[mrein3]

This topic is sort of funny to me.
I'm in the middle of putting factory AC on a non factory AC truck. I modified the firewall, bolted everything together, put the factory harness I bought in all the spots the factory said I should and here is what I got:
low and medium fan speeds only. Sometimes low with the switch in the high speed position.

I got the wiring diagram out and looked at it closer. In low one of the wires that comes from the resistor block has approximately 3 volts (dropped from 12v by one of the resistors). Medium has 6 volts. In the high speed fan switch position there is a seperate orange wire that gets 12 volts. That wire is split at the relay behind the glove box and is supposed to go to another relay somewhere else. The book says "anti-dieseling relay". Of course that didn't exist on my truck by the time I bought it so I bought a new relay. When I wired it up I split the 12V supply to the factory relay (big orange wire in factory harness) to feed the new relay.

It worked the fan on high but it also wouldn't allow me to shut the truck off with the fan on high!

I'm getting feedback through that circuit back to the ignition. Knowing that GMs of this vintage had a seperate orange wire under the hood with an in-line 20 amp fuse that when blown disabled the high-speed fan, and knowing I had an orange wire under the hood with nothing hooked up to it I used it with a 20 amp fuse to feed the power to the fan when the new relay "closed" the circuit for high-speed fan.

I was bumbed that it didn't work perfect the first time but then I remembered that I didn't have a high-speed fan before I tore the truck apart.

Now that I've stepped back from the project for a few days I'm thinking my fan relay is bad. SOMETIMES when working the fan without my new relay hooked up and the high-speed wire plugged into the GM relay as the General intended, I get low, medium, low. Other times I get low, medium, nothing (as expected). The feedback might be occurring in the 30 year old relay I used from a junkyard truck.

To make a very long story short 70 RatRocket, I think you need to get that anti-dieseling relay hooked back up to your truck. Or eliminate it and use another, brand new relay in its place.