Fan Override Switch - Alternative Method for Duratecs?

[theOC]

I've seen a couple of guides about making a manual cooling fan override switch and I thought I'd throw this idea out there which relates to all Duratec cars I believe

How I understand the system works (UPDATED): The 'submarine' mounted temperature sender feeds the dashboard gauge, there is a separate sensor that sends a reading to the ECU. The ECU then controls the fan circuit when the temperature reading is above the switch point (correct?)

Instead of creating a whole new circuit that directly controls the fan power would it be feasible to simply fit a switch that disconnects the ECU wire from the temperature sender? This would send a 'max' value to the ECU and it would then start the cooling fan? (UPDATE - I just plugged in Easimap and pulled the plug off the sender and the coolant temperature reading went to 110 degC so I am guessing this would permanently operate the fan until reconnected).

As far as I can see - fitting a switch that simulates unplugging the sender would work without any issues, please feel free to point out if I have any (more) of this wrong, it is something I may look at doing as a precaution in case the sender fails on a blat.

Thanks, O.

[aerobod - near CYYC]

On my R400D, the ECU and dash gauge use different senders, although they are both close together near the submarine unit. The sender unit circuit also doesn't handle the radiator fan, it is handled via a separate circuit from the ECU to the fan relay. If you grounded the sender unit circuit you would also cause significant problems with incorrect fueling, as the water temperature is a major ECU input to control the fuel in conjunction with the fuel maps.

[theOC]

Thanks aerobod - I must have misread something somewhere. I confirmed my setup is as you described - the OP is updated.

[aerobod - near CYYC]

If the car is fully warmed up, it would change the fuel compensation by about 3% rich by going to the highest coolant temp possible. If the engine is cold, it probably won't run very well due to being about 30% lean at normal ambient temps. I would say if it is just to get you home and the sender fails shorted, just pull the lead off.

[ChrisC]

Both my 360R and 420R are wired as you describe.

If I wanted to have an override switch I would look to control the Fan Relay (or add a second Fan Relay).  But I would want to know why you feel the need for the override in the first place.  Is the ECU not doing a good job?

[7 wonders of the world]

most ECU's control relay by grounding the pull on coil circuit, therefore by putting a simple on / off switch in this wire you can override by providing a manual earth and therefore pulling the fan relay in at the flick of a switch.