Driving 97 kick tach after J18A engine swap

[fixkick]

I guess I should have been more clear, sorry. I have a MK2 Golf which I intend to engine swap with a VW ABF 16V engine, so that's why I started digging around trying to see how people may have sorted the original tach to work with the Speeduino/Mega and COP. I should have thought about that a lot earlier, but I am happy to admit that my electronics knowledge has expanded since opening this thread (Mostly thanks to you and all the info you share). I have ordered one of these Chinese scopes, but it says it's still in transit and I should expect it within a few days. When I get it, I will make sure to check out what kind of duty cycle, frequency and voltage level the ECU broadcasts on the tach line. I read on a forum thread about VQ swaps that a guy was trying to tap into the "RPM Output" wire of the ECU Going to the Power Steering control unit and posted a waveform that had 0-4V levels and the frequency corresponded with the amount of power strokes for the V6, so my guess is that the cause here will be similar. I'm Still kicking myself for not thinking about that earlier (well I did build that circuit with the arduino, but the "tone" library might not have sent the correct types of signals on the output pin).

I will try just the transistor with a current limiting resistor instead of the relay. If I understand correctly, the tach will be seeing a 12V signal, until the transistor is switched on by the "tach" wire on the J18 ECU, then it will pull the 12V to ground through the resistor and the tach will see 0V.

this level of hacking is complicated by the fact car makers mostly do not cover this topic at all , swaps.  nor signals told, some on purpose (odometer laws)
notwithstanding silly PCM pages with key on voltages, only.

The inputs on most ECU/PCM are limited to 15v max.  most  do this 2 ways,  with series resistor (protecting) and CAP filter for noise. and some have zenor diode clamps.  cars are nasty noisy things happening, like AC clutch coil noise, and protection from that and overheating,top the list.
sure scope is best in all cases to see what really is there,  and can be vast things possible.  (even data streams)
But Id not ever send a negative signal in to the PCM  nor go over  +15vdc (alternator max) or risk damage,  and yes the coil neg lug is nasty and is in fact filtered.

BACK EMF is here and off the scale;G16 1991

the 91 this protection and there is no need for the 100v spike to make it work

first note the 2k resistor in the Suppressor  module that reduces this PIKE first. EMF spike. The D110 kills the negative component of the signal and the R106
forms a voltage divider and 2vdc to the fail logic deeper inside (port expander IC chip.? )
The key point here is that the input spick and ringing are not to be set to the logic inside just the 12 to 0volt pulse. and becomes in this one case 2v pulse.
Most store bought Tachometers have complex input selections, not just 2 or 4cycle and cylinder counts, but levels too.
Im not sure what you are doing with VW parts at all but this is how some tacho lines look like , 1991 8v here, G16a. just to show how Suzuki did this clean up.


ON most COP based engines, you have 2 or 4 coils for 4 cyclinder engine, PCM driven.
how the PCM creates a tachometer signal is any ones guess, and I'm sure not all cars are the same.

Most would be a 12v or 5 volt square wave, pulses.  how many per crank turn is no standard either,  the cluster designer talks to the engine EFI designer and they decide that and keep it   a secret but the scope in hand tells the truth, what they did. pulses per Crank turns. CPCT?

On my 2008 jeep it is all serial data, on the CAN BUS. (a network for cars) some signals are encrypted too. yup,  good luck hacking that LoL.
In effect my cluster head is just 1 more network appliance (device) with 3 pins, 12v, ground, and CAN.
I'm sure no GOLF has that. 1983 to 1992 made.  but  Germans do as they want.
NOT saying I know a thing but the filters used yes, and what not to do, best is avoid any negative signal and not over +15vdc, Alternator max.
The links you posted are from folks just  trying so hard to emulate the huge noisy coil signal,  and I would never to that,  id use NO COIL at all .
that 400v spike on my car could blow up vast things easy, for sure other cars parts hacked in or clusters or OTC over the counter tachometers.

try a clean 12vdc pulse. is the most safe way to do this.

good luck with your project.