***edited my above readings because I wasn't reading my multimeter correctly the first time***

Here's the correct coil readings I'm getting:

Primary: 1.0 ohms

Secondary: 12400 ohms

So this coil appears to be within the specs that Zed Head posted.

Edited September 10, 2024Sep 10 by kcpope

I can't remember the exact details but I know that people have fiffled with the loop of wire on the back of the tach and had good rresults.  I think that they might have removed a loop or made it bigger.  It generates an electromagnetic feild and the electronic igntion boxes apparently overload the current sensing system.  They pass more amps.  As I understood what I had read and barely remember.  I think that you can reach it from underneath without removing the tach.

https://www.ebay.com/itm/235464345938?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=sQc_o1-rRnS&sssrc=2047675&ssuid=&widget_ver=artemis&media=COPY

Edited September 10, 2024Sep 10 by Zed Head
bogger

3 hours ago, Zed Head said:

I can't remember the exact details but I know that people have fiffled with the loop of wire on the back of the tach and had good rresults.  I think that they might have removed a loop or made it bigger.  It generates an electromagnetic feild and the electronic igntion boxes apparently overload the current sensing system.  They pass more amps.  As I understood what I had read and barely remember.  I think that you can reach it from underneath without removing the tach.

https://www.ebay.com/itm/235464345938?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=sQc_o1-rRnS&sssrc=2047675&ssuid=&widget_ver=artemis&media=COPY

Thanks for the info. Interesting.

Before I do that, I’m going to try 2 things:

1. Adding a ballast resistor back in with the existing 1.0/12400 ohm coil, and see if that helps the situation

2. I found another coil laying around in my old man’s garage. 1.3 ohms primary, 10600 ohms secondary. I’ll try this coil with and without the ballast resistor as well.

Let me know if anyone prefers option 1 or 2 better.

Well having not messed in depth with the electronics inside the tach, I could be easily convinced that the wave SHAPE and DURATION are also important, not just the peak current.

Point is... I can provide no assurance that the stock tach will be compatible with anything other than points.

If it was mine I'd reach up and wiggle that wire around while the engine is running.  If something changes you'll know there's a possibility.  I might even attach some extraneous metal to the post that the wire encircles.  I don't know what exactly happens as current pulses through that small coil of wire but I imagine that it generates a field in that core that passes in to the body of the tach.  Then magic happens.  

Edited September 10, 2024Sep 10 by Zed Head
splign

Welp, I tried both options I described above. No change.

Only thing I haven’t tried is adding the condenser back in the wiring. I’ll give that a shot.

Also I didn’t realize that the tach can be pulled without removing the dash. That’s a bit of good news.

In the meantime, I’ve found a good deal on a tach from a 260Z. Can anyone confirm if those are voltage driven just like the 280z tachs?

Edited September 11, 2024Sep 11 by kcpope

The 260Z and 280Z tachometers are interchangeable functionally.

10 hours ago, kcpope said:

I didn’t realize that the tach can be pulled without removing the dash. That’s a bit of good news.

It can still be difficult if you have a dash cap.  The cap holes are barely big enough.

The 260Z and 280Z tachs are voltage driven.  They'd work but you'd need to rewire them with a wire from the negative post of the coil.  Probably with the 2.2 kohm resistor.  And you'd need to connect the power wires when you remove the 240Z tach since igntion power runs through it.  Not a big deal really.

And, the fonts are different.

A new thought - it might also be that the matchbox module is failing.  Is it Nissan original or aftermarket?  You could wire in a GM HEI module as an experiemnt for about $30.  Always good to have a backup anyway.

26 minutes ago, Zed Head said:

It can still be difficult if you have a dash cap.  The cap holes are barely big enough.

The 260Z and 280Z tachs are voltage driven.  They'd work but you'd need to rewire them with a wire from the negative post of the coil.  Probably with the 2.2 kohm resistor.  And you'd need to connect the power wires when you remove the 240Z tach since igntion power runs through it.  Not a big deal really.

And, the fonts are different.

A new thought - it might also be that the matchbox module is failing.  Is it Nissan original or aftermarket?  You could wire in a GM HEI module as an experiemnt for about $30.  Always good to have a backup anyway.

Good info, thx. I’ll read up on other threads regarding everything needed for the tach swap.

Yes, my E12-80 module is factory Nissan.

I’m intrigued by the GM HEI swap. Could that be a permanent solution if my tach ends up working with it?

The modules work the same way.  It could be a permanent change.  

23 minutes ago, Zed Head said:

It can still be difficult if you have a dash cap.  The cap holes are barely big enough.

The 260Z and 280Z tachs are voltage driven.  They'd work but you'd need to rewire them with a wire from the negative post of the coil.  Probably with the 2.2 kohm resistor.  And you'd need to connect the power wires when you remove the 240Z tach since igntion power runs through it.  Not a big deal really.

And, the fonts are different.

A new thought - it might also be that the matchbox module is failing.  Is it Nissan original or aftermarket?  You could wire in a GM HEI module as an experiemnt for about $30.  Always good to have a backup anyway.

The mounting is different, too. You have to adapt the 260Z tach internals to the 240Z tach or adapt mounting brackets on the 260Z tach to mount it in the 240Z dash.

The 240Z tach has studs on the left and right.

The 260Z/280Z tach has a bracket at the bottom and a screw that goes through the top front of the gauge.

The 280zx swap in mine is factory Nissan and the distributor was rebuilt by Advance Distributors before installation.  Plus it works verified  up to 4k rpms by my timing light so while I'm not adverse to installing an early 280z Distributor with a face plate swap, it would be interesting yo see why some 240z tachs work perfectly with the same swap and others fail at varying degrees.  Mine is a 73 tach...