AC - No power at low pressure sensor

[289stang]

On my 85 Mustang GT Convertible AOD with CFI, the AC works intermittently. I know it has freon (just added 2lbs) and was working. Now the compressor doesn't engage.

I initially thought it was the low pressure switch, since it was low on freon before. Before when the AC was working but low on freon, I did have power to the low pressure switch. Added freon and worked like a champ for a few days. This is why I decided to check for power here, thinking maybe the switch was bad. So, I turned the AC on Max and blower on high. I pulled the low pressure wire and checked for power with my test light and nothing. I next checked the #6 fuse for AC, fuse was good.

Not sure if the issues is the switch (controller in the car where I turn on the AC/heat) or something else. Did a few searches and there were suggestions about WOT relay. Not sure where or how to test this.

A little history in the car, it's all stock, except for dual exhaust. It sat for 10-12 years under cover before I purchased it. Most everything works (except two rear window and the passenger window switch). I know sitting can take its toll on a vehicle, especially moisture and critter. There was no sign of critters but definitely some moisture. The car is a vert and top was in need of replacement when I got it. No rust, but a mildews smell in the car.

Anyone have any suggestions?

[DrewM]

I had basically the same exact problem and the problem was the relay. In the 86 the relay is under the hood and mine was completely rusted away and the inside of the relay sounded like a rattle. IIRC the relay is in a different location in the 85, but I don't recall where.

[289stang]

Drew I got to thinking and I followed the wires coming off the low pressure switch and they actually go to a relay under the hood, like yours. I did a quick Google search and the relay I have is exactly like the '86 relay. My car is a late 85 production car (6/85) and May have some 86 upgraded components installed. I pulled the relay out and there is a tremendous amount of corrosion and, like hours, it sounds like its full of sand when I shake it. I'll pick one up at work tomorrow (NAPA). Here are a few photos of mine.
.

[DrewM]

Yep, looks familiar, the picture is crappy, but the terminals were basically all rusted away;



The funny part is that it failed the next time I started the car after replacing all of the major AC components (hoses, compressor, dryer, condenser, etc). Everything worked just fine before that (but it had a leak) and worked fine while filling everything up and getting it all setup. Then boom...nothing. Really makes troubleshooting fun when basically everything was changed. I spent a couple of hours jumper'ing everything until I saw that the relay fails often enough to make it worth checking.

[289stang]

Good news, so far... I picked up a new relay and now have power to the low pressure switch. Started the car and compressor engages!! I'll drive it today and put it to the test.