Heater/AC fan working only on high

[grebus1]

The heater/AC fan on my Dad's '74 Burb is only working on high speed. He has checked the fuse and recently replaced the relay. What else should he check? Could it be the fan switch itself?

[NeCrOmAnCeR]

Make sure the switch is getting power from the fuse block, and if it is, also check the resistor block located in (on) the air box in the engine compartment. The resistor block consists of two resistors (made of Ni-Chrome type wire) and is bolted to the airbox so that the wire is in the airstream to keep cool. Depending on the blower switch position one or the other wire will be in series with the blower motor itself (medium and medium-high), if the switch is set to the lowest position, both wires will be connected in series. Usually when it goes bad the user will notice that the blower only works on medium or medium-high and replaces it. So it would be my quess that the switch or wiring is faulty.

[1FaastC10]

your resistor is shot, replace it, and you'll have fixed the problem.

Originally Posted by NeCrOmAnCeR

Usually when it goes bad the user will notice that the blower only works on medium or medium-high and replaces it.

not true, when on high the resistor is actually bypassed, and the blower motor is getting a full 12 volts, when on lower speeds the voltage is run through the resistor, and produces a lower voltage, which slows the motor down. if the resistor is bad, which it is, you will have no speeds where the resistor is used, only a direct connection as there is in high mode. as i said before, replace the blower motor resistor, and your problems will be solved.

[grebus1]

Thanks guys, I'll pass that on to my Dad. It's good to know there is as much knowledge and experience on the '73-87 board as there is on the '67-72 board!

[NeCrOmAnCeR]

I agree that the resistor is something to check as I said in my earlier post. But maby I did not make myself clear, Yes, high is a direct connection, created through the relay, grebus1 said this has been replaced and is working fine. The other three speeds: Low, med, med-high are indeed obtained through the resistor. As I said, the resistor is two different resistors in one. And normally when it goes bad, the owner will notice that he or she has lost low and one of the other speeds and replace the problem before the other side of the resistor has gone bad.

Sorry for not making that clear. The resistor is very easy to get to and you can trouble shoot the switch from the resistor.

Good luck!