[Andy4639]

If the lights go dim on a stock truck with no mods it's the external regulator and the wiring.
The stock system has a built in excitement wire that tells the alternator when it needs to produce more voltage depending on the draw but it has to be above a certain RPM for it to kick on. At idle it kicks off and that's why the truck, & dash lights dim. Plus the fact of how it's wired.

Here is a link to some very good reading on how the GM system works and why to's. They explain it much better than I. I did mine off this page as well.

http://www.madelectrical.com/


With a internal alternator this problem is helped but not always fixed due to the wiring. Some of the earlier internal alternators needed the exciter wire also but later models didn't. The dimming is also caused by slipping belts and worn out belts as well.

Wiring is also a problem depending on how well it was maintained over the years. Take a volt meter and hook it up inside the truck on a hot wire and ground it. Crank the truck up and see what kind of voltage you get at idle with everything off. Then start turning things on and see how it does. This is a quick and easy way to see how the system handles the loads. It should stay around 14 volts with a good wiring system.