Cooling fan manual and backup stat overrides

 A little enhancement on the main engine cooling fan control.

While the ECU fan management is rock solid it is at the mercy of input sensors.

It is straightforward to add a manual switch into the fan relay coil as an override. If my temp sensor ever goes bad again the switch under the dash will let me manually turn on the engine fan.

To make things more interesting I incorporated a tri-colour LED to show on the dash when the fan is on, it sits unobtrusively bottom right of the water temp gauge.

The circuit specifically designed for minimal changes to the main loom - just a piggy back on an existing cable and if my new circuit fails - for any reason - it should not disturb or otherwise upset the existing operation of the ECU to Fan relay circuit.

Off = fan is off

Green = fan on via ECU demand

Red (ish) = fan on via Manual dash switch demand (its actually red + green but the red is more intense)

Blue = fan on via Radiator stat demand

I have a VW Polo radiator with a mounting point for a simple temp sensor which closes a switch, and for me, grounds the cable when it hits a temp limit. This is the automatic backup if the ECU sensor fails again & will light blue on the dash as a warning.

 

Multiple demand on at the same time light green + the appropriate colour.

Circuit

The circuit itself on a Vero board mounted behind the dash with manual override switch beneath. The board is in one of the drier areas of the car but precautionary hot glue on cable ends and the board conformal coated to give it a chance in the relatively harsh environment.

All wire runs were subsequently given a second layer of insulation/soft fabric wrap too.

Emerald Pin 6

Emerald Confirmed that the ECU will not mind an external sink of pin 6 'Fan Relay control' to ground. The easiest place to tap into it on my car is near the ECU connector - a cable spliced on and suitably insulated.

These are all signal current circuits to the coil of the Fan relay - no significant current passing at any time.  Traced pin 6 to the orange wire - double checked by grounding it does turn on the fan - and spliced in my yellow feeder to the new circuit

Wrapped it all back up

That will do.