Delay relay for fan running after shutdown

[MarekH]

You can use the red box relay that comes with the xjs if you modify it suitably. See my old photo albums and maybe bsome previous posts for details.

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Mod the back as indicated and swap out the 22uF cap for a 470uF for a delay of ~4mins, larger cap = longer time constant

1=to Otter switch, i.e. earthing input
2=12v brown wire
3=fan power supply
4=earth

This makes the red box into a delayed changeover switch, so anytime the fan switch grounds, pin2 connects 12v into pin3 to give power to the fan for a time dictated by the size of the big capacitor, then the fan is grounded on both sides as pin3 returns to ground at pin4.

I wouldn’t run the fan for 10 mins - it’s a waste of battery power.

See my post on post shutdown cooling if you want to do this slightly more efficiently by waiting five mins and then swapping the cool radiator water for the heatsoaked water sitting in the heads.

Solving heatsoak - a worked example

I’ll post this in the xjs section, since this is where most people seem to worry about the engines. It applies to all cars, v12 or not. [heatsoak] This is an excerpt from a datalog of a car being driven into the garage and left for five minutes. The top graph shows rpm. The bottom graph shows coolant temperature at the cylinder head water rail in light blue, coolant temperature at the bottom of the radiator in dark blue, ambient engine air temperature in yellow. At t=1107 seconds, the car is…

kind regards
Marek