Rev Counter issue
 

On my 3 litre turbo, 77 model, I upgraded from K Jettronic to a Motech type system

We get a pulse from the box, but rev counter is not working

Car did stand for a few years, not sure if some dust/ mildew is in the mechanism, or if the type of pulse from the new EFI is not compatible with the signal it used to get from the coil

I understand that the pulse from the new EFI is the same as the pulse from the new spark packs, maybe frequency or form is not the same as from the older style coil?

Any suggestions?

You might be on the right track. Can't remember if the 930 has a square wave or a sine wave. You might try an aftermarket tach from Summit or Jegs.

The stock tach is horrible. I put a 5" Stewart Warner in mine. Fits in the stock hole and doesn't bounce around like a pendulum (sp?)

BTW, I think i read the stock tach is square wave

The ignition trigger signal going through the green coax cable from the distributor is a square wave and if the polarity is correct the ignition is supposed to fire at the end of the wave or when the squared off slope drops.

I don't know if its still a square wave in the single black/purple tach trigger wire, but seems like it would be.

You find that black/purple wire at the 6 pin plug on the bottom of the CDI and also the second spade terminal over from the passenger side on the bottom edge of the back of the tach.

If you wire in a shift light or rpm switch for a homemade shift light thats the trigger wire you use for it.