No power to AC compressor

OK, so my 95 Tahoe AC wasn't working again this spring as usual, it usually takes a can or 2 each year since I have a small leak somewhere.
BUT....after starting to add R-134a, I noticed the compressor wasn't cycling. http://www.tahoeforum.com/images/smilies/eek.gif

After doing some electrical detective work, I found that no power was getting to the compressor. All fuses are good, and I had my Master Electrician Dad-in-Law check the relay(which is switching properly with power to that half of the relay), and it was good as well. There is good wiring from the relay to the compressor, but whatever sends power into the relay to power the compressor isn't getting it there.

My questions are:
1) Does anyone know where power to the relay is fed from? I'm assuming there is some sort of sensor since the compressor usually cycles at different refrigerant pressures.

2) Can anyone point me in the direction of schematics that show the entire truck? The crappy Chilton book I have DOES NOT include the climate controls or the AC system in its wiring diagrams.

Any ideas or suggestions would be appreciated. We need to get one more summer out of this old vehicle, and I don't want to spend big bucks at a shop to have it diagnosed and fixed.

I should mention that I did put a temporary fused jumper to the relay, and that the AC system does work (with the jumper only) now that I properly charged it.

Thanks for any help or ideas.

Welcome to AF.
Go to low pressure switch on receiver dryer.
Orange wire should have 12 volts from fuse on it.
Dark green out of it feeds the relay.
If voltage to it and none out jumper switch ans see if compressor kicks in.
You need 28 lbs or more of pressure on low side to turn low presser switch on.
Any ac lights on on ac controls?
Some of them you have to clear low freon code or do a battery disconnect if the system gets low and shuts down and has to be reset to turn back on..

Check you Private e mail for more info.
Let us know how it goes.
MT

Welcome to AF.
Go to low pressure switch on receiver dryer.
Orange wire should have 12 volts from fuse on it.
Dark green out of it feeds the relay.
If voltage to it and none out jumper switch ans see if compressor kicks in.
You need 28 lbs or more of pressure on low side to turn low presser switch on.
Any ac lights on on ac controls?
Some of them you have to clear low freon code or do a battery disconnect if the system gets low and shuts down and has to be reset to turn back on..

Check you Private e mail for more info.
Let us know how it goes.
MT
You da Man MT.
Heres the deal, I do have ac lights on the controls, and there is 12v at the orange wire but none going out the green. I show just above 25 lbs on the low side, so that may be my problem. I jumped the connector at the dryer and the compressor kicks in.
I have noticed that it is blowing nice cold air though, and typically in the past, the compressor kicks on sporadically as soon as can #1 is being added, and its below 25 lbs in the "add" range, while blowing warm to cool air. I also did the battery disconnect and that didn't help.

So....I'm going to add a little more refrigerent and see if that cures it, but if it doesn't, can that pressure sensor be replaced?, or am I looking at a new dryer?
Thanks again MT

You da Man MT.
Heres the deal, I do have ac lights on the controls, and there is 12v at the orange wire but none going out the green. I show just above 25 lbs on the low side, so that may be my problem. I jumped the connector at the dryer and the compressor kicks in.
I have noticed that it is blowing nice cold air though, and typically in the past, the compressor kicks on sporadically as soon as can #1 is being added, and its below 25 lbs in the "add" range, while blowing warm to cool air. I also did the battery disconnect and that didn't help.

So....I'm going to add a little more refrigerent and see if that cures it, but if it doesn't, can that pressure sensor be replaced?, or am I looking at a new dryer?
Thanks again MT

Yes you can replace the switch.
Just screw off old one and new one on.
Also check the wiring on the plug in.

If the switch is working right it will shut off comp below 28 lbs.
And turn on over 30 lbs or more.
But there is a built in delay.
With compresser running and ac shuts off below 28 lbs of pressure the pressure should build back up to around 50 lbs and the it will kick back on and run untill the pressure drops below 26/28 lbs.
Make sure you have enought R-134A in it. and compressor is working good.
You really need a low and high side gauge set to recharge it right.
Post back your low and high side readings at idel and at 2K with compressor running.

If you are having to add gas very often you need to check and find the leak.
And check the system for the proper amount of oil.
Also just adding gas a lot will let a lt of air and moisture in the system.
The best thing is to pump the system down and run a vacuum on it and recharge with the proper amount of R-134-a and ac oil.

Good Luck
MT

Yes you can replace the switch.
Just screw off old one and new one on.
Also check the wiring on the plug in.

Do I have to drain the system down first to replace the switch?

I haven't tried adding refrigerent yet since I worked about 60 hours in the last 4 days. :shakehead
I'm gonna go get some later today if I don't fall asleep first.
Thanks again MT.

Do I have to drain the system down first to replace the switch?

I haven't tried adding refrigerent yet since I worked about 60 hours in the last 4 days. :shakehead
I'm gonna go get some later today if I don't fall asleep first.
Thanks again MT.

No need to drain system .
The switch has a schrader valve under it.
Just screw off and on.
MT

Air conditioning back online!

I added some cold stuff until it was in the 40's lbs range, and it still wasn't working. $17 for a new pressure switch and 5 minutes to swap it out and we're ready for summer.
Thanks MT-2500, your help saved us from the repair shop. :grinyes:

You are welcome and glad you got it fixed.
And thanks for posting back how it went.
MT