It only seems to happen on long trips, after the truck has been running for an hour or more, but as I'm driving my headlights have out of the blue just shut off to the day lamps (or just extremely dim) for just a few seconds, somtimes more, and then return to normal illumination. I've tried turning on and off the stereo when it happens, but it seems to happen independent of what is running. I've asked other people, and they say it's a sign that my alternator will need to be replaced in the near future. My battery still has a good charge on it, and I seem to have no trouble, especially for this cold weather lately, to start it in the morning. What is going wrong with my vehicle?
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How have you determined that the alternator (generator) is working fine?
Whilst the engine is running you need to see if there is at least 12 volts (usually 13.5V) across the battery using a voltmeter or similar tool. Also, whilst the engine is running, what happens when you turn the headlights on? (Does the car stall or try to stall?) This would affect night driving using headlights.
Most importantly, you run the risk of damaging the diodes on the alternator whilst running the
engine with a flat or undercharged battery.
There is also a regulator that controls the voltage from the alternator.
have you checked all fuses, including engine fuses located under the bonnet?
Initially, the headlight relay unit would be suspect (likely). I would suspect corrosion on the terminal connector of the fuse box/relay bank (most likely) affecting the headlight. When damp/cold, the corrosion would be triggering the relays to engaged position. The steering column switch would be the least likely since as you posted headlight still on even if that switch is disconnected. the same may also hold true for the signal flasher. A good approach would be to work your way backwards from the headlight, fuse, relay, switch assembly using a DVM measuring/checking for +12V presence.
Hope this be of initial help/idea. Pls post back how things turned up or should you need additional information.