Joined: Oct 2009
Posts: 490
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11/30/2015 at 5:33 PM
The GM vehicles I've had in the past had a resistor card close to the below motor that allowed the motor to run at different speeds. This resistor had two screws holding it in place and was connected to the electrical controls from the heater. If any of the resistors on this card failed (usually due to over heating of the resistor), that speed would not work. Top speed would work because at that speed, no resistors are used to lower the speed.
Here;s a video showing disassembly on a Tacoma where the owner replaced the resistor AND the wiring harness: