<VV> C-K truck tach question (NO CORVAIR0

[airvair]

Maybe some motorhead here can answer a non-Corvair question for me.

On my '84 C-20 Sub the factory tach has suddenly taken to pointing
straight down at the floor, with no response whether the engine is on or
off. Any ideas as to what's causing this, and what it would take to fix
it? I don't think the needle has come loose, though I haven't had the
dash apart to be sure (I'd think that it would have fallen off, if it
were to merely have come loose.)

-Mark