I have been asked many times about the auto blower control on 2003- 2007 Chevrolet Silverado Classic and GMC Sierra Classic trucks so I decided to take some readings on my truck. This also applies to the Tahoe, Yukon, Avalanche, Escalade and Suburbans of the same era with auto a/c controls. The blower control processor is located under the passenger side of the dash.
There are a couple of 7 mm headed screws that have to be removed in order to drop the hush panel down far enough for testing. There is a third screw that is over the transmission hump that has to be removed if the panel needs to come all of the way out.
After dropping the panel down enough to test, I checked for power on the red wire, ground on the black wire and variable voltage on the purple/white wire. Of course I had all of the proper readings. My blower works fine. I was just testing for informations sake.
I did notice something that seems a little strange. With the key off there is 5.2 volts on the purple/white wire.
With the key on and the blower set to low the voltage was 4.91 volts.
As I adjusted the fan speed to high the voltage was pulled down to 1.632 volts.
My conclusion is that the signal wire from the control head (purple/white) goes to an open circuit when the key is off and the blower control processor is actually feeding out a nominal.
5+ volt signal that it needs to have pulled down by applying a ground from the control head assembly.
The two most common failure symptoms with this part are an inoperative blower motor or a blower motor that will not turn off even with the key off. For testing information please see these repair articles. 2004 Chevrolet Silverado Blower Will Not Turn Off. and 2005 Chevrolet Tahoe Blower Not Working (video).
This part has been updated several times and here are links to a detailed repair article and parts information.
I have a 2003 Silverado. The blower has stopped working altogether so I have begun doing research. I have the auto a/c control like this post shows. When I test it I get nothing thru the purple/white wire. What is my next thing to check?
I am including another repair post link as the original blower speed controller is no longer supplied by anyone. Please do the testing within this post and at some of the other posts that are linked to within these posts. No, you did not ruin the blower motor. After reading some of the additional info you should see that the blower motor may have been heading for replacement anyways.
http://www.sparkys-answers.com/2011/07/2003-gmc-yukon-bower-erratic-not.html
It has auto a/c if I understand right. The box that I thought would be a blower motor resistor definitely doesn’t have a large resistor in it, looks like a couple of mosfet’s instead.
Does your Avalanche have manual or auto a/c controls?
I have been trying to fumble through a problem with an inop blower on my 2002 chevy avalanche. When it stopped working the first thing I assumed was a burnt out blower motor, so I pulled it and put 12vdc to it and it didn’t spin so I ordered a new one, installed, and still doesn’t work. After some extensive, and mostly confusing internet searching, you seem to be the only one in internet-land who has a clue. I have cleaned every contact that looks dirty, checked wires for heat, popped the cover off the blower module to check for burns on the circuit board and it all looks fine. Will checking voltages as shown in this post allow me to rule out the module in the dash? Did I ignorantly ruin the last blower with 12 volts? I’d like to make an informed purchase and fix it right the second time, any advice you have will be very appreciated!