Question about M1028 Charging System

[L B]

So the isolated ground alternator (passenger side) on my M1028 finally went out. Long story short, I was out in the middle of bad flooding when it happened (I'm guessing the water shorted it) and made it back home, so I expected the rear battery to be drained.

I had the alt rebuilt and installed it, but when I turned the key to "ON" nothing happened. No lights, no buzzer, nothing. I check both batteries and my rear battery has 12.5V but the front has only 6V. The front battery tested good and is currently getting charged, but I wanted to ask this question to make sure I don't have another issue.

Should the front battery have been drained if the passenger alternator went out?

My understanding is that the driver alt charges the front battery and the passenger alt charges the rear battery, so under that train of thought I'm very confused as to why my front battery drained.

Both batteries are newer 900CCA, front is 1 yr old, rear is 2 yrs old.
Driver alt is a newer 78amp, the Cadillac sub from the wiki, 1 yr old.
Passenger alt is original (now rebuilt) 100amp.

Thanks in advance,

-LB

 

[cucvrus]

That is because all the 12 volt accessories and lights are being drawn off the front battery. With just 1 alternator the system limped it's way home. Get the batteries both charged and load tested. Then get the proven operational alternators back on properly and test the vehicle. You should be able to answer the question after everything is charged / operational and reinstalled like it was designed. that is a hard question to give a definitive answer to without seeing the vehicle and what modifications have been done. A bad battery could cause the issue you are describing. Good Luck. Be Safe.

 

[L B]

UPDATE: The battery I took to have charged still said to recharge it after the store charged it, even though it still tested as a good battery, so they swapped it out for me under warranty. I installed it and the truck started right up, GEN 2 light went out, GEN 1 light stayed on.

@cucvrus Thanks for the response and I guess you're right, the strain from limping it home seems to have killed both the alternator and battery on that side. I'm going to get the driver alt replaced again, but I still don't understand why the driver alt would die from strain without the other alt - isn't running the truck's 12 volt system what it does normally anyway? The way I'm understanding the charging system (mine is bone stock, by the way, the only thing altered is the start system), the passenger alt dropping out removes both it and the rear battery from the equation so it should have just been like any other 12V charging system for the drive home. Unless it's the fact that the driver alt was a 78amp instead of the original 100amp, I don't see why it would have struggled so much. I'm just trying to understand the process of what happened.