'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:

[spinem]

well the red wire should read 12v when "off" for it is conneced to the + off the batt to starter.

When running the red wire SHOULD still be 12-14v due to is is the "exciter" wire.... the brown should be signal/dummy.....

try splicing into the red wire and jump it to battery while running/ while testing + off back of alt, if line is bad, the alt should jump up as you connect your "jumper circut" (more v than what ever reading before)

next method is to check behing the motor, above the tranny, this is where this red wire meets the + to the cab and +off starter... this is connected by a clamp on lead hunk and is knows for corroding and going bad.

.....if all fails, and i know this sounds stupid, but check the belt tension... if your not using a pry bar to tension it /w/, its too loose.

idk about jumping the brown wire to a + souse for ive never done it.... good luckk!!!

[Corts60]

I know that would probably work by jumping it because the alternator is good. However, I need to find out why when it is wired up normally that the power is not going where it needs to. I think the diesels had a diode or something installed on the brown wire somewhere and I was hoping to get some diesel guys' opinions on this.

[James McClure]

Cort, The charging system between gas and diesel are the same, just the parts are bigger because of the addition of a second battery and you have extra battery cables to accomidate this. The red wire on the back of the alt (bolt on) is alt output. The red wire in the plug in connection is the "reading wire". The alt uses voltage on this wire to regulate output and feed the rotor. The voltage regulator toggles a ground feed to the rotor internally to increase or decrease the magnetic field the rotor makes which in turn increases or decreases output. The brown wire tells the alt the ign is on and is fed thru the gage or idiot light. Check ALL the connections and related fuse links. Also look for ANY green deposits along associated wireing or term ends. IF, the alt tests good, then a problem with wireing is the only thing left. jim

[Corts60]

Thanks for the response Jim. I am going to spend some more time on it tonite after work and I'll post what I find. The fuse links were my next target.

[Corts60]

OK. Figured it out. It turns out some years ago my good friend who owns this truck installed a battery isolator on the firewall so when he is towing he can charge the trailer battery as well. Welp, that thing most have opened up internally because it was preventing all juice from getting to any battery. She is charging at 13.5 now and that is a good thing! Thanks for the help gentleman.

-Cort