How does tachometer work in diesel engine?

A tachometer works the same on almost all cars exept motorcycles.

Now first you need to know what a camshaft is. If you don't search it on google.

The RPM's or Rotations Per Minute,

Are the rotations your camshaft makes.

Quick extra: your cranckshaft rotates twice as fast as your camshaft makes.

So when you are revving at 3000 rpm your cranckshaft wil rotate at 6000 rpm.