Truck A/C

[Czech]

Although not as bothersome as a poscessed horn, my issue is probably costing me a bit more in gas money. The A/C on my 1997 GMC Yukon 5.7L is running all the time, even when it is shut off. This started happening last year at which point I disconnected it. Just for the heck of it, I plugged it back in earlier this year, it seemed to be ok until the last week or so. Now it's running all the time again regardless of being turned on or off. I haven't done any maint on the A/C ever, the truck has 225000 miles on it. Thoughts? Thanks.

 

[Hogwildz]

Not sure how yours is activated, via switch? Dial, push button? Seems to me either switch contacts are worn or something in the wiring may be shorted out.
Its a matter of some kind of switch turning it on & off. Something is not activating, or making contact. Does the heat do the same thing?

 

[Redox]

Might be the same problem as the possessed car horn. I am assuming that you are talking about the cabin fan running all the time? GM uses a high speed relay generally located in the relay box on the fender that connects the blower to a high current fuse. It closes on HIGH to run the fan on the highest voltage possible. The other speeds pass the current through the selector switch and a resistor mounted in the airstream close to the blower. If this relay sticks on, the blower will not shut off, even with the ignition switch off. Unplug the relay (or just stop using high speed) and try it for a few days; it won't hurt anything. If the fan works on the lower speeds OK, then the relay is probably the problem. If you can catch it while it is stuck on, gently tapping the relay might make it shut off.

If it is the condenser/radiator fan sticking on, it gets a little more complicated as there are pressure switches and the computer involved, but this fan also has a relay. From the symptoms you describe, I'd try replacing the relay just to rule it out. One good thing about GM; their parts are cheap and readily available. I would expect to pay in the neighborhood of $20 for such a relay.

FWIW, NAPA has a very good online store that lets you look up specific parts for your vehicle and generally has pictures of the part you are looking for if you aren't sure what it is you are looking at. Give it a shot and let me know if I missed something.

http://www.napaonline.com/default.aspx

Chris

 

[Czech]

Thanks guys, but it is only the A/C, the cabin fan, heat, etc, works as designed. It has to be something with the compressor unit imho? It's a push button on the dash, but the push button doesn't do anything to shut it off when running, but when it is behaving, the button works fine (ie I don't see a relation to the button off and on). I've never charged the system, is it possible the low coolant may do something? It does cool fine when running. I'm a fairly good car guy, do most stuff myself, but I hate electrical and I'm thinking it may be this. Alldata doesn't show much either. Maybe a sticky clutch on the compressor? With gas prices, I may just not worry about it and roll the windows down more often. That said, I rarely use it (something there?), more of a 'I wanna figure it out' thing than anything. Thanks again all.

 

[Hogwildz]

I'd check the relay suggestion.
On a side note, if you research fuel consumption with windows down. You will find you use as much and sometimes more fuel with the windows down than the A/C on. This is due to wind drag.

 

[Redox]

Hmm; blew that one, didn't I? Guess I have to listen to Click and Clack some more...

It's the compressor that's not shutting off? This is a stupid question, I know, but are you sure you don't have it in defrost? GM runs the compressor in defrost for dehumidification. Do you have a rear AC system that might be making it run? I'm assuming you don't have the electronic air conditioner controls.

Refrigerant level wouldn't matter unless it won't come on. Not shutting off sounds like an electrical problem. I don't think I've ever heard of a clutch sticking on, but anything is possible.

Next time it acts up, break the clutch connection while the engine is running without getting your hand in the belt and fan and see if it stops running. If it keeps going, the clutch is bad and the compressor will have to be replaced. The clutches are replaceable, but you need special tools for it and the parts are as expensive as the whole compressor.

If it is an electrical problem, there might be a problem with the button or selector switch. Catching intermittent electrical problems can be a PITA, but if you drive it a while with the dash pulled apart, you might be able to disconnect the button when it is acting up to see if that is the cause. There are other things that can make it run, but most likely it is the pushbutton switch. This may be a GM only part you have to get from the dealer, but it shouldn't be that expensive. Good luck and let us know if you get it figured out! I like diagnosing problems by long distance...

Chris

 

[jabush]

I'm guessing this is a faulty relay switch that's not telling the compressor to turn off when the fan is in the off position. Probably linked to the switches behind the dash. On my '02 Ford, the compressor runs when the fan is in any position other than off. This is regardless of whether you are calling for heat or cool air. If the fan is on, then the compressor is running.
If your compressor is running all the time, it's costing you about 5-6 horsepower and a little bit of mileage.
It might be worth it to you to bite the bullet and let a shop check it out. Chances are they've run into this before wth GMs.

 

[Czech]

My neighbor and I just got done putzing with it, we're thinking vacuum line to one of the relays. Problem is, it is hard to make it mis-behave when we're putzing. Same old story! I'll keep you posted on what we find, cleaned hoses today so I guess we'll see. Thanks all, have a great weekend, just snapped a long neck, it's noon somewhere, right?

 

[jabush]

One more thought Gotz...when I have a problem I can't track down I often visit the boards for that vehicle.
Try a search here...

http://www.truckforum.org/forums/gmc-truck-forum/

hth...

