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Heat only works when I turn it to 90 degrees on module. HELP!

carpdog30carpdog30 Member Posts: 7
edited October 2015 in GMC
Ok so I bought the envoy in the summer and the ac works great. Well it started getting colder out and when I turn the heat to lets just say 80 degrees it blows cold air. But when I turn it to 90 (all the way up) it works just fine. Does anyone know whats going on? Id like to get it fixed before winter. Thanks!
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  • carpdog30carpdog30 Member Posts: 7
    Also I should say I have dual climate control and thinking maybe a bad actuator but which one should i replace first if this is the problem?
  • thecardoc3thecardoc3 Member Posts: 5,848
    If the display will show it, check and see what it reports for the in-car, and ambient temperatures. If the display won't show you that data you will need a scan tool that supports the HVAC system in order to see if those sensors are being reported correctly.
  • carpdog30carpdog30 Member Posts: 7
    I have no check engine light on so will it come up with a faulty code if i dont? Also after doing some more research and talking with local auto parts they said the drivers side actuator is the "main" and both systems run through that one first before going to the others. Is this true? Also would that be the "mode" actuator or the "left air temp actuator. Thanks for the inputs looking to fix tomorrow if possible.
  • thecardoc3thecardoc3 Member Posts: 5,848
    No, especially if it is an issue with the in-car sensor if used. That sensor input isn't used by the PCM, and its unlikely that the ambient sensor temperature is either. Even so, the "sensor circuit" has to fail in a way as to fall outside of the software's limits to generate a trouble code. If one is inaccurate, but not to the point of the failure threshold the module would trust it and try to use that input. Hence the reason the system gives you heat at the full max command, but otherwise gives you cold air at a lower setting. Think about it for a minute, if the computer thinks that it is 140f inside the car and you want 80f, what would you expect the HVAC computer to do?
  • carpdog30carpdog30 Member Posts: 7
    ok so I should get a scan tool from the parts store that reads hvac and see if any codes come about? What are your thoughts to what you think is wrong? Im just trying to get knowledgeable before I go in. My mechanic over the phone said he thought it was the left actuator but dont want to waste money on a bunch of parts. thanks again!
  • carpdog30carpdog30 Member Posts: 7
    Sorry I just re-read your comments and understand a bit more. The ambient temperature is right on point to the outside temp. I cannot see the inside temp so I will need to get a scan tool for that. I will check that first before moving forward with any repairs. If the Cabin temp comes back normal what would be your next step to check?
  • thecardoc3thecardoc3 Member Posts: 5,848
    With a full function scan tool, it would be bi-directional controls and command the mode and blend door actuators for testing.
  • carpdog30carpdog30 Member Posts: 7
    I going to get it scanned this afternoon but I went ahead and checked out each actulator and they all move when i change setting on the climate control is this a sign that they all work?
  • carpdog30carpdog30 Member Posts: 7
    Problem was my Cabin Temp Sensor. I guess it was reading at 86 degrees in the cabin so would exsplain why my heat only worked at 90 degrees.
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