The charge indicator light is wired through the ignition switch, so any resistance in that circuit will cause the light to flicker or remain on, either dimly or possibly at full brightness. If the condition is only the result of resistance in the light circuit, the generator should still function fully. What you describe seems to be low or irregular output from the generator, likely worn brushes, or badly worn or loose commutator bars (worn brushes are the preferred cause).
A quick check on the generator is to ground the field terminal of the generator while you run the engine at a fast idle: if the indicator light goes out, the generator probably is functioning properly; if the light continues without much change, the trouble is either the voltage regulator or the light circuit. Clean the regulator contacts as Hotrod Lincoln says, and temporarily connect a jumper wire from the regulator frame to the battery ground. If the ground wire corrects the problem, remove and replace the regulator, making sure the bolts are clean- those are the ground. Also check for satisfactory body-to-frame grounds.
A quick, but somewhat inconclusive, check of the ignition switch is to wiggle the key cylinder when the light is acting up. If anything changes, there is a loose terminal in the switch- not uncommon.
Without a wiring diagram, all this is somewhat speculative. What year is your truck?
I'm also assuming you are talking about a DC generator, not an alternator. Is that correct? The cause of trouble and the remedy will be different, depending on that.
Harvester
