'78 diesel charging problem

[Corts60]

Hey guys, I'm usually over in the 60-66 section but I am posting this for my neighbor.

He has a '78 Silverado 5.7L diesel (yes, I know but please don't start the 6.2L argument) and the alternator will not charge the batteries.

We have taken the alternator to NAPA and it checks out fine on their machine. I have never troubleshot a diesel charging system and I know they are different than the gas engines so I need some help here.

When the key is "off", the red wire on the back of the alt. and the red wire on the two-wire plug read battery voltage. The brown wire is 0. When the key is in the "run" position without the engine running, the brown wire reads 4-5 volts. When the engine is running, the red wire on the back reads 30 volts, the brown wire reads 26 volts and the red wire in the plug reads battery voltage. Does any of this sound right?

Any and all help is appreciated.

-Cort

Here is a pic: