99-02 Truck E Fan Circuit Mod

Hi All,

Like many with a gen 3 truck or SUV, I wanted to add E fans to my setup. Went with LS1Fans for the harness. Of course, the only way to do that is either tap the AC wire (what I did) and fans will run even at highway speed with ac on, or do the two wire ecm setup and spend another 100 bucks on a new OS.

Well the AC tap has been working great. Fan rarely turn on low, better throttle response, better mileage etc... But what has always irked me is the inability to turn off the fans at highway speed with this e fan setup.

After a lot of time spent staring at the stock wiring diagram. I came up with basically a hybrid of the two styles of e fan harnesses since I'm too lazy to wire in an ac pressure sensor.

Attachment 120658

By adding one extra relay, I believe the functionality would work like this.

AC on + temp below Low fan threshold = One fan on high
AC off + temp up to low fan threshold = both fans low
AC on + temp up to low fan threshold = one fan on high
Temp up to high fan threshold = both fans high regardless of AC

This will also allow for the fans to be turned off over a certain speed since the ground for the ac relay signal is the low speed fans, and the low speed fans are set to trigger with ac activation in the ecm.

In addition, I had a previous version that had a diode between the extra relay and the high speed trigger wire. I was able to do away with that by utilizing the 5th pin on the relay. Turning the ac on removes the series/parallel switching ability that the high speed fan trigger has from the factory, but it can still trigger both fans to high when the ac is off.

Is this perfect? Heck no, especially since you can add a pressure sensor. But that costs money, and so does licensing another OS.

What do y'all think?