tachometer fluctuating

[kepani]

1996 240sx (s14)120,000 miles

my tachometer is powered up, but for some strange reason, the needle fluctuates; sometimes pegged at 4k rpm, then 6k rpm, etc. it's totally independent on what speed the engine is at.

i'm getting a factory service manual soon - hoping there's some troubleshooting in regards to this. does anyone have any ideas or tips on where to start? i did read briefly about the tachometer reading based on voltage range. was it 0-5 volts? i don't remember.

any thoughts or ideas would be appreciated. once i get the service manual, i'll be looking around in there.

kepani