Start with the wiring diagram from the FSM:

It shows one wire coming from the fuse box: blue. There is another wire coming from the resistor: blue/white. The wiring diagram shows this as yellow/white. Finally there is a ground wire.

From this approach, the first thing to check is quality of the ground. Measure resistance to ground on connector on the dash harness side at the black wire. Also make sure there is no resistance from the connector on the gauge to the gauge body. Clean the connector with Caig Deoxit while you're at it.

Next, as I stated before, check the resistance from the blue/white (or yellow/white) wire at the dash connector to the negative post of the coil. You may need to make an extended lead with an alligator clip to reach. You can build your own probe with something like this: https://www.amazon.com/Insulated-Multimeter-Stackable-Connectors-Adapters/dp/B07C535GBD.

Also make sure the dash to engine harness connectors are clean.

 

 

Edited May 23, 20222 yr by SteveJ

Okay, I tested all those connection.  First us3 the continuity setting the the the resistance to verify the actual ohms.  
 

- Black wire on dash harness side to ground shows “short”. With a reading of 1.2 ohms,  

- Blue white (LW) on dash harness to neg post of coil.  Shows “open” with 2.23 k ohms

- Then tested Bluw White (LW) on diagram to the resistor and it  shows “short” with .3 ohms 

- other side of resistor to neg coil post shows “short” with . 3 ohms

-resistor itself tests ar 2.2 kOhms

-when I put a jumper where the resistor it goes I get a” short” reading with 1.6 ohms

This all looks correct to me.  

Of note the thee gauge lights to the tachometer do not illuminate with all the other lights on the dash.  New bulbs in there also.  

 

Researching the web I found a post where a guy had the same problem after adding the ZX ignition.  ZedHead was on this post.  This guy had an MSD coil and when he went back to his OIC coil the problem went away.  I have the same coil I was using before I removed the ballast resistor?   Does anyone think adding a resistor to blue white on the coil would fix this.   Not sure if the old ballast resistor dropped the voltage to the tach.  Or I can try a different coil 
 

Here is another thread on hybrid Z.  This guy changed his coil too and it fixed the problem.   The stock coil is designed to be used with the ballast resistor and  is rated at 3 ohms without a ballast resistor you want a 1.5 ohm coil.    
 

https://forums.hybridz.org/topic/125373-tach-not-reading-below-~1200-rpms/

 

I never had tach issues with ZX ignition in my 260Z. (Same general design as the 280Z tachometer circuit.) It was modified by a PO. I eventually bypassed the ballast resistor and using a coil with similar impedance as a 280ZX coil with an E12-80 module. No problems.

I'm not sure what you mean with respect to adding a resistor in addition to the one that's in the circuit. 

Okay more research to run by you all.  The E12-80 ignition unit needs 12V to function as advertised. The e12-80 module likes a 0.84 to 1.02 ohm primary resistance coil because the currents and voltages match the modules' duty cycle.  So putting in a 3 Ohm coil could cause issues with E12-80 ignitions especially above 4500 RPM.   The 280Z tachometer uses voltage not current to read RPM.   It prefers 5-10v.  280z works great with a 1.0 ohm coil when there is a ballast resistor between it and the tachometer.  The ballast resistor only add the resistance after the engine is started.  If I want

let’s say 8v to the tachometer I need more resistance  between the coil (-) and the tachometer.  I’m thinking 8-10kohms.  This explain why when people put a higher primary resistance coil their tachometers began working properly. 
 

on CaliforniaDatsun’s website for a ZX distributor upgrade he has this blurb in the write up.

 

Edited May 23, 20222 yr by Av8ferg

Have you looked into one of these?

 

https://www.technoversions.com/TachMatch.html

The CA Datsun thing is probably for using a ZX distributor in a 240Z.

The 280ZX's use a resistor inline to the tach also, just like the 280Z's.  The 1978 ignition system is very similar to the ZX system.  Nissan just shrunk the electronics down.  1978 and 1982 both use a 0.84 - 1.02 coil primary circuit specification.  Both ignition modules are designed for the lower resistance primary circuit, and the tachometer system is also.  You could just copy the specs from the ZX system.  

1982

The CA Datsun thing is probably for using a ZX distributor in a 240Z.
The 280ZX's use a resistor inline to the tach also, just like the 280Z's.  The 1978 ignition system is very similar to the ZX system.  Nissan just shrunk the electronics down.  1978 and 1982 both use a 0.84 - 1.02 coil primary circuit specification.  Both ignition modules are designed for the lower resistance primary circuit, and the tachometer system is also.  You could just copy the specs from the ZX system.  
1982

I’ve pretty much done that already.

34 minutes ago, Av8ferg said:

I’ve pretty much done that already.

Couldn't tell, I looked back but I don't think you mentioned which coil you were using.  Maybe in your other thread.

8 hours ago, Av8ferg said:

 I haven’t changed out the E12-80 unit for one of my backups but I really didn’t think that would make a difference.  Any ideas appreciated.  

I had a vague memory of a similar thread (among many, tach problems are common) and found it.  Post #29 is the one.

 

I had a vague memory of a similar thread (among many, tach problems are common) and found it.  Post #29 is the one.
 

Okay, you’ve convinced me to try this. I have another completely rebuild ZX distributor that I’ll try out. I wanted to test it anyway so I’ll see what happens. I may even have an additional E12-80 ignition unit on top of that .

Well, I have figured this out and now and can post this for future Z owners that convert their 280Z to a ZX (E12-80) ignition.  
 

First I tried what ZedHead recommended and replaced the (E12-80) with on of my OEM backups.  Result.. no change tac remained non functional below 1200 RPM.  
 

I’ve kept coming back to my original theory which was the Tac doesn’t like voltage greater than about 12.5V.  I surmised this because I put my multimeter in the tac and measured the voltage charges as I revved the engine.  Voltage drops at the tachometer at higher RPM.  I noticed that around 12.3v it would begin to indicate RPM.   
I then checked my primary resistance on the coil.  It came to .6 Ohms   Using Ohms laws voltage = current x resistance.  I determined that with a .6 ohm coil I was pushing the max alternator voltage to the tachometer.   
So I went out and bought a 1.5 Ohm coil.  Put that one it and the tac magically worked.   I have three coils on hand now and tested each one.  
 

With the 1.5 ohm coil installed I should be reading a lower voltage at the tach   At idle it’s around 10.3 v. 
With the 1.0 ohm coil installed I read around 12.5v at the tach and the RPM worked at idle

With the 0.6 ohm coil installed I read 13.4v at the Tach and no reading at idle.  
 

So I believe the mystery is solved.  I learned a lot on this problem and I also got to test my backup ECU and (E12-80) ignition modules.  
 

Time to move on the the next problem….do they ever end?    Thanks for the help guys! 

Edited May 24, 20222 yr by Av8ferg

To be rigorous you'd try another 0.6 coil.  

Also, it looks like you're saying the 1.0 worked fine.  Not clear why you went to 1.5.