Blower motor resistor keeps going out?

My 2001 mustangs fan for the AC and heat only works on the highest speed but not any of the lower speeds. I replace the blower motor resistor which fixes the problem but within 2 weeks it burns out and again, only the highest fan speed will work. I can’t afford to put a new switch on every 2 weeks. What’s causing the blower motor switch to burn out so often? I notice the dial to adjust the vent air from hot to cold feels burning hot if that’s any help. Also I’m doubtful that the blower motor itself is bad because if that were the case wouldn’t the fan not work at all? Especially at the highest speed

Guess number 1, a blower motor drawing too many amps.

So just replace the entire blower motor unit?

Barkydog is correct; the blower motor is drawing too much current. Odds are if you connected an ammeter to the blower motor you would likely be shocked at the amount of amps being pulled by the blower.

The blower motor is not that much and the blower can appear to work fine even while drawing an inordinate amount of amperage.

Some years ago my wife came home one day and said the blower in out Lincoln had blanked out temporarily. When checked, it appeared to work fine and made zero noise. With an ammeter connected that blower motor was pulling 27 amps on the HIGH setting. That is a godawful amount of current even with a 40 Amp Maxi fuse in place. With a 27 amp steady pull one can safely figure (as with anything electrical) a 50 amp surge current.

A spinning motor generates a back emf (basically acting as a generator). The faster it runs, the higher the back emf. The back emf reduces the current from the startup value of the motor. If a motor (for some reason) is restricted in speed, it will be drawing a higher current than designed for at that fan speed setting. So, if the motor has a bad bearing or the flow is restricted sufficiently, there will be an excessive current, and your resistors are acting as fuses. Pull the motor and check for anything that might be slowing it.

Actually centrifugal fans do the opposite. When either inlet or outlet is restricted, they don’t move any air and with a load reduction, the speed goes up and the current goes down. Block your vacuum cleaner hose to see the effect. The issue with the way it is implemented in the car, the airflow is being used to cool the ballast resistor block and so when that disappears, the resistors overheat and burn up.

That’s one of the fan laws.


Well, I learn something every day! Is the typical squirrel cage fan a centrifugal fan?

Fast or slow? For our OP, are we saying that a blockage is expected, and the fan is running fast and the resistor is blowing because it is uncooled? Or, do we think the motor is running slow so the current draw is high?
Maybe this is the old mouse-nest-in-the-airflow problem. I note that the OP has not really stated reduced air flow.

blower motor could be the culprit, but also check the wiring at the resistor. If it looks burnt or melted at all, get a new connector and wires. I have seen bad wires take out a new resistor.

A fan that takes air into its center and then flings it out is a centrifugal fan.

So yes.