AC/Heater fan won't work on High

My 91 Chevy Astro doesn’t like to blow air on high. The AC/Heater fan works fine at lower settings but when in high it doesn’t work at all or will run for 30 seconds and then stop. I’ve replaced the switch in the dash, but that didn’t help. Any ideas?

You need a new resister block. It’ll be located on/in the ductwork by the blower motor. A Haynes repair manual will show you where it is.

  • mountainbike

[b]It isn’t the resistor block. The voltage for the fan high speed by-passes the resistor block.

This sounds more like a problem with high speed fan relay. On the low speeds, the fan relay routes the voltage thru the resistor block. When you switch to high fan speed, the relay switches over to supply full voltage from the battery to the fan motor.


Tester is right. I didn’t think through the question carefully enough. My mea culpa.

Now if you’ll all excuse me while I go retract my foot from my mouth…

  • mountainbike

Star… you manage to come up with the wrong answer again. Relays are not used to switch fan speeds, a resistor block is.

Plus there are NO electronic commuted motors in a automotive application.

This vehicle uses a resistor block to switch fan speeds. No relays involved in the lower speed circuit.

I can only guess after reading the other posts that a wire break or loose or corroded connector may be causing the problem. Dropping resistors, inside the fan plenum, can sometimes get wet and then corrode. It’s worth a look.

A relay should not be needed for switching to high speed. This can be handled with the selector switch and would be less expensive and less complicated for the mfr.

Tester is 100% correct.

I had this problem last year. The high speed relay is on the resistor block. It is located undr the hood on the left side of the firewall facing the vehicle. The replacements are not a perfect fit and you will have to file a bit on the plastic opening before the new resistor block will fit. Hard to see and will have to estimate by feel.