Charlie;
Welcome to the Forum!
P1800s had two versions of tachs, differentiated in that the earlier ones had a seperate sensor module, usually located in front of the radiator. I have little experience with the earlier one. If this is the type you have, others will have to help.
Later ones had the sensor internal...sensing was done without a direct electrical connection...the points wire strictly makes two loops through a bracket secured by a nut on the backside of the tach, which allows inductive coupling to transfer IGN pulses to the circuit. The circuits are known for quiting as the calibration potentiometer inside opens due to poor connection from age. I have writen this up before...see:
http://www.sw-em.com/service%20notes.htm#Gauges ....but I suggest urge you first assure everything external to the tach is as it should be, before molesting the tach to gain access to its insides. Internal circuits are easily found with a search.
Good Hunting! Greetings from new England!