GMC Truck Forum banner

1 - 10 of 10 Posts

·
Registered
Joined
·
250 Posts
Discussion Starter #1
The electric fan on the front of my '00 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...

IMG00188.jpg


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?
 

·
GMC Owner
Joined
·
55 Posts
I would start by replacing the relay. Its pretty common for those to fail.

#2 in this picture

83294880.gif


Here's a wiring diagram of the circuit. It may help you.

84981162.gif



The auxiliary cooling fan is controlled by the VCM. The VCM controls the ground path for the cooling fan relay.
VCM will turn the auxiliary cooling fan ON when any of the following conditions exist:
ECT above 107°C (225°F) .
A/C head pressure above 300 psi.
Once the auxiliary cooling fan has been turned ON by the Engine Coolant Temperature (ECT) sensor, the VCM will turn the fan OFF when that temperature has dropped about 4°C (7°F) . If the auxiliary cooling fan has been turned ON by high A/C head pressure, the VCM will turn the fan OFF when the pressure has dropped to 250 psi. The minimum ON time for the auxiliary cooling fan is 30 seconds.
 

·
Registered
Joined
·
250 Posts
Discussion Starter #3
I'm not seeing the pictures in your post. Where is the relay for the fan? I saw a relay for the clutch in the underhood fuse panel but didn't see one for the fan.
 

·
GMC Owner
Joined
·
55 Posts
The relay shoud be on the right front corner on the outside of the underhood fuse block.
 

·
Registered
Joined
·
250 Posts
Discussion Starter #5
Oh, ok. I noticed a couple relays hanging there outside the box. I'll go through them tomorrow and see if I can get it going. Thanks a TON for pointing me in the right direction.

EDIT: The pictures just showed up. I know exactly which relay that is. I'll yank it out tomorrow and get it replaced.
 

·
AllOffTopic.com FTW!
Joined
·
8,460 Posts
I checked the fuse box under the hood and one of them says AUX FAN so I'm assuming that has to be it.
Check the owner's manual. The fusebox label is incomplete and may even conflict with the manual. You may find that there are more fuses to check.
 

·
Registered
Joined
·
250 Posts
Discussion Starter #7
Ok I replaced the relay and nothing happened so I checked the old relay (jumped the two pins) and it clicked on perfectly fine.
So I put the old relay back in and was sitting there thinking about the conditions you talked about in your reply... so I unplugged the high pressure harness and jumped the two pins together and what do ya' know the fan kicked on, ran for about 30 seconds then kicked off.

So does this mean there's nothing wrong? I could have swore last summer the fan ran almost all the time (as soon as I hit the AC switch) but should it only run when the engine gets over 225 or the AC pressure gets high?
 

·
Registered
Joined
·
7 Posts
I would like to resurrect this guys thread, cause i have a 2000 Chevy K2500 with a 454, and i'm aware of the factory conditions that have to occur for the cooling fan to come on. Where i live, with the hot weather and stop and go traffic non stop, the A/C doesnt get the chance to get its coldest, so i'm sweatin it out. Is there any way to modify this fan to come on and stay on when the A/C is running? Oh, and the A/C system was recently brought down and recharged cause i wanted it done just for shits and giggles, and the A/C high and low pressures check out.
 

·
Registered
Joined
·
729 Posts
http://shbox.com


check out manual fan switch stuff, i believe it's all the same for 1998 and later GM stuff but be careful and double check yours before cutting or grounding any wires.
the way it's done is 2 wires go to the pcm to control the fan relays. The pcm grounds only the green wire to get both fans on low speed. The pcm grounds both the green and blue wire for fans to come on high speed. So you get a double pole double throw switch and wire it up so one way is low speed, other way is high speed, and in switch in the middle is no affect.
i've done the fan mod on a 1999 camaro i had, and also on my 2002. The 1999 did not throw a check engine light but my 2002 did thinking there was a short to ground for the fan relays because the pcm sees the grounding when it knows it did not command that. A check engine light won't hurt anything but is annoying and the solution is to use 2 diodes from radio shack that are about $1 a piece.
The green and blue fan wires should be next to each other in the harness, and there are fan mod kits from SLP and other vendors for the camaro's and firebirds that come with a plug in harness so you don't have to cut or splice any wires, it plugs into that C100 connector in the pic.

I think the green and blue wires are pins 42 and 33 at the PCM.
 

·
Registered
Joined
·
7 Posts
http://shbox.com


check out manual fan switch stuff, i believe it's all the same for 1998 and later GM stuff but be careful and double check yours before cutting or grounding any wires.
the way it's done is 2 wires go to the pcm to control the fan relays. The pcm grounds only the green wire to get both fans on low speed. The pcm grounds both the green and blue wire for fans to come on high speed. So you get a double pole double throw switch and wire it up so one way is low speed, other way is high speed, and in switch in the middle is no affect.
i've done the fan mod on a 1999 camaro i had, and also on my 2002. The 1999 did not throw a check engine light but my 2002 did thinking there was a short to ground for the fan relays because the pcm sees the grounding when it knows it did not command that. A check engine light won't hurt anything but is annoying and the solution is to use 2 diodes from radio shack that are about $1 a piece.
The green and blue fan wires should be next to each other in the harness, and there are fan mod kits from SLP and other vendors for the camaro's and firebirds that come with a plug in harness so you don't have to cut or splice any wires, it plugs into that C100 connector in the pic.

I think the green and blue wires are pins 42 and 33 at the PCM.
hmm, i believe it would be slightly different for me cause my rig has a clutch fan, and the electric is a back up. i'll check stuff out from info though.
 
1 - 10 of 10 Posts
Top