I wanted to ask you a question, we are trying to wire up the tach output and get it working with the factor cluster (just a side note, I replaced it with the OEM digital gauge cluster). The ECU I have is the Link G4X AtomX and I got a custom harness from MRP to work for my AE86 with the 16v 4AG motor, swapped to COP.


that tach output from the ECU is not working with the factory gauge cluster and I didn’t know if you could help guide us to the right path on wrapping this up. It’s the last thing before we can dyno the car. Let me know if anyone can help.

Does something like this work?

https://www.diyautotune.com/product/axm ... h-adapter/

https://www.justraceparts.com.au/jrp-ta ... al-adapter

If your ECU has a tacho output that switches 12V to ground you can produce a voltage spike
suitable for driving an OEM AE86 ANALOG tacho with a small relay coil. That's easy What
is difficult is knowing what kind of input signal (voltage/frequency) your DIGITAL dash requires.

If the digital dash can accept a voltage spike derived from the coil negative the same as the OEM
analog tach, then you can use this using the coil out of any small automotive relay you have lying
around. You only need the coil, the contacts can be removed to stop them buzzing



If your ECU has a dedicated square wave 5V or 12V tacho output and that does not work with your
digital dash, you might save yourself a lot of pain by temporarily using an aftermarket tacho that
will work with your ECU output.

Cheers... jondee86

jondee86 wrote:If your ECU has a tacho output that switches 12V to ground you can produce a voltage spike
suitable for driving an OEM AE86 ANALOG tacho with a small relay coil. That's easy What
is difficult is knowing what kind of input signal (voltage/frequency) your DIGITAL dash requires.

If the digital dash can accept a voltage spike derived from the coil negative the same as the OEM
analog tach, then you can use this using the coil out of any small automotive relay you have lying
around. You only need the coil, the contacts can be removed to stop them buzzing



If your ECU has a dedicated square wave 5V or 12V tacho output and that does not work with your
digital dash, you might save yourself a lot of pain by temporarily using an aftermarket tacho that
will work with your ECU output.

Cheers... jondee86

Very helpful sir. Going to first try your recommendation...if not; then looks like I wasted a ton of money making this digital dash work for me, haha...

Just to be clear, the simple solution outlined above won't work with a dedicated tacho
output that produces a 5 or 12V square wave. If that is how your tacho output works you
will need to see if you can program a spare aux output to behave as a tacho and pulling
the 12V supply to ground at the engine rpm frequency. The idea is that the small coil
emulates the action of a single ignition coil producing a back emf voltage spike each time
the coil is switched by the igniter.

To generate a voltage spike using a 5 or 12V square wave tacho output from your ECU you
can use this type of emulator. The tacho signal from the ECU is used to switch a transistor
on and off momentarily grounding the coil to produce a back emf voltage spike.



Again, these emulators will only work if the tacho in your digital dash can be driven by a
voltage spike.

Cheers... jondee86

jondee86 wrote:Just to be clear, the simple solution outlined above won't work with a dedicated tacho
output that produces a 5 or 12V square wave. If that is how your tacho output works you
will need to see if you can program a spare aux output to behave as a tacho and pulling
the 12V supply to ground at the engine rpm frequency. The idea is that the small coil
emulates the action of a single ignition coil producing a back emf voltage spike each time
the coil is switched by the igniter.

To generate a voltage spike using a 5 or 12V square wave tacho output from your ECU you
can use this type of emulator. The tacho signal from the ECU is used to switch a transistor
on and off momentarily grounding the coil to produce a back emf voltage spike.

Again, these emulators will only work if the tacho in your digital dash can be driven by a
voltage spike.

Cheers... jondee86

Just a quick update....haven't figured it out yet. Will try to look at my old analog cluster vs this digital one to see if I can get it to work. :/ Trying one more thing to see if I can get it to work...

https://www.dakotadigital.com/index.cfm ... rd1192.htm

I'm interested to see how this works out. There are multiple writeups and videos
showing how to modify and fit digital clusters into various old Toyotas, but not one
of them identifies the required tacho input waveform. In fact most writeups kind
of fizzle out due to problems in getting the dash to work correctly

The few wiring diagrams I have seen show the tacho input coming from the igniter.
But using the voltage spike from a coil as an input does not seem to work. This
suggests that the igniter used on cars with the digital dash modifies the coil signal
into a form to suit the digital tacho.

If this is the case the tacho input is most likely a low level square wave with variable
frequency. If the tacho does not respond to a 12V pulse from your ECU it may want
a 12V or 5V signal that switches to ground. This is the kind of information that does
not appear to exist anywhere on the internet or in repair manuals. Probably because
the digital dash was a thing in the 80's but disappeared in the 90's.

One thing that would be helpful is to see if there is anyone with an original digital
dash car, and scope the tacho output from the igniter. I believe there were some
differences between the JDM and USDM cars, but I would expect the waveform to
be the same. Good luck

Cheers... jondee86