Help!- No high speed on blower motor

[70 RatRocket]

My 70 has factory a/c. I have low and medium speeds on the blower but no high speed on a/c or heat. When I put it on high it drops back to a low speed. I have replaced the relay and the fan switch. There is a feed to the relay on the single orange wire when I turn it on high, even tried another relay with the same results. What should I try next? Thanks for any help. Dave

[fast71]

loose ground???????

[Longhorn Man]

Well, if the ground is good, then i would look at the resistor.

[O'l Buck]

I'd check the switch, the high speed shouldn't use the resistor and is basically an open line from the switch to the blower, so it could be a burned wire on the resistor plate.

[Jemost 72]

1970? When u replaced the relay, does that mean u replaced the high speed fan relay behind the glovebox?
I had this problem when I had my '70..didnt know that relay was there til it was pointed out.

[70 RatRocket]

The relay I replaced was the one behind the glove box. Would a bad ground make it not work on high but I would still have low and meduim speeds?
O'l Buck, not sure what you mean about a burned wire on the resistor plate. Do you mean at the plug? Thanks for the help. Dave

[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.

[71Chev]

there are 2 resistors on a board inside the box under the dash. If you look at the bottom passenger side of the heater/ac box there should be a little flap door you can open for more heat on the passengers feet, you can see the resistors through that hole. I can't remember for sure if the 70 and older have that flap though.

Is the a/c still stock and operating? Are you using the anti dieseliing relay? If not try unplugging it. It should be mounted to the bottom edge of the dash below the cig lighter.

[Terry Peerson]

I had this problem also. After I traced down all of the wiring and replacing every stinking relay and switch on the system, I discovered that the wire that supplied power to the high-speed was the wire that went to the cigarette lighter. I hooked it up and it has worked perfect ever-since.

[Huck]

Terry may have the issue nailed. My 72 had all the classic symptoms discussed---low, med--no high. Changed switch, relay, resistor, redid relay ground etc. Then noticed this big orange wire that was supposed to be hooked to the rear of the cig lighter. Once hooked up with a new fuse---high speed fan works as designed.
Appears them little electrons go round and round in them wires till they hit something they like to work. That's probably why you can add power by straightening out wires so them electrons don't get dizzy goin in circles etc.
Seriously, check that cig lighter fuse, grounds etc. Good luck-Huck

[picklito]

I just rebuilt the AC on my 70, including many "back to stock" repairs to the wiring harness to undo 45 years of bad Mickey Mouse repairs. Everything works great but I have the infamous low-medium-low syndrome. Oddly, I have the cigarette lighter unplugged so I'll have to try that and see if that fixes it.