I had a used VW Variant square back sedan with a generator when I was stationed at Minot AFB in North Dakota. First winter I had it I was led a merry chase trying to trouble shoot/diagnose the genny. Driving the 15 miles from the base to town (Minot, ND) the little red idiot light would sometimes come on for the first 10 miles or so then go out. By the time I got to town the batt was charged some and all was OK. Couldn't find any problem. Brushes and commutator were OK. Then sometimes returning to the base the same thing would happen again, red light for 10 miles or so then normal ops. Batt would get a bit low during the approx 10 miles (+/- 3-4) if it was at night due to draw of headlights but charge back up some after the red light went out.
Standard practice in ND in winter is to put at least a trickle charger on the batt overnight so I never had the batt go flat. Still, It was really bugging me.
Think about it!.................What could it be????????
Post your best guess(es)
What I eventually found and how I cured it, available on request if anyone is interested.
Pat