emptech

This is a very old post, but nobody has answered it.
I have reverse engineered the 260z tachometer. It consists of some protection circuitry and an RC network to differentiate the pulse from the (-) terminal of the ignition coil. The signal receives further conditioning and then enters a potted module, which drives the tachometer, nothing more than a voltmeter. The potted module, made by Hitachi, is nothing more than a one-shot multivibrator. I believe the pulse width was about 1.5ms, with a repetition rate relative to the engine RPM. The actual frequency is the engine RPM divided by 20.

I have seen the one-shot multivibrator fail. It can fail in the conductive stage, in which the tachometer would peg, regardless to the engine RPM. Other modes, no response at all, or a jumpy tachometer.

I built up a small circuit using an NE555 timer chip, right out of National Semiconductor's application notes. I also added a nine volt series regulator, to make the tachometer immune to the vehicle voltage. All this I installed on a small piece of perf board, not worth my time to make a circuit board.

Hope this helps others who are searching this subject

- emptech, for Empirical Technology, also known as Jim