Since diesels don't have an ignition system, how do tachometers work on these things? I'm sure the newer diesels use cam or crank position sensors to measure RPM to operate a tach but how do these older diesels do it? Thanks for any info.

 

My tach has a sensor mounted just off the main crankshaft pulley. There are two magnets affixed to the crankshaft pulley on opposite sides of the pulley that pass across the sensor as the pulley turns.

The tach itself is an ISS Pro aftermarket tach.

 

A magnetic pickup just like tater said. Some have the pickup on top of the injection drive tower and works off the injection drive gear.