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Blower motor running,multimeter set on volts place one probe on the blower motor,ground the other are you showing significant voltage like more than .25 volt if so it's a bad ground,do the same on the power side but this time place one probe on the blower motor hot lead one on battery positive lead,again significant voltage showing on the meter means voltage loss prior to the motor. That's called a volt drop test. If it passes volt drop test,it is the blower motor itself or it's normal for a 40 year old truck.
Just making sure I completely understand, so to test for the ground I turn the blower motor on and put the red lead on my multimeter on the blower motor hot wire, and the black lead on something like the frame or negative battery terminal?
Nope, red lead to blower motor case,black lead to ground, now if your meter shows 2 volts then your ground is only supplying 10 volts, it is dropping 2 volts. Do the same on the hot side. Red to motor positive,black to battery positive if your meter reads 3 volts than your positive hot wire is only supplying 9 volts, its dropping 3 volts. Those are not exact numbers but you get the point. If your meter is reading .2 volts then either you expect more of the motor than 40 year old blower motors actually made,or it is worn out. Don't forget to check for trash in the ducts and blower motor housing. All testing is done with blower motor running.
