My F150 has a issue with its alternator. Last year my alternator died and I had the shop replace it. A few days later my charging light started coming on. Whats unique is when it comes on. When I am at my summer home where it rains a lot the charging light comes on when I first start the truck providing the temperature is 5/6deg C or less. No other time. When I am at my winter cabin where it does not rain but snows, it comes on at first start providing the termperature is 0deg C or below. Generally, once warm there is no issue. When I am driving, it will also come on once the outside temperature hits -10deg C. If I continue driving it stays on. If I park and let idle it goes away. Once I am back on the road it will come on again, but as I say not until the outside temp hits -10deg C. At no time do my lights or other electrical draws dim or slow down. The lights and heater seem to be working as usual even though the alternator light is on. In fact, I have driven up to 40km with the light on and nothing dies or slows. It really seems to be temperature and possible humidity causing this issue.

So in recap, light comes on in moist conditions at +5c or less. In dry cold conditions it comes on at -10c precisely.

 

1st thing to do is have the alternator & battery load tested as a reputable shop that knows how to load test ( not the autozone walk out with the hand tester that is one size fits in X band / all and tests an unknown charge state battery ).

If it has been happening since a few days after replacement, I would suspect the alternator, but testing is the only way to know for sure if it is that or something else.

The other thing you can do in the mean time is to check the battery +VDC cables and the grounds ( and grounds on the truck ) for signs of damage. You will want to make sure to check that the ring terminals have a good crimp on them, and not corroded up inside them ( quick bend of it might break it off ).

 

U may have an internal leak to ground thru a semiconductor in the alternator when it gets to that certain temp. I would have it tested on a cold day for sure. It may be a open door diode when cold. There are several path of current flow with the circuits of the Alternator, the cluster, the ECU and battery. Some are very small but if one leaks?