A/C control , no power

[Layne02]

Hi everyone, been a while since I worked on the truck , but was just starting it up other day and turned a/c on and absolutely nothing happened. So, I checked the fuse. Seemed to be ok, didn't look blown.

So then I took dash bezel and got a/c control out so I could get to wires at back. Wasn't sure which wire (s) would carry voltage so tested all. Nothing at any of the wires going to fan selector or to On switch .

Could I be missing something else? A relay? Any ideas?

Oh, its an 87 GMC Sierra

 

[1987 GMC Jimmy]

The three lower speeds are controlled by the resistor, which screws into the evaporator case, and the high speed is mediated by the high speed relay, which is mounted very close to the resistor. I’d check the resistor before I did anything.

 

[Layne02]

So does power gonto the resistor first. What would be the order of checking for power .

 

[hatzie]

Power enters the Heat/AC control head, then the switch, then the resistor or the AC HI blower relay when the blower switch is set to High on an AC truck.

Heat/AC and Heat-only are a sub harness that was installed based on how the truck was optioned.

There's a wire from the fuse panel to the Heat/AC control head. Usually it's brown but check the GM wiring diagrams for your year... You'll find a single terminal Packard 56 disconnect off the main cab harness near where the radio wiring exits the cab harness. I've found Mickey Mouse installations of radio wiring that co-opted this connection or damaged the HVAC power wire with Scotchlok connectors.

 

[Layne02]

Ok, so if thats case should I be getting voltage reading at plug going into back of control head? Im not getting anything. Wiring problems are really hard for me.

 

[84 Shorty]

Ok, so if thats case should I be getting voltage reading at plug going into back of control head? Im not getting anything. Wiring problems are really hard for me.

If you have trouble locating the brown wire that powers your selector switch you could always tap into an ignition hot on your fuse box and run it with an in-line fuse.

 

[Layne02]

Well Alright, got it going. Sometimes its the simplest thing. Found brown wire and rechecked making sure to have a good ground with tester and had power, checked everywhere else and had power, theeeen, just happened to notice wire going to blower motor wasnt pushed on all the way . Pushed it on harder and theres it is, everything back working, thanks everyone!