Ok so i got a used 3 relay nelson harness for my perma cool dual 14 in fans. Installed grounds, positives, pcm pin 42 and tapped into the ac green wire. Wired the fans, and the pin 42 in turned on. Problem started when i turned on the truck and nothing worked. Waited for temps to raise and still nada, nothing. Turn on the A/C and one fan works on on high. So i start testing with wire tester and when grounding the tester and tapping into the pin 42 the other fan turn on on high. Never both on ob high or low speed. Checked relays and all of them work. What in the world is going on. On its being installed into truck in sig, 03 silverado

 

Here is how the harness works:

1. Ground PCM wire - both fans run at low speed
2. 12V to A/C wire without grounding PCM wire - one fan on high
3. Gound PCM wire and put 12V on A/C wire - both fans on high


Did you have the PCM reprogrammed to run the e-fans?

 

according to allens email response yes they are and the initial tune i recall pin 42 turned on

 

Make sure the contact you put into the PCM connector is seated all the way. I couldn't get mine to turn on for the longest time, then I found that the contact wasn't seated all the way. Use a paperclip or something tiny like that to wedge in there and push all the way in. You should feel it click twice I think, and you will be able to see it on the other side of the connector.

 

yeah i got that snapped into place, what got me confused was when I even was testing the pin the pcm disconnected the yester would activate one of the fans. I am still way confused!!!

 

make sure its the right green wire, allen helped me installed mine when I got my tune and I believe it was doing the same thing you said, so we went back into the wiring and believe we had it on a different one...

 

ur suppose to tap into the dark green one right, not the light green?

 

think i used the light green...it was the same exact color wire as on the harness.

 

i tapped into the same colored wire.

 

It is the dark green wire , will send 12 volts when A/C is on (clutch cycles) , when tested with Ohm meter . As said "same color as in harness" .
Here is a mod that I had to do to mine to get it to stop pulling major voltage when fans were triggered by A/C.
Not a real good drawing , but I used the Nelson 3 relay harness moved a couple of wires and added the 4th relay for my pin 33 (high temperature) 2005 pcm trigger wire , can be used for switch or temp trigger .











GM