2004 gmc (sshhh, close enough) duramax. been having this issue for quite some time. when i first start the rig, the volt meter stays around the 12-12.5 mark for about 30 seconds, after which, it jumps up to the normal mark, lights brighten up, everything works as should. it only does it after sitting overnight. replaced battery, cleaned all connections, replaced alternator. had load test done on system, passes perfectly. amperage doesn't fluctuate or drop, voltage good. there just seems to be a short period, only after sitting overnight, where it takes a bit to get juice to system. rig cranks and starts great. can't find a TSB regarding this, or anyone else who has had this issue. it's not affecting performance at all, so i drive it. just one of them things. ideas?

 

My 03 2500HD has the same problem.Doesn't do it when it is warm outside or the truck is warm.

 

The should be normal on the Duramax Diesel. The ECM turns on the glow plugs and intake air heater when cold. Plus, it juggles the glow plug ON times and alternator duty cycles to prevent the glow plugs from overheating (takes just a couple of seconds to reach ~1,800°F).

 

The dodge diesel does it to when starting. It is warm air injected instead of glow plugs.