Spastic tach

[Cooter]

In the very beginning of your service manual:

http://buellmods.com/content/downloa...ing_manual.pdf

Page B-17

Screen Shot 2023-01-07 at 8.12.04 PM.jpg

The pink wire that drives the tach comes directly from the ECM.
BECAUSE you said your bike runs fine while tach is spastic, all the signals TO the ECM are fine.
the signals TO the ECM are fine.
the signals TO the ECM are fine.
the signals TO the ECM are fine.

BECAUSE you said the speedo is NOT spastic at the same time, it eliminates any power or ground supply issue externally feeding the dash.
it eliminates a power or ground supply issue
it eliminates a power or ground supply issue
it eliminates a power or ground supply issue

SO

You have an issue internally in the power or ground or signal to the stepper motor that the tach needle is on inside the dash. Replace it*.

OR

You have a intermittent open in the pink wire circuit that goes between the ECM to the dash. Replace it.

Period.

Screaming into the abyss




* Buell XB mileage is held within the dash itself on a 'count-up' chip. So buy a dash with similar mileage.