This is a common problem when changing from a distributor to coil-on-plug with many different Toyotas. I had to solve it on my 3S-GTE SW20 MR2.
When there is a wire from the igniter to the tachometer, the wire effectively connects to the ignition coil negative inside the igniter. This means that every time there's a spark, a voltage spike is generated on the wire to the tachometer.
The tachometer then detects the gap between each voltage spike from the igniter to calculate the RPM.
The problem is, a normal tachometer-style output from an aftermarket ECU will simply generate a square wave signal, but it may not exceed the voltage threshold expected from the voltage spikes in the standard setup with the igniter.
The solution is:
Use a relay coil (ignoring the switch contacts) to pull the tachometer output up to 12V. When the ECU pulses the tachometer output to ground it will activate the relay coil, and because it's a coil it will generate a voltage spike as required.
Do a Google search and you will find slight variations of this concept mentioned on heaps of different forums :-)
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