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.

  • \$\begingroup\$ Rather than guess, you need to measure the current when the motor is running. There may be an error in your resistance measurement (meters sometimes are defective), and also there are the issues of back EMF when the motor is running and the phase of the contacts in the motor when you measured the static resistance. \$\endgroup\$ – Digiproc Mar 26 '19 at 10:03
  • \$\begingroup\$ I measured the resistance with the motor off. Can it safely be measured while running? \$\endgroup\$ – pierce.jason Mar 26 '19 at 10:06
  • \$\begingroup\$ UPDATE: Changed out the switch for a new 35A switch, upgraded wiring to 10AWG, still have 20A fuse. Connections at the switch "sometimes" get hot now, but not all the time. Fuse still hasn't blown even if the connections stay hotter-than-comfortable-to-hold even if hot for over 20 minutes. A friend mentioned possibility of fan bearing being wonky, due to the inconsistency of the heat behavior. But if this was the case the amperage would be higher and should blow my fuse. I'm wondering if the hot switch contacts could be caused by some electromagnetic feedback? \$\endgroup\$ – pierce.jason Apr 15 '19 at 1:49

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.

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  • \$\begingroup\$ The motor its self seems to be fine. The question about temperature is regarding the heat at the connection terminal on the motor and both terminals on the switch. I understand that startup current is going to be higher than running current, but my resistance measurement only shows 2A(while running current expected to be ~10A). \$\endgroup\$ – pierce.jason Mar 27 '19 at 4:06
  • \$\begingroup\$ As suggested by @Digiproc, there may be an error in your measurement. If the motor normally draws 10 amps, the resistance can not be 5.1 ohms. Something like 0.25 ohms would be more likely. The fact that the motor seems to run properly with the blower attached indicates the motor resistance is likely what it should be. However it is impossible to properly evaluate the motor without measuring the motor current. \$\endgroup\$ – Charles Cowie Mar 27 '19 at 13:15

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