Dimming lights.. need help

[Super Kaz]

For about ten minutes or so after I started my truck today the headlights and gauge lights kept going dim. It does about every 30 sec for about 5 seconds or so, only when I driving. I have a digital volt meter in the truck and it reads 14.1v at idle, 14.2v when driving and drops to 12.5 when the lights diim. Its been doing it on and off for about a week. Might do it all the time and I don't notice, I don't alway drive at night. Anyone else seen this. Do you think the alt and going out? It is a dual alt, what is the chance that both are going out? I getting ready to head to texas this week (1100 miles each way) and am a little worried about it.

[todd]

when your glow plugs and iah cycle it will do that im guessing just the first time youve driven it in the dark now that the temps have droped. glow plugs still draw amps for a couple mins after the wait to start light goes out

[alberta 7.3]

The dual alternator system is controlled by the PCM, which will switch back and forth between them, it is possible that one of the alternators is starting to go out on you. The best bet would be to bench test both alternators, and see if one is weak or failed.

[dieselcrawler]

Not to hijack... but my truck does something simular... first start it, turn on the headlights, kinda dim, like it's running on only the batteries, then when I take off, once it gets wound out, lights brighten up. Not gradulaly, but as if you turned on a switch, all at once.
My truck also has the dual alt's, if one's going bad, might it act this way?

[alberta 7.3]

Originally Posted by dieselcrawler

Not to hijack... but my truck does something simular... first start it, turn on the headlights, kinda dim, like it's running on only the batteries, then when I take off, once it gets wound out, lights brighten up. Not gradulaly, but as if you turned on a switch, all at once.
My truck also has the dual alt's, if one's going bad, might it act this way?

Your situation sounds normal, after a cold start, the glow plugs will remain energized for 1-2 minutes or so before the PCM will shut them off. Once they are shut off, the voltage will spring back to normal. Glow plugs are a large load, 150-200A, so they will drop the voltage in a hurry.