Anyone know where in the car to look for a tachometer pickup wire? I'm putting in a datalogger (Traqmate) and am searching for a wire to use for the RPM input. I tried using one of the coil signals coming off the ECU, but it is off by 12.5% (i.e., when the engine is at the 9500 RPM rev limiter as shown by the instrument panel tach, the coil is only firing at about 8300 times/second). I verified that the coil signal off the ECU really is running only 8300 with an oscilloscope. Unfortunately the Traqmate only allows integer number of cylinders for the engine so I can't correct for it this way (I would need to set 1.75 cylinders).

Any thoughts? Looking through the wiring manual, it wasn't clear how the instrument cluster gets the tach signal.

Also, does anyone know the math behind the coil firing at only 87.5% of the engine RPM? I assume it has to do with the 3 pulses per rotation of the rotor and the 2:3 tooth ratio on the rotor and the eccentric shaft, but can't get the math to work out.

 

https://www.rx8club.com/series-i-tech-garage-22/question-innovate-inductive-rpm-clamp-174486/

I am not sure how the IPC gets the RPM signal, but I have not seen a hack.

The above works quite well, and it or something like it might be adaptable to your system.

 

Thanks, that thread didn't come up in my search. MSD does an inductive pickup as well:

MSD Inductive Pickup

but I'm guessing it will have the same problem as splicing into the ECU coil signal wire.

 

I moved the tach pickup from a trailing coil to a leading coil. I guess I wasn't expecting much change, and that is pretty much what I got -- the tach pulses are coming in about 12% below what the tachometer in the car is reading.