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If I use a 3 speed fan switch for my car's heater motor, will it make it a 3 speed motor?

I know the switch will add resistance at to make the fan motor go slower, but just want to verify that I won't blow the motor up by adding too much resistance to the motor.

Thanks for your help.

Just make sure the switch is for the correct voltage. The resistors in a 12v switch are approximately double the value of those in a 6v switch. Using the 12v switch on a 6v system will lower all the settings of the switch. The high speed setting will be more like a medium speed in the motor rpms, medium more like low, and the low speed setting may lower the voltage to where the motor barely moves if it does at all. That situation could cause the motor to heat up a bit but unless it stayed that way for hours, most likely no damage. The current would be flowing in the resistor though and that could get quite warm.