 

[taxidermist]

also try automotiveforums.net I get all my info there.

Rob

 

[cncpro]

Is it possible that the AC is supposed to run in certain positions of the heat selector ? I could be way off here but I think some vehicles use the AC whenever the defrost position is selected...

Maybe not... Just thinking out loud... :-/

 

[Highbeam]

You don't have any vacuum hoses on the 97 except for PCV and fuel evap.

That compressor will run when the AC controls are in the defrost mode or even the defrost/floor setting without the little light on the AC button illuminating. So first thing to do is select the dash vent only and turn the fan to Off.

When you call for AC, the AC compressor will cycle. It will not sit there and run run run. My typical cycle time is along the lines of 10 seconds or less.

Unplugging the compressor makes it stop. You said that up front. This is good since you know that the clutch is fine.

Now you have to find out why the truck is telling the AC compressor to keep running. There are two pressure switches in the AC system a low pressure switch that tells the compressor to start, and a hi pressure switch that tells it to stop. I've had to replace the low pressure switch when it died and I knew it because the compressor would never run. It sounds like your hi pressure switch is dead OR you have no refrigerant in your system to compress enough to trip it.

20$ at walmart will buy you a recharge kit with gauges. Your high mileage system is likely low on refrigerant which normally causes a short cycle situation. Does it cool? If so then coolant level is fine.

If adjusting your AC controls doesn't fix it then my guess is that high pressure switch.

 

[Redox]

The high pressure switch is only a safety device and not normally in control of the situation. The low pressure switch will cycle to keep the evaporator from freezing up. It also keeps the compressor from running if the refrigerant runs low. If the system is cooling fine, then you don't need refrigerant. Resist the urge to add more to see if it makes things better. GM uses an orifice tube system that is sensitive to the charge level and more is usually not better. It is also not unusual to have a system that holds its charge for 10 years. They're not supposed to leak!

If you have a known leak, that recharge kit is probably OK, but I personally wouldn't recharge any system without a high pressure gauge on it. It is possible to have a plugged orifice or a bad expansion valve that will make it act just like it's low on gas. Overcharging the system may cause a rupture plug to vent or other equally bad thing to happen. I also wouldn't use anything but pure refigerant as I have seen systems that were so overcharged with oil that they wouldn't cool. Ditto for the "leak stop" stuff, too. It usually doesn't work and can make a huge mess of the system.

Chris

 

[Highbeam]

His 97 is pushing 12 years old. My 98 version of the same vehicle needed its first charge ever after 170,000 miles this year. Done at the GM shop since I wanted it vacuum tested and hopefully they cleaned the refrigerant.

So what happened? Constantly running AC compressors are a problem.

 

[Redox]

His 97 is pushing 12 years old. My 98 version of the same vehicle needed its first charge ever after 170,000 miles this year. Done at the GM shop since I wanted it vacuum tested and hopefully they cleaned the refrigerant.

So what happened? Constantly running AC compressors are a problem.

That is typical for a GM vehicle in my experience. My parents 1974 Blazer never needed gas in the 15 years that they owned it. If nothing else, GM knows how to make a good compressor!

The compressor is supposed to run continuously (whenever the AC is on). It will only cycle off if it is on the edge of freezing up. At idle, it will run all the time, but is probably cycling when you are on the highway. They don't cool unless they are running...

Chris

 

[Czech]

Alright you guys, go figure. I went over all the connections, and again I did mess with vacuum lines, esp the one that goes through the passenger side firewall, there's some sort of value or reply there. It has been fine since, no problems. I'm wondering if the vacuum line is for the blower deflector and that the deflector may have been getting stuck in the defrost position due to a plugged line? Just a thought, you guys know more than I do. I hate this intermittent crap! Thanks everyone for your insight, I love this sight because of that. Just to clarify, when running the compressor does cycle. When it is being naughty, the fan can be off along with all the controls off or in a non-defrost setting, you can feel the compressor cycle and feel the cold air blowing through the vents even with the fan off. Thanks again!

 

[Highbeam]

Your AC system uses electric motors to adjust the settings. No vaccum tubes. You might be seeing the condensate drain which is a tube on the pside firewall.

My AC has always cycled on/off/on/off at idle or running down the road. The length of cycles does vary though.

 

[Czech]

Highbeam, I respect your know how more than mine by far, but this is a vacuum line. Disconnect it with the motor running, it's sucking air. Someone told me it may run the air deflector?

 

[Redox]

Most GM of that era used vacuum to move valves and dampers. The temperature knob is a choke cable that goes to a door that bypasses the heater core, but the selector knob is a vacuum valve. Even my Roadmasters with the electronic AC controls still use vacuum for everything but the temperature. That vacuum line near the firewall is probably going to the hot water valve to shut it off in the summer.

The cycling is load dependent, but most compressors will be on continuously when it's above 90 degrees and you are working them. If you slow the fan down or switch to inside air, you are taking a load off the system and it may start to cycle; moreso at speed than at idle.

The compressor clutch is purely an electrical circuit and will operate even if there is no vacuum. BTW, most GM cars will revert to full heat if the vacuum goes down for any reason. My El Camino would do this going up a hill because there was a bad check valve...

Chris