Reading scanner codes on a 1995 dodge cummins?

the scan tool communicates the the pcm which usually communicates with most other modules. if your pcm was shot then the scan tool might not communicate with the pcm. im thinking that truck gets its tach signal off the alternator, also your voltage is off the alternator, so im betting you have a bad alternator. im not sure if you can replace the seperate sensor that picks up the tach signal or the voltage on the alternator.