testing a tachometer

Hey all, I have a 74 260Z with a tach issue.

First off, its a V8 swapped car with a Tachmatch box which until very recently worked perfectly fine to display the correct RPM. Once in a (long) while the tach would start hunting around sporadically, but it would go back to normal after a restart or two. Well, the other night, it stopped working entirely.

The Tachmatch box has a 'test' mode. With the ignition on (engine off) I have measured this mode to output a steady 1.4v to the signal wire.

I then removed the tach from the dash and measured this same 1.4v at wire (A) red/black

With the ignition on, I also measure 12-13v across wires ( green & black

Does anyone know off hand what the voltage going to the signal wire may be at idle and up? Perhaps the 1.4v in test mode isn't enough to budge the needle? How else might I test this situation to determine if the tach itself is dead? Nothing changed... it used to work, now it doesn't. The Tachmatch box has an LED and some diagnostics troubleshooting which indicate its working correctly.

Thanks all