High speed on the blower fan does not work thru the resistor.
There is a seperate circuit that sould feed 12 volts directly to the motor. If you have access to a schematic it would help.
The connectors for the high speed, burn out a lot. If you cant find I could look it up.
SOURCE: 2005 GMC Sierra 4x4 -
The blower motor resistor is a common to fail and cause your problem. It is located on the passenger side on the underside of the dash. There are two screws that hold it in and very simple to replace. Check the wiring and connectors that go to it. There is an udated version that you can buy if you notice any burnt look to it. The other wires go to the heater motor and will need to be unplugged. Just reinstall the new one and give it a shot.
SOURCE: 02 GMC Sierra-heater-a/c motor only works on high speed
Check or have checked the blower motor resistor. These resistors will cause this problem. I would go this route first as these are relatively inexpensive and usually the problem.
SOURCE: 2005 GMC Envoy SLE blower fan speed #5 is not working
More than likely this should just be your resistor. You have different sized coils on the back of your resistor for all the different fan speeds. if one of them burns out or breaks then that certain speed will not work.
SOURCE: 7 wire harness going to blower motor resistor
There is an updated resistor/connector package. Very expensive from GM, try your local NAPA or other parts supplier for a replacement
Wiring problem of the 2005 GMC Sierra 1500 6
Failure Date: 07/18/2009
The connector that plugs into the blower motor under the glove box became too hot and melted the plastic resulting in the a/c and heat not to work (this includes the windshield defroster). I've been looking up problems and solutions and found that this is a recurring problem with GMC and chevy trucks yet there has not been any recall, in fact the dealerships have an extremely overpriced kit to fix their faulty wiring. About 66,000 mi.
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