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Splitting sensor signal

Practical Harness Construction - Club Level

Discussion and questions related to the course Practical Harness Construction - Club Level


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Hey folks,

Here's a bit of a curious one... one I'm not sure is actually possible. The car in question is an R34 GT-R.

In the GT-R there is an ECU in the boot that manages the ABS, 4WD system, and hydraulically-controlled rear diff. It does so using 4 wheel speed sensors, a TPS and engine speed signal from the engine ECU, and a G-sensor.

Due to the age and street-oriented programming of the unit, it is not uncommon to modify this system to increase 4WD performance. This is generally done through a piggyback-style G-sensor/processor that provides more amplified signals to the primary 4WD ECU with the intent of eliciting a more dramatic reaction, or by adding a separate 4WD-only aftermarket ECU with its own G-sensor and TPS inputs from the engine ECU. In this case the factory 4WD ECU is retained to maintain ABS functionality.

There is a third option; something I've experimented with in the past. By reverse-engineering the tables inside of various aftermarket 4WD ECUs, I was able to build my own 4WD model and tune it to my taste. In this case I'd used a spare Haltech Elite with appropriate inputs from the engine ECU, a motorsports G-sensor, and a PWM output to control the GT-R's 4WD pressure solenoid. It was more of a fun experiment than a dramatically useful development, but it's worth mentioning.

That's a lot of introductory information; on to the actual question. In both aftermarket 4WD ECUs and in my home-brew ECU circumstance, adding wheel speed sensors to the 4WD system would be tremendously beneficial. Unfortunately those signals must go to the factory 4WD/ABS ECU in order to keep ABS functioning.

Is there any proper way to split a speed sensor signal reliably?

I'm assuming the sensors are VR style sensors, generating an AC output heading directly back to the ABS ECU? If your home brew ECU, or the aftermarket unit you're working with has differential inputs, you should be able to tap into this signal without any problems, as the signal strength once the vehicle is up and moving is actually pretty high, and your differential inputs are likely to have quite high input impedance, meaning they wont noticeably effect the signal by measuring it. It's something you really need to confirm with an oscilloscope though.

If the added ECU does have any differential inputs, you'll need to use some sort of interface boards to convert the VR signal to a digital square wave before sending it to the ECU.