2006 GMC battery pack problem

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06-13-2024, 07:16 AM

Filmmaker

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2006 GMC battery pack problem

I have a 2006 GM hybrid truck; I have replaced the batteries and read 36v at the high voltage port. I have also replaced the battery under the hood. The three 12v batteries on the hybrid pack are all showing 12v each. I removed and individually charged them (revived all batteries but 1) 1 battery now is smaller 250ca it balances out to 36v at the business end + 12v under the hood. It doesn't even attempt to turn the engine. No errors shown on dash. No warnings from battery pack. I forced a warning by circumventing the 48v switch so I know its sending data to the ECU. The truck was functioning a few years ago but sat around for years due to a bad battery pack, the owner replaced them with new batteries (1 year ago) but 1 of the new old stock was 11v prevent it from working. This truck is in good condition other than this hybrid system and maybe old gas. I never saw it running so I have my doubts; maybe something else is causing it from turning over.

1- The square water cooled inverter upfront (under the hood - right hand side)... could it be causing it not to start? How does one check this? Can I force a bypass. How?
2- Since I have ZERO response when I turn the key and no errors I'm assuming some safety device is causing this. Is there a way to bypass low voltage switches on the truck to allow it to attempt to start?
3- Does the truck require that the main 12v up front also be 100% charged to start? I left the new battery connected with the bad battery for a few days after replacing (I didn't realize it was adding the 12v upfront to the total voltage on the battery pack - face palm GM). I charged it overnight last night and will try again this morning with the 1 small battery in the pack. I also I have the original battery revived 85% giving 12v this morning so I could put that back and try again.
4- What else can keep this truck from starting if all batteries are charged?
5- Anyplace I can measure voltage / ohms or check continuity? Any suggestions on how to test relays or other elements?

Suspicions. Could the total CCA be the issue? It would not make sense really, if a 48v system wants to see 48v is has that. It should attempt to crank the engine however sluggish. Right now I have two original Panasonic monster batteries and 1 smaller battery.

Please respond with technical info I have required experience.

Thanks in advance.