fan by-pass switch
Team Owner
Joined: Dec 2000
Posts: 90,675
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From: SE NY
Cruise-In II Veteran
Re: fan by-pass switch (85autoxr)
If you only want the fan on control while the engine is running just wire a SPST switch between the fan relay control signal line from the ECM (I believe it's the Dk Brn/Wht wire) to ground.
When you close the switch with the engine running the fan will come on; with the switch open the ECM will still be able to turn the fan if needed.
When you close the switch with the engine running the fan will come on; with the switch open the ECM will still be able to turn the fan if needed.
Pro
Joined: Jul 2002
Posts: 729
Likes: 4
From: Yukon OK
Re: fan by-pass switch (85autoxr)
Find the fan relay just below the brake master cylinder on the drivers side fender well.
The input wire to the relay is green (I think I remember that right). If you ground that wire the relay energizes and turns on the fan.
1 Cut the wire controlling the relay.
2 Put a single pole double throw switch in the dash with a ground on one side and the wire that fed the relay on the other.
3 Common contact of the switch goes to the relay input.
With the switch in one position you have normal control of the fan by the temperature switch. In the other position it will run when ever you have the ignition switch on.
Piece of cake :yesnod:
I will double check the color of the wire when I get home and update if necessary.
The input wire to the relay is green (I think I remember that right). If you ground that wire the relay energizes and turns on the fan.
1 Cut the wire controlling the relay.
2 Put a single pole double throw switch in the dash with a ground on one side and the wire that fed the relay on the other.
3 Common contact of the switch goes to the relay input.
With the switch in one position you have normal control of the fan by the temperature switch. In the other position it will run when ever you have the ignition switch on.
Piece of cake :yesnod:
I will double check the color of the wire when I get home and update if necessary.
Melting Slicks
Joined: Feb 2003
Posts: 2,673
Likes: 810
From: Saskatoon Sask.
Re: fan by-pass switch (LaVidaLoca)
A word of caution if you use this method. By actually cutting the wire going to the relay, you are opening the possibility of no fan activation whatsoever if the switch were to fail! A safer method while accomplishing the objective is to use a single pole single throw switch and simply splice into the relay input to supply it with a ground when you want to. By they way, the wire is dark green with a white stripe. It is in possition C1 of terminal connector C1 at the ECM.
1987 coupe - stock
1987 coupe - stock
Pro
Joined: Jul 2002
Posts: 729
Likes: 4
From: Yukon OK
Re: fan by-pass switch (arbee)
A word of caution if you use this method. By actually cutting the wire going to the relay, you are opening the possibility of no fan activation whatsoever if the switch were to fail! A safer method while accomplishing the objective is to use a single pole single throw switch and simply splice into the relay input to supply it with a ground when you want to. By they way, the wire is dark green with a white stripe. It is in possition C1 of terminal connector C1 at the ECM.
1987 coupe - stock
1987 coupe - stock
color is the same on mine and the idea to splice in rather than cut is valid.
Melting Slicks
Joined: Feb 2003
Posts: 2,673
Likes: 810
From: Saskatoon Sask.
Re: fan by-pass switch (fiberglasscoffin)
The ECM only throws a code when it senses something wrong from an input device. Since you are taping into an output wire(the green/wht) , if the ECM is not calling for the fans to run, the internal solid state switch is open and the computer does not even know that the tap-in exists. If it does reach the point where the ECM calls for the fan to run and you have the manual switch on, they both reference the same thing(ground). No problem.
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Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Re: fan by-pass switch (fiberglasscoffin)
The easiest way is to get an insulation displacement splice at Radio Shack and a spade lug to fit.
Then clamp the splice device onto the Grn/Wht wire, crimp the spade lug onto a length of wire that you run to the switch and plug it onto the splice device.
Later you can remove the splice if desired with no harm to the cable.
And yes fan control is open loop so the ECM will not throw a code.
Then clamp the splice device onto the Grn/Wht wire, crimp the spade lug onto a length of wire that you run to the switch and plug it onto the splice device.
Later you can remove the splice if desired with no harm to the cable.
And yes fan control is open loop so the ECM will not throw a code.
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Re: fan by-pass switch (LT-4 CE)
The fan control on the L98 is fairly simple but I understand it became more complex on the LT1 (& LT4).
I suggest you refer to your GM Shop manual for fan control wiring details.
I suggest you refer to your GM Shop manual for fan control wiring details.