I just bought a 1990 F350 w/ heat & air. The heater fan runs the same speed no matter what position the switch is in. It seems to run slow like maybe low speed. The switch feels normal. Any thoughts on what the problem is? Thanks, Dave

 

I wonder if the blower resistor is shorted, causing all positions to go through the low circuit.

 

It would be a bad motor.
With a short or an open in the resisters you would still have high speed.

 

Yes. Your probably right.