Just took old tach out to do a check on RPM.  Connected the green wire to Dist. and Black wire to ground on batt..  Tach. had no reading.  Went to grab ground and notice small amount of voltage at connection. This does not seem right.  Connected ground to motor still nothing and voltage still on connection.  Then touched red wire to Batt. Pos. side and tach. worked.  Am I missing something, did not think that needed power source just to us.  And don't understand the current through ground connection.  Any help with this.