yesterday while driving, my battery light came on.
When I measure the voltage on the battery, with a running engine, the voltage is 13.8V.
The battery is brand new.
IMHO that’s right on the mark where it should be.
Any suggestions as to why the battery warning light comes on then?
sounds a bit on the low side. With an alternator you should see close to 13 at idle and close to 14 short above that, say at 1000 to 1500 (not at 3000 rpm). That being said: there must be a fault in the circuit as at 3000 rpm and 13.8 V the idiot light should be off for sure - not brighter!
How about the history: when did the malfunction occur first time? What did you do prior to that? Any other incidents?
Strangely, the warning light grows stronger when revving the engine higher.
On idle, it is barely visible at all…to me that indicates a working alternator, but broken wire / short etc.
Had the alternator died, it would either deliver no current or a current that is way to high imho…
(Author of the Book, former owner of an '83 XJ-S H.E.)
Perhaps the alt has lost one phase? I’m not sure what the symptoms are for that, but I know an o-scope shows too much “ripple” in the voltage.
The alternator does, but the current is transformed to direct current in order for the battery to be able to store the energy. So, when connectong a scope to a live feed, it measures dc. When connecting the scope over the seperate phases of the alternator, then it’s a different thing…
More specifically, the light is being grounded from somewhere.
One side of the bulb always gets 12v+ when the key is on. The other side is grounded in the alternator (thus it illuminates) until the alternator begins charging. At that point 12v+ is provided to both sides of the bulb and it extinguishes.