89 Kick Tach signal wave form

[t3tom]

Happy New Year Fixkick!

My question today is about the old 89 Sidekick ECU which uses a tachometer monitor input.

When I looked at your tachometer circuit explanation page, you showed that the tachometer signal wire, from the - coil and after passing through the noise suppressor circuitry, still produces a signal waveform with a 50V back EMF spike.

Does this mean that:

(1) the tachometer in a 89 Sidekick is able to measure the frequency of 50V spikes, and this signal directly makes the analog tach gauge needle move?

(2) Or is the tachometer equipped with some sort of additional digital filter circuitry that converts the spiky signal into some sort of 12V square wave for a digital tach gauge to process and move the needle?


Thanks for reading!

[t3tom]

Second question.

Since the tach signal is used by the tachometer as well as by the ECU, do you think the ECU also uses an internal circuit to convert the tach signal into an actual rpm value, or a digital value such as ON/OFF (yes the engine is running vs. no its not).

I'm trying to determine if this vintage of Kick ECU is using the tach signal only as a fuel injection cut parameter to stop all fuel injection if the engine is not running, and is not cranking.

[fixkick]

89 is not like and 91 and nwwer.
you need to read only 89/90 pages.

yes to all that above
are you going to use some off the shelf tacho.
bTW I have no 89 scope data at all , the spark all happens inside the distributor that year.
the ECU only watches it to see if the spark failed, so cuts fuel instantly
the ECu dose NOT advance spark at all here.


your ECU is 100% fuel only ECU.
but does monitor spark and reports it bad (DTC errors and does cut fuel preventing crash and burn by law it must do that here)

why not just use the neg coil lug minus
like old days , for points ignition, a good OTC tach willl clamp that huge spike and use it, (back EMF spike from when the ignitor goes from closed to open)

[fixkick]

the stock analog tach is very crude
it takes the pulse duty cycle and averages to a needle reading, super simple it is.
the energy in the PLus pulses are simply averaged. (and tuned to that)

the only danger is using some cheap tach that can't connect to a live real coil.