Tachometer jumps when ignition is turned but drops back to zero once car starts.

[Ptero]

A few details on the car. 1973 240z /w electronic ignition. Using ignition module E12-80.

Exactly as the title says. When starting the car the tach will bounce a bit but then drops back down to zero once the car has started. When driving the tach stays at zero.

Today the tach slowly crawled up to 8k and stayed there until I turned the car off. I did however read that this happens when it's hot outside and today was one of those days. So that one makes sense (to an extent).

I searched but didn't find another issue like this one. Is my tach fubar'd or do I still have a hope of getting this thing working?

What I found on the tach going up due to hot weather.

The tach in 240zs are known to malfunction when it gets hot (95+) outside. I believe it is the resistance (?) loop on the back of the tach which causes the problem. Under normal operation, this loop heats up due to the current passing through it.

When it gets hot outside, the heat is unable to dissipate as well and the tach starts to creep up over time. Eventually it will just peg at redline.

[Zed Head]

Did it work before? With the electronic ignition? Or is the ignition system new and has created the tach problem?

[Ptero]

Not too sure if it worked before as I'm the new owner and don't know the history of the electronic ignition change.

[beermanpete]

It sounds like you have the 4-wire type of tach. These tachs might not work correctly with electronic ignition. It depend on the ignition design and how the charging current pulse looks to the tach. If you can, put points back on the car to test the tach. A lot of work just for a test though. Or, replace the tach with a 3-wire type.

The heat related issue is due to the design of the tach. Mine did that once in a while when the weather was hot. I replaced the 2 transistors in the tach and it seemks to work better. Still waiting for a really hot day and the car to be running to stress test the repair.

[Ptero]

I think I've got a couple of extra tachs with the parts that came with the car. I'll check if they are 3 or 4 wire. Is there any way to test them without setting up another ignition system?

[beermanpete]

The easiest way to test either type of tach would be to connect them to a running car that still has points. It does not have to be a 6 cylinder car necessarily. Just know that the tach will not read the correct RPM if tested on an engine with less or more cylinders. It will respond however and that will tell you what you need to know.