I've replaced the tachometer pickup wire since my tach was not working. It would occasionally jump around while driving but that's it. Unfortunately that didn't fix it.
I was driving tonight and out of no where it started working perfectly. I shut the car off and started it back up and again it didnt work. Does this sound like a tach amp issue (cannister under the hood) or something with the tachometer itself? I'm curious if anyone had anything similar happen to them.

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2022 Nissan Frontier Pro4x
1990 Mercedes 420SEL
1950 Packard 8

Previous German cars-
2001 Mercedes SL500
1983 Mercedes 300SD
2011 BMW 328i (manual)
2008 BMW 535xi (manual)
2006 BMW M3 (manual)
1980 Mercedes 300TD
2006 BMW 750i
1996 Mercedes E300D
1994 BMW 740i
2006 BMW 330xi (manual)
1999 E300DT Smoke silver/black 253k
2012 BMW 535i Xdrive M-Sport
1984 300TD-T 304k
1988 Mercedes 560SEL (parts car)
2010 BMW 550i xdrive
2017 Audi A4 Presitge
1979 300SD 313k
2003 E500 189k

Classic symptoms of eminent tach amplifier failure or corroded connections where the tach amplifier pulgs in.
Try cleaning the connections with a good contact cleaner & conditioner, like DeoxIT-D & DeoxIT-S.
If that doesn't help, then the usual culprit for intermittent tach operation is bad solder joints in the circuit board in the tach amplifier.

Sometimes, disassembling the tach amplifier and adding a cigarette filter between the housing and circuit board can restore functionality (albeit temporarily).
The cigarette filter acts like a shock absorber, preventing the circuit board from flexing too much.
The circuit board can be re-soldered, but you have to remove the anti-vibration elastomer encasing the componets.

Or you can just replace the tach amplifier with a good new or used one.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022