Help - Recreating Fan Wiring Diagram Using Derale Controller & GM Stock System

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Joined Jun 24, 2009
Hi everyone,

I was hoping I could tap into some electrically savvy users for this electrical issue. Recently, Derale redesigned one of their Adjustable Electrical Fan Controllers. They switched the unit from being a POSITIVE switching circuit to a NEGATIVE switching circuit. One of the camaro enthusiasts who is no longer with us, designed a wiring schematic that allowed the GM stock fan controller to work in conjunction with the Derale system. Recently, Derale changed the circuit and well a bunch of us who have burned out relays now need a new schematic.

The problem as we see it so far. The GM Stock cooling system is always grounded. The ECM grounds the Primary Fan Relay, which energizes the relay, which then closes the circuit sending power from the battery to the primary fan. The Derale system now wants us to hard wire 1 wire to the positive of the battery, and the negative grounding relay to the negative. In effect, I would have to cut the GM ground wire (disabling it) and run the derale (-) to the the fan in addition to running battery (+). The situation is somewhat complex but I am sure someone with electrical knowledge has a better idea.

Is there a way to safely change a GROUND switching circuit to a POSITIVE?

Below is the original How-To tutorial from

Tech Tips
Dual Circuit Cooling Fan System

Installation of the Derale Adjustable Fan Switch (DAFS)

If you are here you already know why. You want to have more control of your engine temperature since the factory GM default cooling fan temperature set points are potentially to high resulting in overheating and power loss in severely hot climates and under AC operation.

As suggested by 3rd Gen Member "Willie" in his article]GM Cooling Systems ideally there would be a couple of times you might want to revert to the stock GM settings:

1. During vehicle emissions testing.

2. During extreme cold weather conditions for purposes of heater and defrost operations.

Additionally, if you could leave the GM fan system intact, it would provide a nice backup in case of DAFS component failure.

Please read definitions under Auto Electric Basics to gain a better understanding of the terminology I have used in this article. Also, you might consider doing this mod in conjunction with other Cooling System Mods recommended.

Here is a listing of the parts you will need:

Derale Adjustable Fan Switch (Part #16759)
SPST 30 Amp Relay that is Normally Closed
SPDT 30 amp switch
Extra Wires (match gauge and color to Derale unit)
Ring connectors (for 12-14 and 16-18 gauge wire) 3/8" and 1/2"
Heat shrink tubing.
Solder gun and solder.
GM plastic split conduit (Optional)

To the left of the gray centerline is the stock GM system, to the right is the DAFS and its component additions. The beauty of this system is that your stock GM system will still be functional and you will be able to change between the two cooling systems at the flip of a switch.

Derale suggests that it replace the stock GM system and use the following wiring instructions:
Red Wire to attach to the positive (+) side of battery.
Orange Wire to attach to positive fan lead.
Yellow Wire to attach to a switched ignition source (Hot in Run).
Green Wire to attach to positive wire on AC clutch.
Black Wire to attach to good chassis ground.

We won't do that.

We will alter this configuration as follows:
Red Wire taps into GM Hi-Amp hot wire from fusible link.
Orange Wire taps into GM Hi-Amp B/R (+) wire running to fan from relay.
Yellow Wire taps into GM hot in run wire coming from 20 Amp source from fuse box.
Green Wire attaches to one of the toggle switch post.
Black Wire attaches to the out side of the hi-amp output of the relay.

We also add:
Blue Wire (you choose color) from switch terminal to positive side of lo-amp relay input.
Red Wire (you choose color) noted on right side of graphic goes to fuse box and supplies common (+) lo-amp voltage to toggle switch.

Please note: you may choose Hot in Run or Hot all Times, but be cautioned in Hot all Times your fan can run with car switched off. The only reason for providing this option is for drag racing purposes to cool off engine in between runs.

It is recommended you hook this lead to a Hot in Run source to prevent unexpected battery drain.

Theory of Operation

Since the GM system kicks on at about 234° and the DAFS is factory pre-set at 170° (totally adjustable ±) the best that could happen is the GM system never again kicks on. It is a moot point that the GM fan system is concurrently hooked up because the DAFS will over ride it and control when the fan operates.

Only in the event of DAFS failure or by your choice (manual over ride switch) would the GM system be deployed. Additionally by using a normally closed relay and a toggle with the middle position (off) the DAFS is the default system and it is with no current running through the switch and the new relay not energized.

This should provide a more stable, dependable, safer circuit. The relay requires a (+) & (-) lo-amp source to energize, and when it does it breaks ground to the DAFS disabling it and returning your system to the stock GM operation.

Lets look at what happens when we toggle between different switch positions:

Function 1: Flip the switch one way (red wire):
Power from a fuse box source goes into the switch, out of the switch via the blue wire, into the positive lo-amp input of the relay, energizing solenoid, breaking ground (disabling) the DAFS for stock GM operation
Function 2: Flip the switch the other way (same red wire):
Lo-amp, (+) power is routed to green wire of DAFS which results in manual fan operation mode. Since the GM system was never disabled your AC clutch operation works as it always did, allowing you to use the AC operation green wire of the DAFS for manual fan control.

Function 3: Middle toggle:
Relay de-energizes and closes (switch current is off), therefore, the
DAFS takes control of the cooling fan.

That's all there is to it!



Joined Mar 24, 2008
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