It sounds like the blower motor resister is bad. How it works is this. When you select a speed, power is sent to the resister. In the resister are different wires that cut the power down (hence a resister). If they go bad inside, the resister wires melt down and no power goes through. However, the direct wire that supplies 12 volts directly to the motor and runs it on high usually is still good. As a result, the blower motor only works on high.
It isn't an expensive part.
Let me know if this helps you or if you have other questions.
Joe
PS: Sorry for being so wordy. I think it's easier to understand a problem if one understands how things work.
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Thursday, September 15th, 2011 AT 2:04 AM
