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AC/Heater Fan Control-Help Please-Loosing my mind

Discussion in 'Chevy C/K Truck Forum' started by Keeta, Nov 28, 2012.

  1. Keeta

    Keeta New Member

    [SIZE=+1][SIZE=+1]1999 Chevrolet K1500 4x4 Ext.Cab LS
    To say the least I am lost! I have a problem with my [SIZE=+1]fan speed control on the Heater/AC controller. This is what happens and what I have tried.
    [SIZE=+1]When the AC/Heater fan speed control is turned of[SIZE=+1]f it still runs. If I turn it to low or high and the other 2 speeds there is no change. So in order to turn it off I need to pull the fuse. I went and got a used unit and hooked it up and it worked for about a week or so then back to problem. The AC button, recirculation button and all other controls work just as they should. So I took [SIZE=+1]the control unit back and swapped it out for another one and got the exact same result. I would be so appreciative for your help with this. I would also enjoy being able to turn the heat or ac off/on and select the speed to my liking. Can you please help me with this issue. Thank you in advance.[/SIZE][/SIZE][/SIZE][/SIZE][/SIZE]
  2. Pikey

    Pikey Moderator Staff Member 3 Years ROTM Winner 1000 Posts

    I could be a problem with the resistor, although I have never heard of a bad one not letting you shut the fan off. If it in a NBS truck then The resistor is easy to pull out and inspect. Just crawl on the passengerside floor board and remove the plastic shroud on the bottom of the dash. Just like you were changing your cabin air filter. you should see the resistor bolted into the HVAC system. It is in between the blower motor and the in cab filters. Disconnect the wire, remove the bolt and pull it out. Look at the printed circuit board. Many times they are melted or the solder has melted off.
  3. RayVoy

    RayVoy Well-Known Member 2 Years 1000 Posts

    As Pikey suggests, the manual HVAC blower resistors usually act the oposite to what you are experiencing; However, if you have the automatic (digital) HVAC, the speed control module can act as you describe. The manual units use resistors to create different voltage drops that change the speed of the blower motor. The auto units have a FET (field effect transistor) that varies the speed of the blower (much like an amp varies the loudness of a speaker).

    If you have the automatic HVAC, the cure is the same as the manual unit, you need to replace the speed control module (different name, same function).

    BTW, the FET is usually what fails, if you have a friend that can work on electronic equipment, if you change the FET yourself, you can save some $$
  4. Keeta

    Keeta New Member


    I luckly work on computer motherboards and commercial motor controls. I have heard that what you both are mentioning could be the problem, however, since you both and the personal friend both stated it usually acts the opposite of what is happening I dismissed it. But since now three people are stating the FET I will pull it and repair it or replace it. If you have any other thoughts while I am messing with this project I would appreciate hearing from you guys. Thanks for your quick response, winter is creeping in and being able to adjust the speed or turn it off while driving would be great.
  5. Pikey

    Pikey Moderator Staff Member 3 Years ROTM Winner 1000 Posts

    I did the same thing on mine (until I could afford to buy a new one) one of the runs had completely melted off. I soldered a pad and a jumper wire across that section and it worked fine for a few months until I bought the new one
  6. RayVoy

    RayVoy Well-Known Member 2 Years 1000 Posts

    Hope you hung on to the first one, it could be a great spare.
  7. Pikey

    Pikey Moderator Staff Member 3 Years ROTM Winner 1000 Posts

    Nope! Sold the 02 so it is not my issue any longer
  8. RayVoy

    RayVoy Well-Known Member 2 Years 1000 Posts

    Haha, good answer

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