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
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)
Okay, now on to the good part, lets look at the circuit:
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:
to attach to the positive (+) side of battery.
to attach to positive fan lead.
to attach to a switched ignition source (Hot in Run).
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:
taps into GM Hi-Amp hot wire from fusible link.
taps into GM Hi-Amp B/R (+) wire running to fan from relay.
taps into GM hot in run wire coming from 20 Amp source from fuse box.
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:
(you choose color) from switch terminal to positive side of lo-amp relay input.
(you choose color) noted on right side of graphic goes to fuse box and supplies common (+) lo-amp voltage to toggle
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
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!
Be sure to check your cooling system to verify all components are
operating according to GM specifications before performing the DAFS installation.
I am always open to critique and suggestions for improvements, so feel free to email me your comments. I'm sure there
are other ways to achieve the same results.
Thanks to Matt_Ky & Willie (fellow Third Gen Members)
, contributors to this article.