BJ 42 tach working intermitingly

[caper]

tach works sometimes and not , is there a sensor somewhere on crankshaft , hopefully just needs to be cleaned or something

 

[Dieseler]

Tach should have a hologram sensor on the bell housing and gets its reading from the flywheel on one revolution IIR there is a magnet inserted in the flywheel and the sensor reads it when it passes.
There are some Tachs which get their reading from the IP but this is not the case on a 3B