Auto Air Conditioning & Heater Repair: AC Blower, silverado hd, chevy silverado

Question
I have a 2004 Chevy Silverado HD (Diesel) My fan will only work on the highest setting wondering what would cause this

Thanks Jim

Answer
Jim,
Your blower motor speed works like this...

When you select a speed from your control panel, voltage flows through the switch, through a resistor block, and then to the fan motor. The speed the fan turns depends upon the amount of voltage it receives. The resistor block is what knocks down the 12 volts to whatever it needs for a lower speed. For high speed the motor needs 12 volts, This travels directly to the motor itself, it is not wired in with your resistor block. Every other setting travels through this resistor block where it loses power before it gets to the fan making it turn at a slower rate of speed.

It is safe to say your resistor block is bad. This is typically a fairly easy replacement. And usually costs around 40 dollars at an auto parts store. It is usually mounted directly on your blower motor housing. In some vehicles, if you take out the glovebox, it is easier to access, not sure on yours, though.

I hope I was able to help a bit, if you need more, please let me know. Thank you for the question and good luck.