This does not always work, especially if the bearings are getting tight. When it does work, your diagnosis is done and all that is left is to replace the motor.
When it does not work, we will need to step back and start with a proper diagnosis. The place to start is by measuring the voltage to the motor. The exact voltage values you find can vary and appear to be somewhat misleading. The important points are you must find something, (as opposed to 0 volts) on one wire, and 0 volts on the other wire, and to be valid, you must take those readings with the motor plugged in. Most of the time you can pull the plug partially apart just enough to stick the voltmeter's probe in there to take the readings. I really disapprove of poking through a wire's insulation to take readings. You can unplug the motor, then check for voltage on the wires feeding the motor. Most commonly, with the motor unplugged, you will find 12 volts on one wire, regardless of the speed you have selected, and 0 volts on the other wire. The problem is that doesn't check the ground wire. With the motor plugged in, there still must be 0 volts on the ground wire. If the ground wire is open, (bad / cut / corroded), voltage will appear on it, but only when it gets there through the motor.
A different way to approach this is to use a pair of jumper wires to apply twelve volts to one wire on the motor and ground the other wire. Polarity is not a concern. If the motor runs, it is okay. If you have the polarity wrong, the motor will just run backward. If it runs, we will need to work backward to the heater controls and resistor. If the resistor for speed selection is defective, on almost all cars the fan will still work on only the highest speed. That alone proves the motor is okay. If it comes to this, you will need to tell me if you have simple levers and switches for heater controls or a more complicated computerized control panel, then I will dig up a wiring diagram to figure out where to go next.
Tuesday, November 22nd, 2016 AT 4:08 PM