Peterbilt 386 starts and dies immediately

[JimT]

You said earlier that the batteries were just replaced due to old ones only having 8v... now I'm not an expert here but maybe you have a short somewhere else (not the ecm) that is draining the batteries? I would start by checking all the cables in the aluminum battery box as I've had shorts in there myself. Next I'd check all the grounds again starting at the battery and starter. While you're at it how is that starter? Is it drawing too much current to turn over? how about the solenoid, connections clean, no problems inside or out? Also, have you checked the alternator output?

I'm no expert, but all those voltages you mentioned seem too low. 11 instead of 12, dropping down to 8.5 while cranking instead of 10.5-11

Also, I've mentioned this before and seen others mention it as well. But on my truck, a 2012 Pete 587, when the key is off and all lights off I still get a slight glow from my brake lights (they are LED's so a resistor might 'solve' my problem). I've heard from others that this is common and it's something to do with the CECU or body controller, I forget exactly. I bring this up as that might be another place to check for a short or parasitic drain.