Today I opened the magic black box and found that there's not much magic.
It's wired up more or less like this:
- There are two relays, let's call them R1 and R2,
- R1 is wired to top inputs 1 and 3 (see post #1 for input numbering), and there is a ~65ohm resistor in parallel,
- R2 is wired to top inputs 2 and 3, and there is a ~65ohm resistor in parallel,
- Top input 4 connects to bottom input 1 (B1) through a 1000ohm resistor,
- Closing R1 connects B1 and B2 (bottom inputs 1 & 2), resistance between B1 and B2 is ~0.1 ohm,
- Closing R2 also connects B1 and B2, with resistance ~1 ohm.
The only magic part is that I couldn't find a connection between R2 and B1. R2 connects B2 to the diode next to the big coil in the external housing. There are four tabs in that housing, only two actually connect to the internal workings, yet all seem to be connected to each other (according to my multimeter) and to B1, but I couldn't visually see any connections.
When hooked up, B1 connects to the radiator fan's + input, B2 connects to the battery's + output.
So my guess is that when the coolant temperature exceeds 95°C the ECU sends a signal (via top pin 2) to close R2 and engage the radiator fan through the big coil resistor. When coolant temp exceeds 105°C the ECU closes R1 (via top pin 1) to run the fan at full speed, bypassing the resistor. Top pin 4 is probably for the ECU to check whether the relay closed and whether there is voltage in B1 going to the fan.
Question: The difference in resistance between B1 and B2 is either 0.1 or 1 ohm, depending on which relay is closed. Is 1 ohm enough to noticibly slow the fan, or maybe does the coil in increase its resistance when the radiator gets hotter (when installed, the external housing is right up against the rad)?
This gives me a new way to get a signal for a fan indicator LED in the dash: tap into the top inputs 1 or 2. I'll have to test that when I get some time.