Two Radiators, One ECU

So typically, in these scirocco radiator installations, the logic for fan control is um...dumb. I mean that literally: The thermistor (coolant temp sensor, aka CLT in Autogeek terms) that screws into the radiator turns the fan on at 196°F and off at 191°F or thereabouts depending on which CLT you buy.

For a dumb, un-coordinated system circa 1974 (when it was designed) it works. One radiator, one fan, one thermistor (CLT.)

But when you put two radiators in a series coolant loop, yet still hand over simple minded fan control to separate uncoordinated CLTs, you've got a case where your cooling fans are not working in concert with each other: they still think they're each in their own single car and they manage cooling their radiator independently.

As an example of bad things that can happen: the coolant exits the engine past the factory CLT probe, but since we don't use it for fan control, it can't help us. The coolant enters the left radiator, and does one full pass before the thermistor CLT (LEFT) gets hit, giving an artificially low reading. If the artificially low reading is still hot enough, CLT (LEFT) turns on its FAN (LEFT.) RAD1 (LEFT) cools better for the help of the fan. By the time CLT (RIGHT) sees the temperature, we've already done 3 passes through the scirocco radiators. That means that CLT (RIGHT) may read just cool enough not to turn on its FAN (RIGHT.) So the left hand fan gets a work-out, and the right hand fan gets...less action.

Instead of trusting these blind switches that were not designed for this engine, let alone this daffy application (A Subaru in a Bus?!?) I'm keeping all of the control with the Subaru ECU, because it is using a HELL of a lot more sensor information to determine cooling requirements than just simple thermistors.

Look at the diagram below, and then read the description at the bottom.


There are two types of fans available for these housings: single speed and dual speed. They get two speeds by having two windings. Power one winding: low speed. Power both windings: high speed. In the factory scirocco applications, the low speed winding was connected to the AC compressor; When the compressor comes on it automatically switches on the low speed winding to cool the condenser (inline with the radiator) as well as to slightly over-cool the engine, since running the AC produces more load and more heat. The high speed side still operates for coolant over-temp situations triggered just by the screw-in thermistor. Cars without AC had only a single speed fan, triggered by the thermistor.

With its stock fans and radiator, the Subaru ECU controls two single speed fans, a smaller fan (AC ON and/or low heat load) and a larger fan (over-temp conditions.) Unlike the blind thermistor however, the Subaru ECU uses multiple sensors to determine conditions: the CLT sensor measures coolant temp at its hottest point where it exits the engine, and the VSS which measures the Vehicle's Speed and can take into account how much ram air the radiator should be receiving and therefore whether to trigger the small fan, the large fan, or both for maximum cooling.

If you follow the diagram, you'll see that I'm only just twinning the number of relays that FAN1 & FAN2 low amp circuits control. When the ECU triggers for slow speed cooling (FAN2, in blue) then FAN2A & FAN2B are both energized, cooling the flow through both radiators concurrently instead of relying on the blindly independent control of the thermistors screwed into each radiator.

The Subaru ECU can compute needed cooling based on load (MAF data vs MAP & BARO correction,) plus the vehicle's speed (VSS) and the CLT temperature value all at the same time. If FAN2A & FAN2B aren't doing the job, the ECU will add FAN1A & FAN1B (the green circuit) for maximum flow. In this way, the ECU waves the baton for all four circuits on both radiators concurrently. No chance of two different thermistors deciding to do different things with their fans.

That's the concept in a (very verbose) nut-shell. The wiring is actually very simple (which is good, because I'm sort of rubbish at wiring) and Jeff Robenolt fabbed up a fuse and relay box for me from Subaru parts based on my design. It all works, and spending 60 seconds with it would allow the idea to make sense to someone who can't follow my ersatz wiring diagram. I'm going to do a slightly higher end build using new Cooper-Bussman fuse panels and relay blocks to really clean up the appearance and make it very simple to mount and diagnose. It also removes the need to harvest the mounts, fuse panels and relay blocks from the donor Subaru which always look bodged together when hacked into a new vehicle. Better to spend the extra $50 on an off-the shelf solution of common industry parts than unnecessarily harvesting brittle old plastic from a 20 year old Subaru.