I've searched and read, but haven't found the answer to an issue I've had for a while. At this point it isn't so much an issue as something that I'd like to fix and understand.

My OEM tach doesn't work properly.
The shift light that is connected to the back of my gauge cluster does work, all the time.
My ECU has a "tach sweep" feature that sweeps the stock tach needle at startup - and the sweep works.

With the engine running, the tach is dead (stays at 0), but the shift light (with integrated digital tach) works perfectly.

The car & wiring harness are 1996.
I'm running Toyota COPs (though I don't think it is relevant)
Vi-PEC ECU (Link G4 Extreme)

...and here's the very strange part. The tach startd working all on its own the other day. Later that day I realized that the alternator wasn't putting out any voltage. I replaced the alternator, and now the tach is back to not working while the engine is running. I've disconnected the alternator a few times and get the same results:

With car running & alternator working (~14.3 volts), the stock tach does not work but everything else does.

With car running and alternator disconnected (~12.5 volts), everything works, including the stock tach.

So, any help on:

A. Why
B. How do I 'fix' it?

 

Problem is solved, at least for my needs.

I actually had 3 devices connected to the ECU's tach output: the stock tach, the Autometer shift light, and an output to my data logger. Seems three is one too many for the ECU's internal pull-up resistor. The stock tach sprang to life as soon as I disconnected the wire that ran to the data logger.

The wire was an old connection anyway, and not necessary, as the logger and ECU now speak to each other via serial cable. That wasn't always the case, so the RPM lead to the logger was necessary at one point. According to Vi-PEC tech support, I could have used an external pull-up resistor to make 'em all work. Anyway...

Thought I'd update this in case someone else ever searches to solve a similar problem.