Jumpy tachometer, any fixes

[nukegoat]

I just noticed this on my '96 3 weeks ago. Only happens when stopped and tach will jump down to 200 RPM then back to normal. Engine isn't changing it's idle.

No big deal, just more character.

Same. 600rpm to 400rpm back to 600rpm. Actual engine speed is smooth and steady at 600. Obd2 reports 600 as well.

 

[Bambusiero]

Does anyone know what kind of signal is used to convey tachometer info to the instrument cluster?
i.e., is it a dc voltage or current proportional to RPM, direct frequency signal at a nominal voltage amplitude, etc.?
What would you see if you probed that signal with an o-scope?

 

[nukegoat]

Does anyone know what kind of signal is used to convey tachometer info to the instrument cluster?
i.e., is it a dc voltage or current proportional to RPM, direct frequency signal at a nominal voltage amplitude, etc.?
What would you see if you probed that signal with an o-scope?

Good question

 

[minsanity]

Sounds like a faulty ground.

^This....Checked the ground by the batt terminal. Twas warming up on load. Terminal was securely bolted to the inner fender. The culprit in my case a loose splice on that cable. Proper resoldering steadied the jumpy tach.

 

[sogafarm]

I've just recently noticed my '94 tach with the random twitch at idle and around the 2000 rpm mark. Actual idle doesn't change, but the tach needle jumps randomly about 200 rpm then goes normal. I notice no performance loss at all. After reading this thread, still not sure what the underlying cause is. I don't necessarily want to replace my tach without knowing for sure that's what it is. I'll double check my battery ground just to rule that out.