02 2nd Gen 2500 6sp 4x4

Having an issue at startup with a sudden voltage crash after the grid heater shuts off. Everything seems normal at first, the wait to start happens and after the grid heater shuts off there is a giant voltage crash about 10seconds later and the check gauges light comes on. I’m getting the following codes: P0380 P0382 P1689

I have replaced the alternator, grid heater, grid heater relays, checked all associated fuses, my batteries are newer, load tested and fully charged (currently sitting at 12.76v) I have checked all the cables, connections and grounds.

I replaced the grid heater and relays because of the P0380 and P0382 codes

If I try to start the truck before the voltage crash it is still slow to crank. I have gotten it to run once or twice in its condition and it runs and drives normally if you can get it started.

I have posted a link to a video of the problem as it happens

 

If you have a multimeter you could check if the voltage drops at the battery or if you have the voltage drop in the rest of the system only.

If the voltage of the batteries stays up while it drops on the dash indication, you need to continue to seek the point of the voltage drop. To me it looks like you got a bad connection somewhere that gets worse as it heats up under high load.

Possible culprit is the main battery wiring both plus and ground, these cables often do corrode inside and lead to the problems you have.

If the voltage drops at the batteries too you need to get them load tested. If they are good you will need to find what is consuming this inordinate amount of power.