Electrical mystery - '02 D'max

[davewald]

My stock '02 2500HD started showing under voltage for about 1 minute whenever I start it cold. It doesn't make any difference if the outside temp is 20 degrees or 60 degrees, and I always wait long enough for the glow plugs to heat up. Once the truck has warmed up, the voltage is normal (about 13-12 volts) whenever I re-start. I replaced the batteries with top of the line Sears batteries about 10 months ago, the serpentine belt is tight and in good condition, and the truck has about 69K miles. I have tried disconnecting any add on's, like the compressor for the air springs, with no luck. The only other anomoly is that my block heater started tripping my GFCI plug in my garage, but that was a couple of years ago and I haven't used it since.

Any ideas?

[DmaxMaverick]

The voltage drop you are seeing is normal. When you start the cold engine, the glow plugs will cycle, even after starting. If cold enough, the intake heater will also draw juice, and the "cut-off" temp is above 60

[davewald]

I mis-typed the voltage. Once the truck runs for about a minute, the voltage jumps to the 13-15 range, which is what I consider to be normal. It's weird that I didn't notice the cold start under voltage until this year, though. I'm pretty attentive to the truck. Is it common for the behavior to start as the truck ages? I would be disappointed (though not surprised) if GM shipped the truck with this behavior, since a simple fix would be to use a larger alternator or some sort of capacitor to handle the short term load of the glow plugs.

[Turbo Al]

I have the 2001 and it doesn't surprise me that you missed the voltage changes when the air intake heater is on and the glow plugs cycle. I have only noticed it once or twice since I bought this truck, the first time I noticed it I dam near had a heart attack seeing the needle way down there then I realized what was going on when it bonced back to normal.
Al