A/C compressor runs during heat mode

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by wclark160, Feb 6, 2013.

  1. wclark160

    wclark160 New Member

    Hello guys, I think I 've come to the right place based on the threads that I have read so far. I even found one forum site that recommended that I come to this site. Therefore I have a challenge I think. I bought a 2003 GMC Sierra with dual-climate, automatic HVAC control. I never use the Auto function on HVAC , as I like to adjust it all myself! I guess it's a guy thing. Anyways, here is the scenario:

    Truck running at normal operating temp.
    HVAC in manual control with the fan on.
    If I select anything other than 90 deg f. (Max), the AC turns on and the air gets frigid.
    This occurs no matter if I select 90 on the passenger or the driver side.
    If I lock out the A/C compressor (with the A/C off button), the temp control fine all the way through the range.
    If I select 80 Degrees F., the temp blows about 80 degs, until I release the lockout on the AC comp and then it starts to run and cool it way down.
    The A/C comp. cycles noramlly (probably on evaporator temp or something).

    Therefore, I have a way around the frigid temps by locking out the A/C function on the keypad. This works until I have to use defrost mode and then it blows freeaing cold air in the truck and cools it way down.

    I have checked all actuators and they are working. The recirc,the two blend doors, the mode control door work.

    I guess my question is, why does it seem as though the air conditioner is always on. Is this a controller issue do you think?
    Any advice would be helpful, I'm learning this stuff quickly but there's always someone who knows alot more, sitting right behind another keyboard!!

  2. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    When you press your lockout button does the display show something that looks snow flake with a line thru it? I had an 03 yukon xl for a while. I would have the same issue that you are describing. Every time I got in the truck I had to press the button to display the snow flake with a line or my compressor would cycle on and off. I never figured out if that was how it was supposed to work, I totaled the truck. I now have an 05 xl and the control looks exactly the same. But, if I press the button it turn the AC on. I am just wondering if for some reason GM did that year backwards.
  3. wclark160

    wclark160 New Member

    It does have the snowflake with the line through it when you lock the compressor out. It works perfect if you lock it out during the winter. But when in defrost mode, it should direct the hottest air you have to the windshield right? Problem Is, defrost mode blocks the ability to lock the compressor out, therefore ice cold air is being used to defrost the windshield. Because of that, I feel that it is an issue with truck, not a manufacture mishap! Did you have a problem defrosting the windows in you're truck?
  4. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    No i did not have an issue with defrost. Why don't you try recalibrating your hvac actuators? Basically you disconnect the battery for 15-20 minutes. Then, when you reconnect it, turn the key forward but do not start the truck. Turn on the hvac if it is not already on. Do not touch any of the hvac controls once it is on. I have been told different amounts of time to wait. I usually wait around 2 minutes. you should hear the actuators moving during the cal cycle. After 2 minutes use the controls as normal. I hope that this helps.
  5. wclark160

    wclark160 New Member

    Thanks for your help Pikey. I have tried re-cal the actuators with no resolution. I don't know if it is, but I'm going to check the resistance of the temp detectors just for giggles. I wonder if one of them is reading way high temp, therefore the A/C thinks it need to run to cool it down?
  6. j cat

    j cat Epic Member 5+ Years 1000 Posts

    you could go to the dealership and have them pull the ac codes. this will reveal the problem. not free or cheap.

    I would replace the ac computer now since you did the power cycle reset with no change. many have had these issues with the fully automatic ac climate control.

    when I ordered my 2000 SIL I made sure no auto climate control.

    it maybe possible that the outdoor air temp sensor is the problem . if the computer sees that the sensor is reporting a warm temp with the defrost on , then the ac compressor will turn on.
  7. pmf608

    pmf608 Epic Member 5+ Years 1000 Posts

    I also have a 2003 truck that functions the same way. I've never had a problem with the air being cold on anything other than max. The Air Conditioning is always enabled in automatic, but mine always keeps a comfortable temperature as long as I keep it set 1-2 degrees above where I want it, to account for the fact that the drier air feels colder. For this problem, I can't help you. The max (when set to metric units in canada) is 32 degrees celsius. I always keep mine set at 24 in the winter in Auto - unless I need extra defrost/defog power. This keeps it a comfortable 22 or so degrees. In the summer, I keep it set at 17-18 degrees on Auto so that the air conditioning keeps it a comfortable temperature without freezing me.

    On many newer vehicles with OR without automatic climate control, the air conditioning is always on if the defroster is turned on. This isn't to make the air cold. It is to dry the air, which is why the defroster is so good at getting fog off the inside of your windshield (or preventing it) while other settings can cause fog on the windows in some weather conditions. This is normal, and your defroster wouldn't be nearly as effective if it allowed moist air to create a layer of fog on the windshield.
  8. shibby2oo8

    shibby2oo8 Member 2 Years 100 Posts

    X2 on the defroster.

    A defroster works by allways bringing fresh air in (no recirc) abd runs ac so the evap core will catch the moisture and drain it out. Now i have manual controls on my truck so i xan only help with dystrm operation. First notice where the blend door actuator is at wgen at max then go down a few degrees to like 85 and make sure the blend door only moves slightly not all the way to cold. What i rhink off the top of my head is the inside air temp sensor is bad and the computer thinks its 188* ( i think thats the top but you get my drift) so even if its 1 degree below 90 the computer thinks its 180 and turns the ac on and moves the blend doors to cold. So to sum up notice the blend door position at full cold and hot abd see where it goes after temp input.

    Sorry about spelling I use my iphone and its a pain to respond.
  9. wclark160

    wclark160 New Member

    Thanks to everyone for the response. I understand the function if the a/c during defrost, i guess i should have said that i have no control of temperature on defrost mode(or any other mode). With the temp at max, the blend door is at full heat, with it at anything less than max, it's full cool. IMHO, I find that an intermediate temp is perfect because it defogs the window, but doesn't cool the air too much.
    Heres the latest: I agree that it's possibly a sensor issue. I spend alot of time with all kinds of controllers at work and know that one bad sensor, throws the process out the window.
    For an air temp of 55 deg f. All air temp sensors should have a resistance of 5000 ohms. This was the case with the exception of one, which was open (high temp). I jumpered a 5400 ohm (all i had)resistor into the harness for the one bad temp sensor and them plugged everything in and started the truck and allowed it to calibrate the controller and actuators. I immediately noticed control of the system. Before all my checks were complete, the temporary resistor fell out and it started to have the same issues again. Before I declare victory, I'm going to make a more permanent connection. I will let everyone know how it turns out. I love prints! Electrical it's always a fun troubleshooting experience!
    Last edited: Feb 8, 2013
  10. shibby2oo8

    shibby2oo8 Member 2 Years 100 Posts

    So it was the inside temp sensor wire you added the resistor to? Is it an internal part to something and where is it? I think i would get a new sensor to get the temp settings back to normal if its not too expensive. I like electrical too but they can turn into nightmares fast and they can be very annoying without a diagram and system operation description. I remember an explorer sporttrac that had a inop airbag system, tested everything i could (cant really test the bags themselves) took 3 days until i found a connector melted internally just enough to arc. Wanted to drive it off a cliff.

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