Just thought I'd pass along some info in hopes it helps someone having issues with their tachometer.
The tach on my 79 SR5 has been cutting in and out for about a year now. Occasionally the needle would bounce all over the place for a bit before finally cutting out completely. Recently decided I'd finally try to figure out what was wrong with it.
I'll save all the troubleshooting I went through to land on the solution, but the short version is everything on the board tested fine except the little IC in the tach unit labeled "C1006". (Note this is the board on the tach itself, not the motherboard for all the gauges). I found an IC on eBay labeled "C1006C" for $15 and swapped it out and the tach seems to be working fine now. I don't know what is going on inside that IC. It's old enough that there isn't any documentation on the web about it, but my guess is it converts pulses from the coil into a steady voltage to move the needle. During my search I Found a thread from an Isuzu forum and it looks like the same IC was used in the tach unit for some Isuzu trucks as well, so it must have been a fairly common chip - meaning they should be available in some form for a bit longer even without documentation.
Based on my troubleshooting and talking to people with similar problems, my guess is that these chips develop internal shorts over time. Before I pulled the old chip, I noticed the 47ohm resistor coming off the 12v supply was getting very hot. Many people with failing tachometers have reported finding that resistor (and sometimes the zener diode next to it) completely cooked. To add to that, when I first installed the new chip, I forgot to solder the pin which is on the same trace as that zener diode, and the needle bounced around all over the place before maxing the gauge out at 8k. I went back and re-soldered that pin and the tach has been working great since then. That pin must have something to do with voltage regulation for the tach unit.
Hopefully this saves some people some money gambling with junk gauges on eBay. There's not much to these tach boards so as long as the gauge itself is working there's no reason not to try and fix it.

 

Great. Thanks for posting your resolve, so many people post problems they're having, and people jump in with possible fixes, and you never hear back which one actually works.
Thank you.

 

I know what you mean, and it's very frustrating. I've been searching high and low for solutions to the tach problem, but never found anything. I know lots of people have this particular issue, so I wanted to make sure there was something documented to try next time somebody starts googling for a solution. These tachs are getting harder to find, and it makes me cringe when I think about how many have just been thrown away because people assume they're unfixable