74 260Z tach grounding problem
 

Hello,
I just got this 260Z and it has a tach problem. When I turn the key to ON, the tach goes flying to over redline.... I think this got to do with choke wiring since there's one broken wire on choke area so I try to ground this wire with chassis and the fuse blew. if anyone knows how to bypass choke wiring for tach please let me know! (I don't need choke, it's not connected to carb anyways... thanks!!!

260z tachometer needle pegs
 

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