I started this thread in the interior section
https://www.thirdgen.org/forums/inte...em-gauges.html

Maybe I should have started here instead.
At this point I'm trying to find info on how much voltage I should see on the dash connector for the Tach? White wire coming from the coil.
With the engine idling, I'm measuring 13.5vdc. My tach sits at 7000 rpm.
Anyone have any info on what I should be reading?

Should be batt, with about a 3 millisecond pulse to ground for every cyl firing. Which wouldn't show up on a DC meter at idle. So what you're seeing is probably right.

Sounds like your tach is busticated, same as everybody else's.

Read a few posts on this forum about "tach" and see what you think.

Yeah, I think you're right.
How far away are you to building a fix?
Is there any info on what exactly is bad in these?
I can pull it apart and re-solder parts...

So I came across this thread

https://www.thirdgen.org/forums/elec...-required.html

What is the open 4 - 10 that its referring too?
I haven't seen that in another thread yet

He had the schematic for 2 things conflated... the LM1819 chip, and the factory's resistor array. It is of no consequence.

Are you saying that no one has figured out why these tachs go full sweep?

does the tach return to minimum when ignition switched off?-IIRC '82-89 tachs can get "stuck"at upper RPM section of dial if power is cut while tach is reading high RPM-as I recall,high revs can correct this condition.

It's not a question of "no one has figured it out"; it's just electronics after all, and just old Stone Age analog circuitry at that, pretty much a no-brainer. Problem is more a matter of ever getting it back accurate again since the part that fails is the calibration resistor. All of which is done in the most pathetic weenie failure-prone way I can even imagine for doing it. Just altogether a poor design.