I have seen several folks speak of their problem with the interior fan speed control, but none regarding their fixes. So, here is what I did when mine failed and stayed stuck on HI speed. There are two componts in the fan speed control circut----the dash mounted AC Control Unit and the Variable Blower Control Unit mounted on the blower plenum, just above the passengers feet. Determining which is at fault is very quick and easy, and just requires a voltmeter. Open the passenger door and look at the blower, just above the passengers feet---there is a small rectangular block on the bottom of the plenum with a four wire flat connector on it. This is the Variable Blower Control unit. Three of these wires are large gauge and one, the second from the left, is a smaller gauge wire that carries the control signal from the dash control. Place the voltmeter between that small control wire and ground and set the voltmeter to be able to read up to 5 volts. DO NOT UNPLUG the connector to do this or you will get a FALSE indcation of dash control unit failure !! Now, with the ignition at ON, and the fan control at OFF, you should read zero volts. Slowly turn the fan control towards HI and the volt meter should rise, up to 5 volts when the fan control reaches HI. If this happens, the failure is in the Varaible Speed Control box. No signal here indicates the dash AC Control Unit is the culprit. I hope this helps folks....and I am once again enjoying all 14 fan speed settings and was able to avoid replacing the much more expensive dash control unit.
On edit, I should add that this is for the manual control AC system. I don't know if it would apply to the auto-temp control option also, and I have not gone through that particular section of the shop manuel because it doesn't apply to my truck
UPDATE, One year later Well, it has been a year now, and my blower speed control is still working fine. On occasion while running at a lower fan speed on a very hot, humid day here on the Texas gulf coast, the fan will shift to high for a very short time, and then return to it's previous setting. I believe this may be a "fail safe" mode to avoid overheating the VBC module circuitry, and is not (to me anyway) a distraction or problem.
On edit, I should add that this is for the manual control AC system. I don't know if it would apply to the auto-temp control option also, and I have not gone through that particular section of the shop manuel because it doesn't apply to my truck
UPDATE, One year later Well, it has been a year now, and my blower speed control is still working fine. On occasion while running at a lower fan speed on a very hot, humid day here on the Texas gulf coast, the fan will shift to high for a very short time, and then return to it's previous setting. I believe this may be a "fail safe" mode to avoid overheating the VBC module circuitry, and is not (to me anyway) a distraction or problem.
