2010 GMC Sierra 1500 V-8; tonight I could not turn on A/C and defroster did not seem to defrost but heater worked like an easy-bake oven; Help; input appreciated!

You’re going to need to elaborate - its not at all clear what you’re describing as a problem. Its not twitter. You’re allowed to describe in more than 140 characters.

Why thank you cigroller; driving with defroster on last night; windows continued to fog, windshield barely clearing even in full “on” mode; moved to heat, had plenty of heat and fan; shut everything off, back on, same thing; then decided I would try to turn the A/C on just to check; turned knobs to proper position, then pushed A/C button; button blinked twice, would not come on; fan blowing, but not A/C mode; got home, shut car off, did alla this again, same situation; gave it a 2-hour rest, went out and everything is working proper; possible short or just a bit of a gremlin? Truck has 51,000 miles (I drive a 3-county area in Houston); suggestions, thoughts, input? Thanks again.

and, by the way, I don’t twitter; text regularly but new to this format; trying not to be a dialogue hog!

Lift the hood, and locate the A/C compressor.
Start the engine, and observe the rotation of the pulley on the compressor.
Now, push the A/C control button on the dash, and observe the pulley.

Does the compressor clutch engage?
Is there a change in sound from the compressor area after pushing the A/C control button?

If not, you can try tracing the wire leading to the compressor clutch, to see if it is broken or disconnected.

Unfortunately there are several reasons that you can hit that A/C button & have it blink at you and turn back off. I say unfortunately because that means the most straightforward way to get an answer is to have someone put a Tech II scanner on it. As a 2010 w/ 51K I’d have to assume that it is under warranty and I would march it into the dealer. At this young age & mileage I can’t actually think of a problem that wouldn’t be covered by warranty.

What were the outside air temps during this episode & did they change between the times it was & wasn’t working? Do you recall where your engine temp gauge was? Did anything else weird happen?

It could be that you have low coolant in the AC system. It has a High/Low pressure switch to prevent the compressor from coming on if you have too much or too little coolant. The AC light blinking is your clue that something may be wrong with the system.

One way to check it out is to raise the hood. Open all the windows and turn the AC onto Max with the fan up all the way. You should see the AC compressor clutch pull in and say in. Keep watching if the clutch clicks in and out a few times over the next 10 minutes you have a problem with the pressure.

Good luck

temp guage normal @ close to 210; temp outside about 38 degrees steady; ran out of warranty @ 36k, except for powertrain; not worth $2k to buy extended warranty that only goes to bowt 125k miles max (3 years for my driving); AC clutch seems to be working now; gonna run by autozone and see if they can do a quick diagnostic on it then ask my mechanic friend to see what he thinks;

by the way this was isolated incident; AC light only blinked twice when I pushed it, not for long term; as said earlier, all seems to be functioning well now; maybe it just had a “moment,” but I don’t need these “moments” to return; thanks for the input;

GM mechanic tells me AC will not engage on defrost mode if 38 degrees Farenheit or less, so I got one answer about the behavior;

Make sure you don’t have the vent set to ‘Recirc’. You need to pull drier outside air to defog the windows. The heater seems to be working OK, since the second post mentioned ‘plenty of heat’. Hopefully this will solve your problem.

VDC said it…se if the compressor is actually engaging… If you listen you can hear the clutch CLICK…and you will see the RPM’s change a little bit… I can tell from inside the car, but its easier to just have a peek under the hood…

If it is not engaging the A/C clutch you probably have an A/C leak…and the pressure has dropped too low to allow the A/C compressor from turning on… Then you need to locate the leak…fill the system…then test again…prob will work fine… Its the leak thats the bitch to fix


I think that the GM mechanic is right. The computer controls the compressor by sending a ground to the compressor clutch relay. It may be possible that the relay froze on you, but I suspect that the computer just decided to not send the ground.

Busted Knuckles is right about the recirc button. 32 degree outside air, even at 100% plus relative humidity, still does not have much actual moisture in it. Cold air can’t hold as much water as warm air. But your inside air, being much warmer, holds a lot more moisture that condenses against the windows. The water vapor comes from the occupants in the vehicle.

The A/C button will flash three times if the user pushes the button when outside temperatures are near or below freezing.
From the owners manual;

If this button is pressed when the air
conditioning compressor is
unavailable due to outside
conditions, the indicator flashes
three times and then turns off. If the
air conditioning is on and the
outside temperature drops below a
temperature which is too cool for air
conditioning to be effective, the air
conditioning light turns off to show
that the air conditioning mode has
been canceled.

Do you have a garage attached to your house? If so, the temperature will be in the 50s even if it is in the 30s outside. You can test it in the garage. If you don’t have an attached garage, maybe a friend does.

Recommend against running an engine in an enclosed (and attached to the house) garage without an exhaust evac system. Good way to mess up your day. Open the door if you need to do this. It’ll still take a bit for the inside to get cold. Then, leave the door open so the fumes can all escape.

Especially if you don’t have carbon monoxide detectors in the house.


Boy you said it Chaissos… That reminds me of a year ago…in my neighborhood a couple decided to run a generator during a power outage that lasted like one day… or less…

They put the genny in the BASEMENT…then they went to bed… They woke up Dead… WHat a terrible thing…but SO AVOIDABLE… That story saddens me… They lived up the street about 1/4 mile from me…

Ventilation if you are in a garage PLEASE…