99 dodge diesel tack work part of the time when it stops service engine light comes on any ideas.

 

Most likely your CPS (crankshaft position sensor). The ECM uses it so it knows when to time the injections. It also uses that sensor to know if the engine is running and for the tach signal.

2 other symptoms should show up when the tach dies. Your A/C compressor (if on) will stop and your voltmeter will show 12 volts instead of 14. The reason for that is the ECM controls the when the A/C clutch is engaged and also controls the output of the alternator. When the ECM loses the signal from the CPS it thinks the engine is not running and shuts off the A/C compressor and the alternator output like if you were to turn the key off.