I have a 12 valve cummins The alternator won't charge the batters but the alternator worked befor I took it off to change the pulley. We have had it tested and is good put on and it did not work. The rpm gauge stopped working about the same time.....?????????..????????

 

Check for battery voltage at the post on the alternator (8 gauge wire) with the key on.
Either a fuse, current limiter or wire broken probably.
You can also check the plug on the alternator one of the two wires should have twelve volts on it which excites the alternator field winding's.

 

Mine did the exact same thing and it was a bad battery. No tach, no charging, alt tested good on the bench. Thought it was the PCM (which is also possible, as it's where the regulator is- but rare, as they almost never go bad) but it wasn't.

 

Engine speed sensor.... Mounts just above the harmonic balancer.

And its a bit picky to set.

 

Thought I was having that problem once too, but it turned out to be a 24V balancer.

 

jberar stated a good point. if you get a damper to replace the old one or bad one you have to make sure it has the two notches that are grooved in the damper if you dont then the sensor can not sense the pulses going by on the damper also want to make sure that the sensor is gaped properly .049-.052 i bilieve is the correct distance.

 

i grinded the two notches in but still wont work i checked to see if its getting power at the connector and it is ive also gapped it many times and still wont read im guessing its not gapped right again