Driving 97 kick tach after J18A engine swap

[fixkick]

Hey there!

So I tried hooking up 12V to the tach and a mosfet (since that's what I had on hand), as a switch, pulling the tach wire to ground through a resistor when turned on by the ECU signal wire. The needle does jump a tiny bit only when cranking, as soon as the engine starts, the needle stands still.

My Chinese scope has arrived too, but I don't really know how to use it yet. I wrote a simple arduino program using the "tone" command. It should output a 5V signal at a set frequency and 50% duty cycle. I set it to 250Hz for the test and when I hooked up the scope and pressed the "auto adjust" button I got some funny results (so this is arudion out put)/? if yes then the driver in the Arduino is set wrong,
i know that arduino and computers like this very well,  and there are many output modes in the Arduino possible
what works best is not known yet , impossible to know  at all; you need to do the below tests first then with those facts learned we design the ardiuno sketch to do the right thing, both signal type and polarity  and all that.
We even do not know the output of the current cars tacho signal at all
nor what the meter wants. 2 unknowns.

the real problem is there are 2 devices here,  old PCM to old tach  and not a different engine,? to different meter>? IDK>
I am not clear at all what this means
"MK2 Golf which I intend to engine swap with a VW ABF 16V engine,"  what does this have do do with anything suzuki? IDK?
are you saying there is no J18 engine, no J18 PCM , no J18 car here?
What car is the target car.? what is the donor card, all parts moved tell.
what PCMs are used and what tach meters used in target car
none if this is clear at all. and very very confusing.




here:
https://photos.app.goo.gl/rRjqmkx7gT6qkiPp9
Why does it display a negative pulse and nothing near a square wave? I also tried manually setting the pin to output 5V, then 0V with a manual delay and I got pretty much the same result. Either this scope is crap, or I can't set it up right.(I'm leaning more towards me being dumb).

What could be causing that?

to convert from one simple out put to another device,  simple one must know 2 things.
The ECU sends the tachometer output
and the actual tachometer input wants some other unknown signal.
the problem is both are alien to each other,
first we scope the car ecu first. tacho pin,

those narrow spikes are buss capacitance changing voltage states,  means wrong settings on i/o port  of the Arduino
but we need to answer the above green questions first.

the path is simple (pure electronics only)
the tacho meter must get the same signal it got off the old cars PCM. in the exact same way , voltage , polarity and wave form type.
the conversion can even be simple in some cases but we do not know what the tachometer wants yet.

more later.