessing that a borderline part had finally given up the ghost, and with the heat of summer coming, especially when you get out to the desert surrounding San Diego, it was time to fix it!
I tell you, troubleshooting issues is so much easier with forums and with Google than it used to be. I searched around and learned some things it could be.
A quick note for anyone troubleshooting electrical problems. You need both a meter AND a test light.
Meters are awesome. I spent about an hour one day troubleshooting my sister's 1981 Celica and why the headlighs wouldn't work when they had 12v running to them (13.6 actually). Scratching my head. Taking more readings. Scratching my head some more. Finally I broke out my test light and tested a headlight connector. The problem was immediately noticeable as it barely lit up. Following the wires back to the connector at the battery showed a wire that had corroded nearly all the way through in a hard-to-spot way, 2 meager strands providing the 12v it took to fool my meter into thinking all was well.
The FSM (Factory Service Manual) is a must for anyone working on this stuff. This one is a 1988 version instead of 1987, but usually that's close enough! The wiring diagrams in books like these are great, but what's missing is... where is that silly relay/connector/fuse block/switch/etc. actually located on the truck! I have a Haynes to go with the FSM.
So... back to the AC.
In my case, the AC button lights up, but nothing happens. The fact it lights up at all was a little weird I thought, but it turns out it seems to just mean the button is pushed in.
Since getting the switch out looked to be a pain, and it lights up 100% of the time it's depressed, I figured I would start with the AC relay. Hopefully it was easier to get to and I could check for current from the switch with my handy-dandy test light.
The AC relay is directly behind the glovebox, on top of the evaporator housing, just to the right of centerline (unlike the left a forum post had indicated - that's the AC Amplifier). The glovebox was easy to get behind, pulling down the flap (gently!) and popping the box itself down past the left and right catches (gently again!). When I flipped the switch on and off, the relay clicked merrily. Hmm.... That's good, but not for me. :)
I borrowed this picture from 4Crawler
The little black button is the light switch for the glovebox.
He's marked the AC low pressure switch connector.
The connector below it is for the thermistor.
My thermistor connector was jammed behind the glovebox switch, wearing through the white/blue wire's insulation and causing a short. Since no fuse was damaged, and looking at the wiring diagram, it appears to be on the ground-side. However, it could certainly explain the intermittency since it runs back into the AC Amplifier at that point. The AC Amp controls compressor lockup.
I taped the wire up, then moved the connector up higher and used some of the wire clips available to keep it there.
Unfortunately, that did not change the behaviour - the compressor still wasn't coming on.
Well, the low pressure switch is right there, let's check it. Hmmm, power to only one side (red wire, incoming).
I pulled a cap off the compressor and just touched the schrader valve. Nothing. Held it down. Nothing.
The system is totally empty (standard pressure). Crap, time to find the leak... It's been converted to R134, so it'll be easy to rectify once I've sorted out the leak. When it was converted, it held vaccum and worked for over a year, so this is a new problem.
Hopefully it's not the compressor (cha-ching!!).
It wasn't a total loss, I got to tear into the truck a bit and learn more about it.
At the same time, I found the heater and AC fuses behind the dash. No, really. Not hard to get to, but you have to pull the glovebox out (2 minutes) to get to them.