Driving the fan from an aftermarket ECU

[20vTurbo]

I'm currently running an Autronic ECU on my ABY S2. The next refinement in its installation is to let the ECU control the main rad fan - currently, the cooling system is completely standard.

After doing some reading up, I thought it would be sensible to keep the after run pump function, so my aim is to try and retain the factory functions as far as possible.

So from what I understand, there are a number of thermal switches that control various functions...
1) The 3-wire thermistor on the bottom of the rad controls the fan speed (low/high) based on the temp seen at the rad
12v is seen on the green/black wire at lower temps triggering the fan's low speed
12v is seen on the green/red wire higher temps triggering the fan's high speed
2) The after run pump uses the coolant temp gauge (MFTS) on the water rail (only enabled with the ignition off)
3) The ECU uses the coolant sender on the side of the block

I think this means that essentially the different temp sensors and associated circuits seem to work independently.

If this is indeed the case, then my plan would be to supply 12v (via the Autronic ECU through a relay) to the high-speed wire on the thermistor (green/red). This should mean all the factory functions are retained, but the ECU simply triggers the high-speed fan based on what temp it sees via the coolant sender on the side of the block (the trigger temp is configurable on the Autronic ECU).

Can anyone confirm if the above logic is correct?

Cheers

Mick

[newsh]

The ECU uses the coolant sensor on the back of the head.

I’m not completely sure but I think the afterun pump uses the other small sensor on the water rail.

But in any case your plan should work!

[mcandmar]

^ yes that little one on the rail is the afterrun pump, triggers at 100c or thereabouts.

From memory the large sensor that drives the temp gauge also has an overtemp wire to the ECU which can limit boost. Cant remember the exact details, but not relevant in your case.

[20vTurbo]

Thanks for the replies. I’ll give it a go and see if cooling is improved.

However, for curiosity, first I’ll just rig a light up to the ECU fan output so I can compare when the ECU would trigger the fan compared to how the factory setup triggers it.

Cheers
Mick


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[S2central.net]

From my memory - the S2 ECU has nothing to do with fan control - during normal running or for after-run... there is a plethora of relays for that. The main fan during engine running (speeds 2 and 3) are controlled by a dual thermal switch at bottom of the radiator with two relays and a hefty resistor pack in the circuit. Meanwhile the after-run mode (speed 1) is managed by a control relay above the drivers pedals which is triggered by a thermal switch on the water rail (and another relay in the main panel for driving current through the fan and resistor pack).

The only connection to the ECU I can remember in all of this is that one pin is a gating input to the after-run control relay to prevent the after-run operation (which pumps water the wrong way) during engine operation... The after-run pump circuit is tested every time you turn ignition key to ON position with the ABY (and other variants I presume) - the pump runs for a few seconds then stops - everyone thinks its a fuel pump priming noise but its not.

Just have a look at the after-run control circuit on the S2/RS2 and replicate something like that based on OE relays and wiring positions in the fusebox - simples - no interaction with ECU required - just make sure the after-run pump is gated OFF when ignition is ON.

Important - none of the above has anything to do with the MFTS sensor on the water rail... the 'T' signal drives the temp gauge, the 'C' signal shuts off aircon if water temp is too high and the 'R' signal connects to ECU (not AAN) for boost reduction when water temp is too high. The after-run temp sensor is a separate thermal switch on the water rail.

[20vTurbo]

Ah ok, noted - thanks!

So I think my plan should still be ok - ie use an Autronic ECU output to simply trigger just the high speed fan, in order to provide additional cooling. This will only be enabled with the ignition on, so shouldn’t interfere with the reverse after run function.

Mick


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[Airwolf]

What kind of fan are you using?

[20vTurbo]

It’s the factory fan


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[Rusty]

I did a how to for a fan bypass switch for high and suoer high speed operation. Have a search if youre interested.

[20vTurbo]

I did a how to for a fan bypass switch for high and suoer high speed operation. Have a search if youre interested.

I did yes thanks... used your post as a part of the information gathering.

Cheers

Mick

[20vTurbo]

As a follow-up, I can confirm I now have the Autronic ECU controlling the high-speed fan whilst all factory functions are retained.

The negative trigger from the ECU was used to power a relay and send 12v down to the green/red wire on the thermistor on the bottom of the rad. I've also wired a manual switch to trigger the relay/fan also - thought this may be useful when stood queuing.

The ECU now triggers the fan slightly earlier than factory slow fan engages - it did seem to keep temperatures down on my last track day... which involved a fair amount of queuing on a 30+degree day.