I just got a brand new Alternator and put it in on my 1996 12 valve Cummins. It is all hooked uo correctly but when I ground it to a bolt or something just metal to know it is grounded properly it reads 0.01, 0.02, 0.03, or just 0.00.(It is normally grounded to what its metal mounting bracket) So the Alternator is not putting out power and should be about 13.5v. So i know it is not the alternator because its brand new and tested. The batteries are good because they just got a charged by a battery charger, after it ran they were dead though. At the batteries it reads 12.3v so they are basically dead and getting no juice at all. What could it be, the only thing that came to mind was the Power cammand module or PCM that is the voltage regulator that tells the alternator to charge. What could be the problem with the PCM, or do i just need to replace it.

Thanks a bunch Tom.

 

Here is a good writeup I saved from a couple of years ago. PCMs rarely go bad, and if it did, assuming it is just the charge section, I wouldn't replace the PCM I would just externally regulate it. That info is available here as well.

BTW, This is assuming your Tachometer is operating, it is isn't it?

"I had to troubleshoot my charging system recently and came up with a fairly simple way to isolate the problem. Here is a step-by-step way.

First you will need a DVM to measure voltage. Also the batteries need enough charge to start the engine, so use a battery charger to get them charged enough.

The charging system is made up of the PCM mounted on the firewall, alternator, and don’t forget the 120 amp fuse in the PDC on the driver’s side fender well.

1) Start the engine and measure the battery voltage at one of the batteries. It should be more than 13 volts but probably less with a charging problem and that is why you are here.

Measure the voltage at the alternator output terminal (B+) leaving the DVM negative connected to the battery negative post. It the measurement is greater than 13 volts then the fuse in the PDC should be checked or the nut holding the heavy gage wire on the alternator is loose.

If you get this far then the next step is to measure the voltage coming from the PCM. The two small nuts on the back of the alternator are the ones to measure with the lower one positive and the upper one negative. These are the field terminals on the picture. If the voltage reading is greater than 8 volts the PCM is calling for full charge and your alternator is bad and not putting out. If the voltage is less than 3 volts your PCM is toast or the wiring between the PCM and alternator is broken. The normal voltage reading at the field terminals will vary from 3 to 6 volts when working correctly.

It’s always a good idea to clean battery terminals and check for bad grounds before spending money on replacement parts.

My alternator turned out to be bad and I had 9 volts at the field terminals. It was the diodes inside that were bad."

 

There are alot of posts about replacing the voltage reg. He is correct they rarely go bad.

http://www.cumminsforum.com/forum/94...tall-pics.html

Floyd