I have a 98 GMC Jimmy with automatic climate control. When the AC works it works great. Most of the time it doesn't. When I hit the AC button it blinks 3 times. If I leave everything on (fan, vent setting etc) sometimes the AC will come on by itself run for a little while and go off. I have had a mechanic check the freon and everything is fully charged with no leaks. The compressor was replaced on it by the PO a little over a year ago. Any ideas?

 

This could be a problem with the relay or with the HVAC control module itself. If the relay checks out fine or you swap it with another relay of the same type in the fuse/relay center under the hood and the problem persists, then it is not the relay.

The next thing I would try is resetting the battery power. This will cause the HVAC control module to go through its learn procedure. After disconnecting the battery, waiting 15-30 minutes, and reconnecting the battery, put the key in the RUN position and wait 5-10 minutes for the HVAC control to cycle through its settings. If the problem persists after that, the control module itself may be at fault.

Have a look at THIS ARTICLE from ImCool.com. I know it is for a Malibu and the control module in our trucks is different, especially for the ATC models, but some of the information may apply and heck, if it is already broken, I suppose you are not going to break it any more... LOL

 

Thanks for the link. I'll tackle that tonight. The relay your referring to, is that the AC compressor clutch relay? Would running the control module in manual override the problem and "force" the AC to run? Its hot out there today

 

Well I took the control module apart last night and cleaned everything and everything tested ok. Put everything back together. Found out something this morning though... the AC will work fine until the engine gets to running temperature then it shuts off and when you hit the AC button the light blinks 3 times. I also swapped the AC compressor relay with a known good just for good measure.

 

This could be a sign that the system pressure is too high. When was the last time you cleaned the A/C condenser (what is in front of the radiator?

 

Sorry for the delayed response. My internet at home is out. That's kind of what I was thinking. I took it to the car wash and cleaned it as best I could. The condenser has a few small pebbles and bugs in it but nothing major. Does it need to be perfectly clean?

 

No, not perfectly clean, but it cannot be clogged. I have had some look clean from the outside, but not flow water through them. If you can push water through the core, then it should be fine.

 

I took it to the car wash yesterday, pulled the grill off and cleaned out the radiator. This may have done the trick. I drove it home yesterday in the 95 degree heat and it didnt die. Thanks for your help.

 

Good deal! Glad it was something so simple. Hope it all works out for you!

 

Well heres one for you. I drove the Jimmy to the store yesterday, got back home and went inside for a few mins. Came back outside to move it and the AC wouldnt work. Popped the hood and felt of the lines on the compressor. They were both extremely hot! So I got to thinking (yeah I know, scarry isnt it?) Maybe the heat from the engine is heating up the AC lines, pressurizing the lines and wont let it start. So I pulled the plug off the high pressure switch and (ok dont freak out on me here, Ive done this before on many diferent AC units, I was a maintenance Technician for an industrial plant for 7 years) shorted the prongs together to get the unit to run for just a few seconds. Plugged it back in and the AC started working fine. I know the PO had replaced the compressor a year or so back. Could the high pressure switch just have to low of a setting?