After a few years of having the tach operate intermittently, hang up at a random rpm , read too high, read too low, etc., I decided to have a look at it. It doesn't come apart as the trim bezel is swaged over the outer case, so it would be ruined if I attempted to open it up. I figured I'd have to buy a new one, but I did a few other things first and it works great now. Here's what I did:
1/ Fix the ground. The ground wire to the tach was between the nut and the mounting bracket on the stud marked GND. I scammed a pic off the net so you can see, even though I think it is a speedometer shown. There was no support behind the bracket, so I added another nut. Once everything was back installed, I tightened the extra nut up against the back of the bracket firmly sandwiching the ground connection. I know this at least helped my panel lighting which is now as bright as the other instruments. It was slightly dimmer before.
2/ There is a 4-6-8 cylinder switch on the back of the tach. Thinking it might have dirty internal contacts, I rotated the switch CW and then CCW several times. The switch doesn't have much in the way of detents, so once back in the car, I fired up the engine and adjusted the switch to the middle of the range it worked in. I suspect this is what helped the most.
3/ Contacts on the SIG and BATT lugs were male spade terminals, so I cleaned them with a bit of 800 grit paper. They looked more or less OK to begin with, so I doubt this did much.
So I'm sorry I can't narrow it down more, but if you have a wonky tach, and you know your ignition system is working properly, try these three things and maybe save yourself the cost of a new tach.....at least for a while.