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Fan control using ECT and IAT


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Hi Gents,

I would like to set up my engine fans to run with ECT and IAT, due to two radiators being used. I've dug up some useful information from Clint, and the method mostly makes sense.

Standard setup:

Aux2 set to Engine Fan function, Low Polarity, On 90, Off delta of -2


Modified setup:

Aux2 set to GP Output:

switch off 5 seconds, Logic Condition 1 and 2, Switch conditions are Vaux1 and Vaux2 ports.



Vaux1 set to GP output:

Cond 1 ECT > 85c

Cond 2 Aux Virtual value = on 0

Cond 3 ECT > 84C



Vaux2 set to GP output:

Cond 1 IAT >32c

Cond 2 Aux Virtual value = on 0

Cond 3 IAT > 34c



So I see that Aux 2 which is wired into the rad fan, changes to GP output, then sets the two switching conditions to look at Vaux1 and Vaux2 as reference.


Vaux 1 sets switch on condition / output to Aux2 when ECT is greater than 88c, and when it is greater than 84c, and Vaux sets the IAT temps, however this is what confuses me:


- Appears to be no way to specify the fan polarity on.GP output at Aux2, Will it output by low to default? I don't want to reverse the fan.

- What does condition 2 do? Does it just state 'this is active'?

- I can't understand the switching logic with the temps by using AND for each condition. I see the output will stay switched as long as the temp range is above both lower and upper values. But since both can not always be true (above 32 and 34) surely the output will stop as soon as the 34 condition is no longer met (IE temp goes to 33). This seems to me to be the same as just setting 2 conditions with one > temp, and doesn't allow for a range of cooling? Can anyone clarify please? My concern is just a 1 degree buffer the fan will be constantly cycling on and off.


Many thanks for your help :)





By the way, the reason for this is I have fitted a new large charge cooler core at the front of the car, this is mounted directly in front of the regular rad (about a 2" air gap).

Rather than fit further thin lower cfm fans between the two and obstruct flow, I would rather just run the rad fan with ECT and IAT. The MR2 roadster already has a dual fan shroud (for air con models), so although my engine temps are fine (<90) I would like take advantage of the extra flow from the twin fan unit, and have this activate to pull air through the charge cooler also when this warms up. I plan to piggy back the fans from the same relay / 12v supply.

Testing with just the single fan already I feel quite some nice flow through the front charge cooler to the back of the engine rad, so it seems this may be a much better solution.


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Set up below should do what you want assuming I have understood what you are explaining.  For my virtual aux 2 I have just used the standard Engine fan function to save messing around with more conditions.

In this example the fan that is connected to aux 2 will turn on the fan when ECT goes above 90 and will turn off when ECT reduces by 2deg.  The fan will also turn on whenever IAT goes above 35deg and will keep running until IAT reduces to 30deg.  If that is not what you want please explain the conditions you want the fan to switch on/off under and I will see if there is a way.


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Adam, thanks for the quick response, perfect.

It makes sense having one set to fan, makes things a little more simple, although I guess both should work, I'll add one of the Vaux's to show engine fan. I set the switch off timer to 5 seconds, as I assumed this would take effect when the other condition is no longer met, IE overrun for 5 seconds, just to be sure it's within the temp I want and doesn't just flick into it for a second and back up.

However, I still wonder what the polarity of the fan will be seeing as there is no option unless you chose the engine fan under aux2?


Many thanks,



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11 hours ago, SteveX said:

However, I still wonder what the polarity of the fan will be seeing as there is no option unless you chose the engine fan under aux2?

The "polarity" doesnt matter for a GP aux as yo have full control over the conditions.  If the conditions are "true" the aux will be turned on.  If you want it to act in the inverse of that then you just change the conditions to the opposite.  

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