I am diagnosing an issue in an automotive HVAC system and have been testing the blower motor. I would expect somewhere approximately 8A to 15A of current draw. I have test wired the motor with 14AWG wire, a switch that is supposed to be rated to 30A, and a 20A fuse. It seems to blow the expected amount of air and not blowing the fuse, but the spade connectors at the switch and at the motor are getting uncomfortably warm to the touch. I measured resistance across the motor's terminals and I am seeing 5.1ohms. Wouldn't this imply that the motor should only draw 2.4A at 12v? Note, I do not currently have access to tools to directly measure the draw when running.
The resistance of a DC motor determines the maximum current during starting. As soon as the rotor starts to turn, it generates a voltage called the counter electromotive force or back EMF. The back EMF opposes the supply voltage and controls the running current. Since the maximum starting current is much higher than the normal running current, measuring the resistance doesn't tell you very much unless you have other information about the motor.
Motors can safely run uncomfortably warm to the touch. If a blower motor is designed to move air over itself when properly installed, it may run warmer than normal when tested on a workbench. Here again, it is difficult to tell if the motor is running too hot without specific knowledge of the motor.
Blower motors in a car often have a speed switch that switches sections of a resistor in series with the motor for three or four operating speeds. Failure of the resistor is probably the most common failure. The resistor element can open up and cause the motor to run only on one or two speed settings or not at all. If that or a broken switch or connection is not the problem, the motor is probably bad. If the motor runs and does not make a lot of noise, it is probably ok.