It's those little things that go wrong that eventually bug you. A number of years ago I was driving my 240Z and all was well. Then one day on a cruise out of the blue the needle on the tach started slowly climbing until it was pegged! Shut the car off, restarted and all was almost well. At idle through about 3200RPM the tach works fine, but then will go no higher than about 3200RPM. Didn't really mind too much as I normally shift by ear and feel anyway. One thing that was changed about 1,000 miles before this happened was the dizzy was replaced with a 280zx distributor (electronic) and the ballast resistor was eliminated. I've gone over the connections and all appears to be well and the car runs and drives just fine. This is pretty minor, but has got to the point where I thought I'd ask if anyone out there has a clue where I might look?

Thanks,

Wilkes

Just a guess but maybe it's a sign that the ZX module is gong bad.  Shorting to ground lightly, causing constant current through the tach circuit.  If you have a spare module a swap might show something.

Or the higher current flow from the ZX module and the coil is damaging the tach.

From what I can see it looks like the LX-516 module is the same thing as the E12-80 module... Thoughts?

https://www.ebay.com/p/Standard-Motor-Products-Lx-516-Ignition-Control-Module/1022975711

 

That looks like the right part but I think it's a scam like "New" alternators.  It should be flagged as deceitful on eBay.  Look at the words and compare it the real part on Rock Auto.

https://www.rockauto.com/en/partsearch/?partnum=LX516

 

Are you saying this isn't the correct part? Or are you saying this isn't a "new" part? From all Google searches LX-516 shows as correct for a '81 ZX. 

I doubt it's doing damage to the tach since it's been like this for over 3,000 miles and since the initial tach run up to pegged has never happened again. If the module was going bad would there be any other symptoms besides the tach not reading correctly from 3000 RPM and up? 

It's a "knockoff", not actually made by Standard Motor Products.  The eBay ad is a lie.  The one on Rockauto shows a true manufacturer, Ultrapower.  The eBay ad says SMP, but is false.

Notice how they avoid defining "manufacturer" and instead use part number or brand.  It's a con and eBay is enabling it.