I've replaced my fan switch, resistor, and relay on my truck. The fan is only getting 7 volts. The fan works fine on straight wire. I want it all to work originally since I've worked so hard on it. Seen on a old forum where it was a ground issue. Haven't tried it yet but, I've had to replace everything in the dash because of incompetence and winter is coming soon. Really need heat! Any help would be appreciated.
The resistor drops the voltage, try by passing it but first, take the blower our of the heater and spin it by hand ~ there should be very little resistance and it should keep spinning at least two 360 degree turns after you let go of it. These were made with oilite bronze bushings and often can be taken apart and dribble some light machine oil into the bushings, let sit over night and do it again , there's often a felt ring that holds a bit of oil so it keeps working for decades longer . If the brushes are shot, don't turn the commutator, just clean it and replace the brushes, measure them and fine new ones cheaply on E-bay or Amazon or in your local tool supply house if they're any good . Clean carefully and work slowly and gently over a clean, large light colored towel . There are usually very delicate flat fiber thrust washers, take care not to damage them, I find new nylon ones in the small parts drawers in big box home hardware stores . Adding 14 gauge (not thinner) ground wires to well cleaned up and polished (toothbrush sized wire brushes) studs on the motor to the cab proper and also make sure there are cab to frame ground wires/ straps, there should be some for each headlight too .
