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  1. #1

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

    1999 Chevrolet K1500 4x4 Ext.Cab LS
    To say the least I am lost! I have a problem with my fan speed control on the Heater/AC controller. This is what happens and what I have tried.
    When the AC/Heater fan speed control is turned off 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 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.
    Thanks in advance,
    Graham

  2. #2

    Default

    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.

    1995 Silverado 4x4
    6" BDS Suspension Lift-3" Body Lift-Add A Leaf in rear -Trailmaster SSV Shocks-Duel Steering Stabilizer Kit -AirAid Cold air intake-
    4.56 Gears with Detroit Auburn Locker-Pro-Comp Traction Bars with duel shocks-Aluminum Skid Plate Kit-38.5" x 16.5" Mickey Thompson Baja Claws-Constant Dropping fuel gauge

    2005 Yukon XL Jet Power Programmer, Bilstein Shocks, Bilstein rear springs, Helwig Anti-sway bars, EGR Window Visors, EGR Hood Shield, Denali Headlights, Headlight harness upgrade, GE NightHawk Bulbs, White Night Rear lighting system, Russell Braided SS brake lines, PowerStop Brake pads, PowerStop cross drilled and Slotted Rotors, http://www.gmtruckclub.com/forum/sho...5-GMC-Yukon-XL
    2002 Silverado ext cab 2wd (Sold)
    2003 Yukon XL (Totaled)

  3. #3

    Default

    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 $$
    Ray

    '09 Avalanche LTZ - Black
    '05 Envoy XL (sold)

  4. #4

    Thumbs up Replying

    Quote Originally Posted by RayVoy View Post
    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 $$
    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.
    Thanks in advance,
    Graham

  5. #5

    Default

    Quote Originally Posted by Keeta View Post
    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.
    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

    1995 Silverado 4x4
    6" BDS Suspension Lift-3" Body Lift-Add A Leaf in rear -Trailmaster SSV Shocks-Duel Steering Stabilizer Kit -AirAid Cold air intake-
    4.56 Gears with Detroit Auburn Locker-Pro-Comp Traction Bars with duel shocks-Aluminum Skid Plate Kit-38.5" x 16.5" Mickey Thompson Baja Claws-Constant Dropping fuel gauge

    2005 Yukon XL Jet Power Programmer, Bilstein Shocks, Bilstein rear springs, Helwig Anti-sway bars, EGR Window Visors, EGR Hood Shield, Denali Headlights, Headlight harness upgrade, GE NightHawk Bulbs, White Night Rear lighting system, Russell Braided SS brake lines, PowerStop Brake pads, PowerStop cross drilled and Slotted Rotors, http://www.gmtruckclub.com/forum/sho...5-GMC-Yukon-XL
    2002 Silverado ext cab 2wd (Sold)
    2003 Yukon XL (Totaled)

  6. #6

    Default

    Quote Originally Posted by Pikey View Post
    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
    Hope you hung on to the first one, it could be a great spare.
    Ray

    '09 Avalanche LTZ - Black
    '05 Envoy XL (sold)

  7. #7

    Default

    Quote Originally Posted by RayVoy View Post
    Hope you hung on to the first one, it could be a great spare.
    Nope! Sold the 02 so it is not my issue any longer

    1995 Silverado 4x4
    6" BDS Suspension Lift-3" Body Lift-Add A Leaf in rear -Trailmaster SSV Shocks-Duel Steering Stabilizer Kit -AirAid Cold air intake-
    4.56 Gears with Detroit Auburn Locker-Pro-Comp Traction Bars with duel shocks-Aluminum Skid Plate Kit-38.5" x 16.5" Mickey Thompson Baja Claws-Constant Dropping fuel gauge

    2005 Yukon XL Jet Power Programmer, Bilstein Shocks, Bilstein rear springs, Helwig Anti-sway bars, EGR Window Visors, EGR Hood Shield, Denali Headlights, Headlight harness upgrade, GE NightHawk Bulbs, White Night Rear lighting system, Russell Braided SS brake lines, PowerStop Brake pads, PowerStop cross drilled and Slotted Rotors, http://www.gmtruckclub.com/forum/sho...5-GMC-Yukon-XL
    2002 Silverado ext cab 2wd (Sold)
    2003 Yukon XL (Totaled)

  8. #8

    Default

    Quote Originally Posted by Pikey View Post
    Nope! Sold the 02 so it is not my issue any longer
    Haha, good answer
    Ray

    '09 Avalanche LTZ - Black
    '05 Envoy XL (sold)

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