It is COMMON for them to stick al the time in the lower readings rather than pack up.
They can and do pack up though and stop the engine too!!
It is only a VOLTMETRE with na tacho face stuck on, it reads instead of analogue signals it reads DIGITAL SIGNALS.
The wiring to it is very simple, it reads from the coil throught he same green wire that feeds the coil, try bypassing original wire with one straight between coil(green) and tacho(green) and see what happens.
tabetha