I also have a 4JH Yanmar. I have had intermittant problems with the tach. I cleaned all the connection associated to the Tach and it slowly came back to life...for now.
A good idea is to identify the injector 'kill' lever and paint it red. I have to actually climb into my engine room to get to it. I don't want to be screwing around a turning engine looking for a lever in poorly lit conditions.
__________________ "Life is not a journey to the grave with the intention of arriving safely in a pretty and well-preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming: Wow - what a ride!"