u/MetaphysicalEngineer

New CV axles for the ol gal. Can't complain about 100k miles on this set before the boots started to split.

I really liked the specs and reputation of SPAL brushless fans, but I struggled to find solid info on how to control them. My options seemed to be either the SPAL proprietary controller, or a standalone ECU with configurable PWM output. But what about the rest of us with stock ECUs, or old school setups with no ECU at all?

So I did the smart thing and bought an expensive fan to experiment on. Unit used for testing is model number VA207-ABL523P/R/A/N-65A, or 30107328. A 500W, 2400 cfm monster, sometimes called The Beast. I used an Arduino to generate a PWM signal, with a small NPN transistor as a driver/inverter stage.

Here's what I found:

1. Many of these fans, but not all, have two control wires. One is Enable/PWM, and the other is an analog input. The fan can run at full speed with soft start by connecting the PWM wire to battery negative and analog wire to battery positive. Any relay or switch of sufficient current capacity will work.
2. Pulling PWM to ground wakes up the fan drive. Letting PWM float puts the fan drive to sleep. Unit consumes 32mA when awake but only 35 micro amps (0.035 mA) when asleep.
3. When the fan drive is awake, voltage applied to the analog input will control the speed, with soft start and ramp up/down. Speed control is linear from 4V for minimum speed to 10V for maximum. See second image for the graph. Notice how there’s multiple modes!
4. Anything that can provide a variable voltage will work on the analog input. A set of relays, diodes, and resistors could allow an older ECU with two stage fan control to run the fan. Or, a pair of temp switches with different temp ranges could do the same thing. Even more speed stages are doable, limited only by your patience and complexity tolerance. See third image for example diagram.
5. There's a section between 1V and 3V where the fan will suddenly run full speed reverse! Beware accidentally triggering this mode. It's loud as hell and does not move air very effectively!
6. PWM control is similar to the analog mode with a few more quirks. See graph in fourth image. The PWM wire is pulled up to battery voltage by the fan module, so it defaults to effectively 100% duty cycle. However, this causes the fan to stop, not run at full speed! Pulling PWM to ground also makes the fan stop.
7. Pulling PWM to ground and releasing it again rapidly creates the "off" and "on" part of the PWM wave, respectively. Any small transistor or MOSFET driver can handle this. However, the logic will be inverted from the POV of whatever is generating the signal for the transistor. 10% duty cycle input to the transistor will present 90% duty cycle at the fan, for example.
8. There's a range of PWM between 7% and 18% that also makes the fan go full throttle reverse, similar to the reverse range of the analog mode.
9. Speed control is linear between 25% and 90%. Above 98% or below 7% stops the fan.
10. Minimum speed is about 27% of maximum for the fan I used, but this will vary somewhat for each fan model.
11. Analog and PWM inputs can be used at the same time, with fan prioritizing the PWM signal if both are present.

Here's the video if you'd want to see me struggle through mapping this out: https://youtu.be/1kRsGcjU1Ck

With this, I was able to design a custom PWM controller so I could use multiple temp and pressure inputs to control the single fan on my car. Video on that coming later after more testing.