Driving 97 kick tach after J18A engine swap

[fixkick]

First off, happy holidays!

Finally! I was able to hook up the scope to the tach output on the ECU. It seems to be sending out Vbat pulses at every ignition event (~30hz at idle, 2 pulses = one revolution, so divide by 2, multiply by 60 to get the revolutions per minute ~~ 900RPM at idle). It seems though it's not anywhere near 50% duty cycle, in fact the scope says it's mere 5% at idle and it increases as the RPM go up. Though if I recall correctly you said that  the duty cycle does not really matter to the tach.

Video: https://photos.app.goo.gl/AzgY9mYyDX2EuC9J8
The signal gets fuzzy at the end because of the crappy ground, I just put an alligator clip on the metal on the clutch pedal and moved it on accident.

This confuses me though. It would seem that the Tach does not really like the 12vDC.

Do you have any thoughts on why that could be?

I apologize if I sound dumb or my statements don't make sense. For me it is a great chance to learn about electronics and I am making progress, albeit kind of slow.


Now I'll curve the topic a little bit.

My friend has a MK1 Golf with a 16V engine out of a MK3. The MK3 has a single coil with a distributor. In order to get the tach working we were told to hook up the wire to the ground pin on the 3 pin coil connector. That did not work, so another guy told us that there is a little cover that we could remove and that uncovered 2 solder points. Though I don't remember which one we soldered the signal wire to, the fact is that the tach starter working off of that wire. Turns out, the MK3 has the coil driver built in, so the ground on the connector is "filtered" (I guess) and that wasn't sufficient to drive the tach. The solder point under the cover I guess is the actual coil ground before it gets filtered by the driver circuit.

My point being:
Is it possible that the Vitara's tach is configured in a similar fashion?

I came across this: https://www.pirate4x4.com/threads/faq-2-...x.1003571/
You have probably seen it already. I am not too familiar with filter circuits, so I can't say what role the resistor plays in that particular circuit.

Once again I want to thank you for all the knowledge shared so far!

Enjoy your holidays!

For sure easy (IF)
if current signal is known from newer ECU or older if be the case.
and what the stock TACHo meter wants to see, we all know there is no standards here. ever.  the oem does what they please end to end.
but  3 to 12vdc signal is common, and on ours a filter so the 100s of volt back EMF pulse does not wreck the tachometer.
on COP  engine , they do not use the COP huge pulse they just drive a 5 vdc signal out of ECU called tachometer. out.  no nasty HV pulses there.
and in the right polarity too. (and duty cycle,  our old tachometer is analog and works on the total energy of the 12vdc pulses. (integrator)
the more pulses per second there are the more RPM is presented (a DC voltmeter it is) , crude as crude can be. (but cheap)

we do not know the , 1989-96 G16 sure do. but not the others. well not me, but I can scope them all an photo my screens and learn any car made.


nobody has a scope so all they do is guess. and guessing can work. wild.

my guess is the 6k ohm reduction is the input resistor voltage divider.


that makes the signal stronger to a wrong tach and  now works.  

he changed the input circuit , in the tacho.

the PCM is must  simple transistor, some cars do not use totem pole driver only and open collector transistor and must have  resistor to 5vdc or is dead.
they did that because nothing is more cheap than that method.  so adding pull up to 5vdc can cure a dead tach too.
or the tacho already  has that resistor.
in fact the one he changed (no schematics) may be the pull up to 24k , made the signal stronger. and worked,  of the 2 ways it could.

good luck finding what works.