I am at a loss. 1989 camaro with factory air. My cabin blower motors runs on 1, 2, 3 settings however it will not run on 4 (high settings). It doesn't matter if I am on cooling, vent or heat. What would cause this? I have ohm out the switch on the dash and each setting has continuity when turning the dial. I checked voltage to the blower fan motor and I get 12 volts on settings 1, 2, 3 but when i put it on 4 i get no voltage. Can someone lead me in the right direction to figure this out?

 

If you look at the upper right hand portion of this wiring diagram, you will see that the highest speed is run by a relay, instead of the multi tapped resistor that runs the lower speeds. It's highly likely that something is amiss with that relay (which is located under the hood, in the area of the blower motor) .

 

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