How do you Hook up a tachometer on a diesel?

The easiest way in many cases is off of the engine's alternator. This is done by wiring a terminal into one of the leads on the diode and rectifier pack.

http://www.midwesternmedicalmachine.com/~damonfg/alt_rebuild/017.jpg

If you look in this picture, you will see the three terminals that the stator and diode hooks to on the rectifier. The diesel versions of alternators with tach outputs will have a wire from one of these terminals to a terminal on the outside of the alternator casing. You can wire one through a vent hole in a casing, but be sure to make sure the wire will not rub through. This gives the alternator a pulse to work from. Remember, the alternator is generally spinning faster than the engine, so you will need to calibrate the tachometer to the alternator's pulses, and to do this, you will need some type of photo or mechanical test tachometer.

Another option is to use a magnetic sensor which is mounted to take readings off of the flywheel or the front pulley. To have this work, you will need notches of some type for the sensor to create a signal from. To see an example of this type of set up, look at the set up of ABS wheel speed sensors on modern cars.

Yet another option is a mechanical drive tach. To do this, you will need an engine that is set up to drive a mechanical tach. They generally drive off of the oil pump or cam. Some tractors used a mechanical tach driven off of the rear of the alternator, but those are rare and hard to find.

A final option would be to use a photo sensor on the flywheel or front pulley. Generally these work by placing a reflective strip on the rotating object which reflects a light beam back to a sensor when it crosses in front of the light beam from the sensor, and that provides the impulses to determine engine speed.