Embedded: Tach… Fail?

So I’ve been using the aforementioned tach input for my pneumatic gearshift system with moderate success. However a problem has arisen.

I run a spark cut on upshifts so you can keep the throttle planted and just hit the paddle to get the next gear. This is all well and good but I also need a tach signal during this period to ensure the clutch comes up once the engine speed is correctly synced with the road speed; otherwise you get a massive jolt through the drive train and break differential housings (I’ve broken two!). The target RPM is based on the engine RPM when the shift is requested, the current gear ratio and the target gear ratio; simple.

RPM_{Target} = RPM * \frac{Ratio_{New}}{Ratio_{Old}}

The tach signal I had been using comes out of the CTO pin on the Ford EDIS unit (pin 11, clean tach out) which is a fancy version of what comes out of the IDM pin (pin 2, diagnostic signal). As it turns out this is based on an “EMF Flyback Circuit”, which uses the reverse voltage of the triggering coil when it is grounded (A good reference can be found here).

This is all well and good, but in short: No Spark, No Tach Signal. This is annoying because the ignition unit still knows what the RPM is from its variable reluctance sensor, it just doesn’t output the correct RPM.

To solve this problem I’m going to have to take an RPM reading directly out of the ECU or setup my own hall effect crank speed sensor…

I’ll let you know how I get on.