I decided to replace the blower motor on my truck because nothing happens when I turn on the fan. After I took off the vent cover that exposes the evaporater and condenser I noticed the blower motor plug was melted some. So I decided to stop right there and see if I had power to the plug and nothing. Next I went to the resistor. There was no power to either one of the plugs on the resistor. Next I went to the relay , the relay has power on one of the prongs. So I thought maybe it was the relay, replaced it with a part number I found on another thread from oreillys and still no power at the resistor. Can someone tell me what the chain of power is on this whole process. Which plug is suppose to have power on the resistor and where does the relay come in to play? Thank you

 

Go here:
https://www.ford-trucks.com/forums/1...then-some.html
And here:
1973-1979 Ford Truck Wiring Diagrams & Schematics - FORDification.net
If you go to Fordification, you can use the '76 wiring schematics. The A/C wiring is the same as your '77.

 

I looked at all those before but they don't tell what feeds what. I know how it all hooks up just not sure what is suppose to be hot and so forth. Thank you though

 

Let me rephrase that, I think that one schematic may show what feeds what but I can't understand it lol.

 

Okay, go to page 4 of the '76 wiring diagram, upper left you'll see the A/C circuits. Look at the "safety relay", that's what Ford calls it. Everyone else calls it the blower relay.
The wire that you showed was hot should be the orange one. The one that controls whether this relay turns on or not is the yellow w/ red trace wire, circuit #753. With the A/C on check whether you have voltage at the relay plug. If you don't show any voltage, go to the A/C mode switch (on the A/C control panel) and check for voltage at the orange wire, circuit #181. If you show voltage there you need to replace the switch, this is part of the vacuum control switch and isn't available separately.
This is what the switch looks like.

 

Here is some more a/c help.

1979 F100 Air Conditioning

NORTH COUNTRY SPECIALTIES ®

 

Would you have the part number I can't make out the one on the box. Thank you

 

Ok I have power at the yellow wire on the relay, but that's it. The power goes off when I turn the ac off. You can hear the relay click when I turn it on and off so I think it's good, plus it's brand new.

 

Update

I checked the vaccum control switch and it seems to be working properly. Does anyone know what all wires should hAve power going in to the relay?

 

Boom !! Finally figured it out the orange wire ran to the starter relay ( which the previous owner just replaced) and he had it on he negative side. So after buying a new blower motor, relay, and heater core it was just a crossed wire lol. Thanks everyone for the help.

 

Glad you found it .
Quite often the worst thing that can happen to one of our trucks is a "previous owner".