I saw this in the RockAuto magazine. They actually carry new blower resister assemblies in their catalog. Generally under $40 and a bit easier to find than at your local junk yard.
Blowing Air at Only One Speed
If the blower motor for the heat and A/C system starts blowing air at only one speed or stops blowing altogether, then there is a good chance that the blower motor resistor has failed.
On most cars the blower motor is a simple electrical motor with only two wires coming out of it. The speed of the blower motor and its attached fan is determined by how much electrical current comes through the wire.
The resistor(s) in the blower motor resistor act like a valve to control the flow of current to the motor. When the fan switch on the dash is at its lowest setting, then the current is routed through the highest ohm resistance in the blower motor resistor. The resistor uses up most of the current before it gets to the blower motor and the fan turns relatively slowly.
Move the switch on the dash to the highest fan speed and typically the current is not routed through the resistor, but goes 100% to the blower motor so the motor and fan operate at maximum speed. This is why blower motors will often operate at top speed even if the blower motor resistor has burned up.
The blower motor resistor is usually mounted to the plastic heater case in the dash or under the hood. The blower motor resistor turns the current flowing through it to heat and the air flowing through the heat & A/C system case helps cool it down.
The photo is of the very simple blower motor resistor found in my wife’s ’87 Mazda. The maximum resistance and lowest blower motor speed occur when the current is routed to run the full length of the coiled wire resistor before getting sent out to the blower motor. The resistor wire looks like the filament from an incandescent light bulb and it can burn out similar to a filament. Where the resistor wire breaks determines what if any speeds the blower motor will still operate at.
Find blower motor resistors for your vehicle by ACDelco, Airtex/Wells, Dorman, Four Seasons, Motorcraft, and Standard Motor Products under "Heat & Air Conditioning" in the RockAuto.com catalog.
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Guest Message by DevFuse
Blower motor resistor
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