PCU PROBLEMS!!!
 

My 97 cummins has a charging problem i had the alternator, starter, tested both were good.. Tested the battery's one tested bad, so i got a new one after that i noticed that my alternator was still not charging and my speedo or tach will work.? I hear that pcu is a common prob with these trucks and when it goes bad that neither one of these things will work, I am going to get another one tomorrow, But.... I wanted to double check with you guys and confirm yea or nea???? Thanks for the help!!

My 97 was doing that same thing but it was the engine fuse blown. It blew a couple of times over six months then it quit one day and wouldn't start. I had to change the shut down solinoid and relay and it hasn't had a problem since.

Replacing just one battery will cause the other to have issues. Always replace both batteries at the same time.

So the alternator tested good?

Yes, I believe the computer controls regulates the charge rate. But you can wire in an external voltage regulator.

If your speedo and tach don't work, it is almost certainly NOT related to the pcm. The pcm will not turn on the alternator or speedo/tach (or a/c for that matter) if it does not get a signal from the crank sensor, which is located on the front of the engine right over the balancer. If the sensor or it's wires are bad you will get EXACTLY those symptoms. This happens much much more often than a bad pcm. You should probably check those wires carefully. There is a plug that connects the sensor to the rest of the harness-it may have come undone.