LS Swap Controlling Electric fans with AC

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LS Swap Controlling Electric fans with AC

Post by brandtjohnd » Wed Jul 14, 2021 12:07 pm

This post is for informational purposes only. Hopefully it will help one of you down the line when/if you need to control electric fans when the A/C is commanded on. I've recently completed an LS Swap on my 89 Grand Wagoneer. General details:

Motor = Gen III 5.3 Liter LM7
Trans = 4l60e Late version
Xfer = NP229 (Adapted using Novak converter)

I was able to integrate the new (LS) 134a compressor and the existing A/C lines in the Wagoneer to get A/C working. The issue was that my version of PCM does not control fans when A/C is requested. This causes an issue because Hi side A/C Pressures can build and cause problems is air is not moving over the condenser. The solution was to use a relay to turn on the fans when the A/C request wire gets 12+ volts.

I used a 4 pin automotive relay to ground the fan circuit when the A/C has been requested.
Pin 30 = This pin is grounded to chassis Ground
Pin 85 = This is the pin that connects to the wire coming from the A/C in the cab that get 12+ when A/C is requested.
pin 86 = This pin is grounded to chassis Ground
pin 87 = This connects to pin 42 (Fan 1 control) on the LS ECU.

How it works:
The ECU grounds the Fan 1 Control pin when it wants to turn on the fan (typically based on coolant temp). In this case we are passing grounding this wire when A/C is requested. So when A/C is requested and pin 85 sees 12+ and Pin 86 has ground, then relay is triggered. When triggered, the relay connects pins 30 (Ground) to 87 (Fan 1 control) and the fans spin up! This simply taps into that ground wire for Fan 1 control and allows us to retain the Fan on/off based on temp via the PCM.
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