TL/DR: I think I need a pullup resistor to let my stock '94 coils drive the rev counter properly, but does anyone know if that resistor should pull up to 5V or 12V?

Ran a DIYPNP for years with the Tacho Out on as instructed even though the stock '94 gets tacho pulses from its 4-pin coils on the same wire. Never understood why I needed both but it worked anyway.

Just switched to a 36-2 trigger wheel and it runs just fine except the rev counter shows double speed (Tunerstudio shows the correct rpm). I set Tacho Out to run at half speed which forces it to read correctly but something's not right. I asked over on MSEXTRA forum and Matt Cramer said they normally don't do that (though I think he'd usually expect the coils to have been upgraded so the tacho would only be driven by the MS).

What I guess is happening is that, in "Miata 36-2" trigger mode, the Tacho Out signal coincidentally happens to be high when the plugs fire, so the coil momentarily pulls the signal low, turning the positive-going pulse into a double pulse. That doesn't happen with the stock CAS and "4G63" spark mode so my guess in that case is the plugs fire when the tacho out signal happens to be low so the extra pulse is lost.

The easiest fix seems to be to let the coils drive the rev counter and stop the MS from interfering. If I switch off Tacho Out that kills the rev counter. If I disconnect the MS Tacho Out wire, that also kills the rev counter. So I guess I need a pullup resistor instead of the tacho out signal, but I don't know what voltage it should pull up to.

 

Pullup to 12V.

 

Thanks, Rev.

<edit to add> That fixed it. Rev counter now works normally instead of double speed.

Switched the Tacho Out pin to be a 4k7 pullup to 12V and the coils now drive the rev counter happily with the MS just supplying them with volts instead of tacho pulses.