Need some help from a Chevy Truck Mechanic

[BRD-PREY]

Hi, I have EFI Live for my LS1. I own a 03 Duramax with an Edge, exhaust and AFE intake. Work truck.
Heres what it started doing today.
OK I've noticed that the Volt meter doesn't go to 13.8 or so until the trucks been running for about I don't 20 seconds or something. Its always been like that since new. I figured they have a relay or a delay built in to the alternator field flash for some reason.
Today I got a not charging message and a low voltage on the volt meter. I got out to make sure I didn't pop a belt. Belts fine I wiggled the Alt plug. No luck about 2 minutes later it started charing. Wierd. I shut off and started 5 or so more times through the day and the same thing it takes betweeen 1 to 3 minuted to start charging.
Is their a relay or something going bad. My Batteries are fine.
Thanks Ed

[blackdeath]

get a new alternator

[joecar]

If this works like the car alternators...

Check that the alternator turn on wire has voltage... if it does, then the alternator is bad.

Otherwise, with a fused jumper wire, provide the alternator turn on signal, and see if the alternator works (13-15V)...
if it does not, alternator is bad, otherwise the turn wire and/or the PCM have a problem.

[BRD-PREY]

The reason I posted this instead of just replacing the alternator is the following. I have never seen a vehicle where the voltage doesn't go to 13+ volts immediately upon starting. I made the comparison\conclusion that since voltage is below 12.8 vdc that the alternator was not producing any juice. Point is, I'm not sure if it's alternator and not field flash or maybe some other large electrical load is keeping the voltage down right after starting (glow plugs etc).
Since it has done this since new and the wait time for the voltage to come up is about 30 secs or so. I didn't care and just though it was as designed.

Now to catch up to 5 years and 153,000 miles. I started the truck the other day and 5 seconds after the start I got the battery not charging message. I figured great a $150.00 or so alternator. But not so fast after running about 2 or 3 minutes the message cleared and voltage came up. 6 more starts since then and the time to come on kept getting longer and the battery not charging message didn't clear yesterday when the wife used the truck.

So why post here; I was explaining to my wife that if a person really understood the charging system for my truck that those above symptoms would probably point with most certainty at the problem. That and it just does't sound like an alternator to me. maybe theres a ect/iat*cetane pid for alternator voltage LOL

thanks for your time Ed

[joecar]

Ed,

If you look at the various STATE pids, there may be one that has the state of the alternator bits.

[NelsonDiesel]

How old are your batteries?

Ya, believe it or not.... good possibility.

What happens is when you first start up the truck you have the glow plugs drawing massive power as well as the intake air heater. After a bit these will shut off and thats where your voltage goes back up. Haviong some weak batteries wills how this more because they aren't able to retain the power like they did before.

There is a GM bulletin on this as it is a very common problem.

[Sparky8370]

My pass side batt had an intermittent short in it that would cause the light to come on every now and then. It would also drain the other batt sometimes. When it got to happen too often, I replaced them.
The route I would go if it were me is first replace the batteries if they are more than five years old and see if that cured the problem. Most of those chain stores will also load test your batts for you, as well as check your alt. Load checking my batts would not have helped because one just had an intermittent short inside that became worse. I noticed when I disconnected and reconnected when checking connections. The other batt was still hooked up so I shouldn't have really gotten very much spark, but I could tell it was under a load when I reconnected.