Hi I also have the long standing problem of a non working Tacho in my 1963 P1800S.
A great place to start is to read Ron's excellent pages on the various types of Tacho's used in the P1800's,
http://www.sw-em.com/Smith's%20Tachometer.htm
My own one is a Generation 1 with a remote sender which I think is the reason it doesn't work. I recently purchased a modern replacement sender unit from Spiyda which I hope will resolve the problem, but I've not got around to fitting it yet.
https://www.spiyda.com/tachometer-electronics.html
There is I believe a Canadian firm which will repair the Generation 2 instruments with the electronics in the gauge rather than a remote sender.
http://www.volvosolutions.com/1800inst_tach2.html
Your first step is to determine which generation of gauge you have and Ron's web pages will help you do that. Please be aware that I think there is an oddity in the transition from P1800 to P1800s before the the introduction of the Generation 2 unit, see my post on this
https://www.volvoforums.org.uk/showthread.php?t=274712
hope this helps
Dave