Volvo tach shows reading when engine is off
 

My tack now jumps to 2500 RPM when I turn the ignition on without the
engine running. I checked the grounds and the wiring, all looks OK. How
do I tell if the tach is bad or is it in the wiring? The input to the
tach (gray) wire seems to have about 1.2 v on it when I turn the
eignition on. Whne the engine starts the tach maxs out. Anyone know any
tests I can make to narrow the problem done?

Paul