Where does the AC condenser fan draw signal from?

[Avery4jc]

The electric fan on the front of my k3500 that's supposed to come on with the AC isn't kicking on anymore. I checked the fuse box under the hood and one of them says AUX FAN so I'm assuming that has to be it.
I checked the fuse for continuity and it's good. The rest of the AC system works perfectly.
I unplugged the fan and ran power straight from the battery and it kicked on and ran like a champ so the fan is good.
I traced the harness and found this plug...

If you unplug it and energize the left side the fan comes on so the harness from there down to the fan is good, including the ground. I tried to trace the right side of the plug but it's an absolute cluster of wires and almost untraceable without tearing apart all the looms.
I did try to do a continuity test between the two prongs in the fuse box and didn't get anything so at this point I'm trying to figure out where that right side of the plug gets the signal from so it knows when to turn the fan on?

 

[chevydan3]

For the control circuit, my Haynes book shows a dark green wire coming out of the "heater and a/c control module" then it splits and one dark green goes to the vcm, the other dark green goes to the "recirculation door motor." The recirc motor has a brown and black(ground) coming out of it too. The brown splits a couple of times and leads back to the "htr a/c fuse 25a." It says it is fuse #12 in the dash fuse block.

*Edit*

For the "aux fan motor" it shows a red going to the "aux cooling fan relay." 2 oranges and a dk blue also come from the relay. One orange goes to the "a/c comp mini fuse 10a" in the underhood fuse box. The other orange goes to the "aux fan mini fuse 30a" in the underhood fuse box. The dk blue goes to the vcm.

Hope this helps.

 

[Avery4jc]

I'll double check that fuse tomorrow (I know which one you're talking about since I pulled it this afternoon).
A guy on GMFS said it's not uncommon for the relay itself to stop working. Apparently it's the relay hanging off the side of the under hood fuse panel so I'll pull it tomorrow and test it.

 

[Thunder]

Yeah probabally the relay. Common for them to go bad. That is my first thought.

 

[Avery4jc]

I replaced the relay today and nothing changed so I put the old one back in and started thinking about the conditions at which the fan kicks on. According to a guy on GMFS it kicks on when the coolant gets above 225* or the AC head pressure exceeds 300psi. So I jumped the two pins in the harness together where it plugs into the high side of the AC and sure enough it kicked the fan on.
So I guess it's good to go but I could have swore that fan ran all the time last summer.

 

[lectric80]

It may have run all the time last summer, and has now bled a bit of refrigerant so it no longer gets to 300 PSI. If there is a small leak it could easily bleed down a bit and prevent proper pressure.