This thread is to discuss the factory fan/relay/trigger system and how to best control it using an elite ECU.

In the oem FD fan system there is a resistor built into the fan motors that drops the input voltage down to change the speed (low/medium/high) where any of 4 fan “triggers” cause the fans to jump a speed level. Any time 1 of these is triggered fan speed = low, 2 triggers = fan speed medium, 3 triggers = fan speed high.

1. ECU Coolant Temp Sensor – Green 2 wire sensor located on the back of the water pump housing. ECU reads coolant temp and at a set temperature (OEM ECU was 105C) the first fan input was triggered and fans come on at low speed.
2. Fan Thermoswitch – Black 1 wire sensor located on the back of the waterpump housing under the ECU coolant temp sensor. Second fan relay is triggered at 107C by this sensor powering a relay and medium speed.
3. AC Input – Anytime the AC is on the AC fan relay is triggered.
4. ELD System – box behind ECU monitors engine load (headlights, defroster, etc.)


The patch harness only drives one thermofan output, which runs to pin 3D on the oem harness and corresponds with the fan relay triggered by the green 2 wire coolant temp sensor reading exceeding a set value in the ECU so then you go to fan speed = low.

So, in theory, you are limited to the "low" fan speed via the ECU without a custom harness that is set up for ECU outputs to trigger each individual fan relay. The AC button can be used to step the fans to medium speed when required. Given the oem FD thermoswitch only triggers it's relay at 107C (224F), this won't likely ever come into play to get you to high speed. I know some have swapped in an FC switch to trigger this relay at 95C (205F), but I still feel this is too high. My v-mount keeps the car running at around 180 when cruising and at 185 the fans cut on which typically only happens at extended red lights. That being said…you never know when you may end up in a situation where you want more, or need more fan cooling. If the relays are there and the ECU is capable, no reason not to take advantage of it. I'd say low speed at 185F, medium speed at 195F since low speed obviously didn't stop the temp increase, and then high speed by 205F for max cooling.

Which leads me to the fact that I'm going to need to setup individual trigger wires from the ECU to each fan relay as mentioned above.

Now here is where it gets interesting…maybe.

I noticed the I/O for the FD TT 2500 has 3 active thermofan outputs.

A28 – INJ6 = Thermofan 1
A1 – DPO2 = Thermofan 2
B26 – DBW1P2 = Thermofan 3

I’m not sure why but the elite 2500 twin turbo basemap has these 3 thermofan outputs. The only fan trigger the ECU is capable of controlling through the factory harness comes from Pin A1 on the ECU, which runs to pin 3D on the harness. Pin 3B on the oem harness interfaces with the ELD box and may could be uses as an fan trigger to over-ride the ELD making the decision, but I have no idea if or how that would work.

So in conclusion I'm thinking that outside of wiring individual relay triggers from ECU outputs or relying on the thermoswitch you are limited to only driving the system through the ECU at the "low" fan speed. I may very well be wrong, but the logic makes sense to me. I aim to figure out how to best control the fans to the max of their ability as it doesn't matter how strong or well-tuned your motor is, an overheat can still trash it.

I'll post the marked up wiring diagram when I can.

Skeese

 

I stopped reading at "limited to the low fan speed".

With the patch loom you have identical control as what you would have with the OE ECU. The single output from the ECU triggers low speed. The thermoswitch in the back of the water pump triggers medium speed. And high speed is triggered by the AC switch, if the other two inputs are already on.

On most of our harnesses we use two outputs from the ECU to replace the thermoswitch circuit. This allows user programmability of the medium speed so you can bring it in at a lower temp. This can easily be done through, I believe, the X14 connector, might be the X05. I'd need to look to confirm. Would be pretty easy to add with a patch loom.

The 3rd speed is directly controlled through the AC switch with no ECU interface in the OE application. Truth is, the 3rd speed is overkill IMO and draws HUGE amperage. I've measured over 100 amps in-rush and consistent 20+ amps of running current. I feel if you have cooking issues the first two speeds won't control you have basic cooling issues that need to be addressed.

 

Thanks, I figured you had already sorted it out. I'll look into eliminating the thermoswitch and instead using the ECU to drive that output as well, with the contingency option to then use the AC to hit speed 3 if ever needed. I highly doubt I'll have any issues.

Also

If I setup the clutch switch to be one of the AVI inputs, what should the switch on voltage and switch off voltage be? I can fiddle with it and find out too see what works.

Skeese

 

No idea. Wire it up and open a window to view the voltage on adjust accordingly. I always use a digital input and just set it to falling edge.

 

or you can just modify and make the fan speed continuously variable like i did

 

I see zero point of having a variable speed control on a radiator fan. Air passing through the radiator isnt going to have an immediate or sensitive enough effect on coolant temp in the system to justify a variable speed. If its getting warm, turn on fans. If its HOT, run fans ***** out.

Skeese

 

Not to mention with any real fan, you're talking a 100A Solid State Relay + Heatsink + Good Thermal Paste + Remote (cooler) location + Huge Fuse. Using a standard Hella 30/40A SPDT Relay is asking for problems on PWM.

 

Measuring the current, havnt seen anything above 6 amps per fan motor. the nice thing with motor controllers is that you can control the initial current spike with pwm. so you dont need a 100 amp ssr in that case. I have another video showing the current spike at startup and continuous draw.

and of course you cant use a standard automotive relay for pwm.

 

Stock FD fans pull more than 6 amps continuous draw in my experience.

I do agree with soft start being a big bonus of PWM control. Controlling the in rush allows you to not have that big voltage dip at power up and that makes everything in the car work better. PWM also allows you to find a happy spot for consistent coolant temps. Instead of engine temp cycling up and down you can hold the temp at a consistent temp. Again, making everything work better. There's a reason the OEs use PWM control.

 

Can the OEM fans handle PWM control without issues? What's a good frequency for them? I'd like to stay under 150hz if possible so I can use an MSD solid state relay block.

 

I've done it, though I can't remember if this was pre or post Rx8 fan motor swap. You'll have to play with your frequencies a little bit but I found good soft starts without rattle below 100hz.

 

Perfect, that’s well within the ballpark