Pulled my tach out to check my connections. There are five connections on the back (and bracket) of the SW tach:
POS
NEG
SIG
Light
Ground is a tab on the bracket side
SIG goes to the (-) coil
POS goes to a fused power source
Light, well, that's the light connection
Ground is a shared ground wire from the fuel gauge
NEG ---> This needs to be connected to ground.
I had left NEG floating, having assumed the case and bracket provide adequate ground. They probably do, but NEG cannot be left floating. Tie it to ground.
Tach works fine.
