Maybe the ecu is doing a positive only trigger, and if the tacho used to be points then it's looking for a 12V- Ground pulse. I'd try a "pull down" resister- wiring the 12V tach output through a 560ohm resister and to ground. Wiring the tacho to the 12V side the tacho will now get a 12V - ground pulse, simple but it works in some instances.
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