[79TRANSW72]

Can anyone tell me why when my blower switch is set to high blower is off, when set to med blower is on high and when set to off blower is on low. Any input would be appreciated.

[tjs72lemans]

On my 72 Lemans, it was the high speed blower relay. Mine was on the firewall passenger side. It was faulty and acted the same way.

[rolling money pits]

I believe, on AC cars, the blower never shuts off. There is no “off” setting, it’s for keeping air circulating through the whole system so mold doesn’t build up?

Non AC cars did have an off setting for the blower.

So what you’re most likely experiencing is what was mentioned above, the hi relay is bad, or something in that particular circuit.

[Goatracer1]

Also for high speed there usually a fuse at the back of the alternator.

[VCho455]

GM commonly used two power sources for AC systems. One for low and medium blower speed as well as supplying the rest of the AC system. Then a second separate power source for the blower motor when high speed is selected.

This was done to reduce the amperage seen at the switch when running the blower on high. The blower motor on AC systems turns at a higher RPM and it generally has a larger fan. Both of which increase the amp load.