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I have an issue with my heater fan. When I first got the truck the fan would come on sporatically. It would come on and stay on even though I had it in th eoff position. I replaced the fan switch ( one the push controls go into. No the fan worked for a whiel on three speeds. No it wont work at all. I don't know if it is the fan switch lever, or something in the wiring. It did work fro a few days no it wont come on at all. Is my 3 position fan speed switch bad or what? really need a diagram of the wiring for the heater controls. I found one but it did not cover the fan cand geater controls. My truck has a A/c with it also if that matters.

short of loose connections, fuse, bad ground and a bad blower fan motor, there is a resistor that sits on top of heater box. If resistor is bad, blower will work on high only.

click to enlarge wiring diagram of 70 with ac

Thinking more about your problem, the heat, ac, def, off switch controls the blower fan switch. Most likely your problem if blower doesn't work at all and your fuse is good. Put a jumper wire between the brown and light blue wire on back of selector switch connector, behind the heat and defrost selector. this will bypass the selection switch and send power to the blower switch.

The 3895790 switch is an active number if it should be the problem. Rock auto has them as well as other suppliers. (Just checked ,Item currently not in stock at rock auto)