Fan Override

02/01/2022

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	Electric Fan Override

This is another mod I have been asked to detail, it allows you to manually override the electric cooling fans fitted to the supra. Based on yUK 1991 Car, other vehicles such as pre 1990 & USDM will differ slightly.

This involves the fitting of a switch and running one small wire into the engine bay. It will allow you to turn on the electric cooling fans manually however retains all stock automatic control as well as killing the fans once the ignition is off.

The fan switching system sadly is all undertaken in the engine compartment, at no point does the wiring end up inside the car, for this reason we need to run a single small cable through the bulkhead and into the engine compartment.

The system uses a couple of temp switches to energise a relay mounted below the engine compartment fuse box. The switches just use a grounding signal to do this which makes our job far simpler!

The sender we are going to use is the AC Pressure Switch, this is the easiest point at which we can interface with the fan control system and keeps cabling simple and easy to install.

The photo to the left shows the location of the pressure switch (RHD-UK) and its connector, its at this connector that we will interface with the system.

Once you have found the correct connector you should find 2 wires in it a White / Black & a Blue / White wire.

The wire we need to tap into is the Blue / White, this is the control signal to the fan relay.

Simply splice your override wire into this wire, solder and insulate it.

Don’t cut the existing wire as this will still be used by the AC system to automatically control the fans as normal.

With this wire now connected and run through into the car you can connect up a latching switch to override the fans. Because the fans are relay operated the switch does not need to be high current.

REMEMBER this is a ground switching system - do not send a + 12vdc supply to the fan system!

Several types of switches are availible on the market these days from simple on off types to ones with LEDs or bulbs fitted.

Below are details on how to wire a number of different switch combinations depending on what you require.

Basic On/Off switch without indicator.

This is just a simple latching switch, no indicator lamps etc.

One side of the switch goes to ground the other to the fan control wire.

Switch with LED Indicator

This circuit incorporates a switch with an LED indicator. For this particular application you will require a switch that gives you access to both legs of the LED.

This configuration requires a Switched Ignition Supply to power the LED. The + supply MUST BE SWITCHED otherwise you will get feedback issues resulting in your fans staying on permanently regardless of switch position.

This switch configuration also has the added bonus of the LED lighting when the fans are switched on by the car system as well as being overridden by the switch so, you always know what’s going on under the hood.

If you don’t want the LED to light whenever the car switches the fans on a diode can be inserted into the control line as per the “bulb” switch diagram below.

Switch With Bulb Indicator

This configuration used a switch with a standard bulb as an indicator.

Because of the way bulbs work a diode is required in the line to prevent feedback through the bulb causing the fans to come on permanently once ignition has been cut.

A standard diode such as IN4001 is suitable for this application.

Switch with Built In Lamp

If you already have switches or want to use a switch with an indicator that does not allow you access to both ends of the light source allowing only a switched + command to be used, then a relay can be incorporated into the circuit to allow this.

The switch can send out a + volts signal to a relay mounted eiteher inside the car or in the engine bay.

The relay contacts are then used to send a grounding signal to the fan system.

This circuit is particularly useful when using specific switches to match existing ones on a custom dash or using a switch that only allows switching of positive going signals.

All the above switches can be mounted anywhere. The grounding points can either be picked up from an existing circuit with a ground wire present (usually white/black wires) or simply grounded out to the chassis.

A switched ignition positive line can be obtained from the back of the heated rear screen switch - YELLOW wire. This point is also already fused down to a safe 7,5 amp by the GAUGE fuse.


	Below is a guide to illuminated switch configurations available to give you an idea of which types can be configured to switch grounding signals and trigger an indicator directly and ones that can only trigger an indicator lamp with a + signal thus requiring a relay.

The below type of switch/indicator configurations allow you to switch ground based signals to operate the fans & drive the indicator lamp directly avoiding the need for a relay.

Here are links to a few easily obtainable suitable switches.

Without lamp Small rocker type.

LED Indicator push button type (requires resistor)

The below type of switch/indicator configurations are pre wired within the switch and can ONLY switch + signals to retain indicator operation so, would require a relay. Personally I’d avoid these types of switch!!

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