Blower fan only works on high speed — what to check
A bad blower motor resistor can cause a blower fan high only works on high speed condition
The blower motor resistor is what’s responsible for giving you three blower speeds. The resistor block contains three different high resistance coils. Think of these coils like a filament in a light bulb. The coil resistance causes them to heat up, thereby reducing the amount of voltage that can flow to the blower motor. When a resistor coil burns out, you lose that fan speed. A blower motor resistor can fail in several ways; since many are designed in series, if one resistor fails, you loose speeds 1-3. In other designs, the failure of 1 resistor means you lose only one speed. However, most car makers bypass the blower motor resistor completely when you move the fan speed switch to high. If your blower only operates on its highest speed, chances are you have the series type of blower motor resistor. This is the most common failure mode.
Find the blower motor resistor
To repair the problem, simply replace the resistor block. It’s usually located in the ductwork (to keep the coils cool) below the glove box. Unplug the electrical connector, remove the screws and replace the unit.
Replacement blower motor resistor blocks are usually available from any auto parts store or the dealer. Prices range from $15-$40 dollars, unless you have a variable speed blower motor. Those require a new solid state controller. You guessed it, they cost a lot of money.
A clogged cabin air filter can cause blower motor resistor failure
If your vehicle has a cabin air filter, check that before putting everything back together. A clogged cabin air filter reduces airflow through the ducts, increases current draw on the blower motor and can cause blower motor resistor overheating and premature failure.
If you replace the resistor and it burns out again, check the current draw on the blower motor. Even a few extra amps is enough to melt the plastic connector at the resistor. So check for signs of scorching and melted plastic. If you find that, buy a new connector pigtail from the dealer and splice it into the harness. Replace the blower motor and the resistor and you should be back in business.
© 2012 Rick Muscoplat
Posted on by Rick Muscoplat