How to override the Prius (gen3) Battery Cooling Fan.

I look forward to learning about this because Gen3 cooling fan is a three wire system and Gen2 is easy with a 2 wire system that pulls about 6.5v DC when it is running at its peak. If someone on here, maybe @Elektroingenieur could post how the wiring works on Gen3 cooling fan so we don't have to depend on Prolong for their expensive fan controller box they sell with their overpriced harness that'd be much appreciated.

 

Covered here.

At the time I wrote that, I hadn't found information on the SI0 waveform in the manual, but it's in there, I just missed it. Waveform 5 at the front of the Power Management Control ECU diagnosis section. Looks like about a 400 Hz square wave (by eyeball), and I'm guessing the % of duty it is pulled down gives the requested speed (given that grounding it seems to work on the bench to make the fan run).

 

As a general note, sometimes great care is needed to keep details from blending together.

The connectors in the car's electrical system are given names with a letter and number. When Elektroingenieur mentioned connector S14 at the blower motor, that was the name of that connector.

The individual circuits running through a connector also have names, in addition to their pin numbers. Their names are memory jogs to suggest their purpose; IG0 and GND0 are good examples. Sometimes the names aren't very obvious: SI0 (and notice those are the letters S and I, and digit 0) might stand for something like Speed Input, and VM0 might be something like Voltage Monitor ... it's the signal returning from the fan that shows how fast it is turning.

In particular, in this example, the "S14" (name of a connector) and "SI0" (mnemonic name of a circuit) look sort of similar, but are totally different kinds of things.

In normal operation, the ECU tells the fan how fast to run by sending pulses to it on SI0. A train of pulses will alternate between ground and some higher voltage, and I think this signal is arranged so that the more time the pulse train spends at ground, the faster the fan runs. That would explain the suggestion to simply ground SI0 for testing on the bench, and that would make the fan run full speed. (I'm still drawing inferences that aren't completely spelled out, though.)

Elektroingenieur was suggesting a method to test the fan on the bench, where you would just put +12 from a battery across IG0 and GND0, and then ground SI0 to make the fan start.

I think I would be more careful about just blindly grounding SI0 while the fan is installed in the car though. You'd not just be supplying an input to the fan then, you'd also be pulling down the ECU's speed signal output. Maybe that's ok, but it merits some thought.

It kind of sounds like the pullup in that circuit is on the fan end, which would make it probably ok. But I shouldn't set your comfort level modding your car.

 

Kinda late but it can be done as shown by @CatNinja


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