Twist it however you want. And you should have said Pre-03.

 

I have OEM fans with an OEM harness on my 2001. I just added an additional relay to the AC compressor clutch wire to convert the positive voltage from the clutch to a negative signal to trigger the relay harness. I know that sounds kinda janky, but its been like that since 2011, and has given me absolutely ZERO problems ever since I installed them. They have held up just fine in this hot *** triple digit Louisiana summer heat for five solid years now, this is now the sixth summer.
Here is my install thread.
https://www.performancetrucks.net/fo...-chevy-484892/

 

Theres a lot of ways you can do it. I just made a harness for a guy that has a 2000 Silverado, but has the newer 2002 pcm OS with the dual fan controls. Those years did not come with a AC pressure sensor, but if you want full fan functionality based on AC pressure and coolant temp you have 2 options; add the AC pressure sensor in there and update the PCM, or make a harness that incorporates some type of AC control.

I did the later option using 4 relays. It has fully PCM control of high and low speed based on coolant temp, and AC control for high speed (3 trigger wires). Installing the pressure sensor could take the place of the 4th relay.

PCM low on, AC off - both fans on low
PCM low + PCM high on, AC off - both fans on high
PCM low + PCM high on, AC on - both fans on high
PCM low off and high off, AC on - 1 fan on high
Everything off - fans off

 

That's a LOT of extra wiring to accomplish basically the same thing. I just added a relay to the AC clutch to trigger the factory harness. So now my fans works as follows.
Coolant temp above 210° - both fans on low
AC on - one fan on high
Coolant temp above 210 AND the AC on - Both fans on high

This was very simple to wire up, took me less than an hour to solder everything up, install it, plus install the fans. And its been working flawlessly like this for 5-6 years now. It has no problem keeping my truck cool in these triple digit Louisiana summers.

 

That's the exact same as mine, except I didn't have a stock harness to start from so I had to make that too. Be careful you don't back feed your ac trigger wire to the compressor, you don't want the fans to turn on the compressor.

 

Yeah, I'm good. With that extra relay on the AC clutch, its basically relay isolated. The fan harness at most could only back feed to terminal 30 on the relay that's on the AC clutch. No way any current is getting through unless the coils are energized. The AC clutch energizes the coil with a positive trigger when the clutch is engaged. Once that coil is energizes, it closes the relay to connect 87 (ground) to 30 which is the negative trigger wire on the factory harness. The only purpose of that additional relay is to convert a positive trigger to a negative, but at the same time it also isolates it from any back feed when the clutch is not energized.