Niehoff tachometer

I don't know but be careful lest you ruin it. There are several possibilities...........

May have a battery hidden inside needs replaced
May need an external sender like the older Suns
(If this is the case try hooking your ohmeter to it and swap lead polarity both ways, look for any meter movement of the tach)
May hook to coil + (and ground)
May be set up to trigger on mag/ alternator pulses on a 2 stroke such as outboard