I know that when the battery symbol lights up on the dashboard it indicates a problem with the electrical system, which is commonly either an ageing battery or a broken alternator, but for the purposes of diagnosis, what does it actually mean in a circuitry sense? Does it come on when the alternator charging voltage is too low, or when the voltage supplied by the battery is too low?

I've had it come on in two different cars, for different reasons - first one the alternator was fine and the battery needed replacing, and second one, the alternator was worn out.

1 Answer 1


The "classic" warning light was connected between battery positive and the alternator output.

This would then exhibit various states of illumination, which with experience, would tell one what was wrong without having to do all the tests stated in the workshop manual.

So, if the light was on solid bright then the alternator was not charging and that could be the alternator or a broken wire between alt and bat.

If the light had the merest glimmer then it was a failed diode - there were 3 for the warning light and 6 for rectification.

Some would have the light flickering and that would be brushes, but all of these would vary between the different makes of alternator - as what was one fault on one would be a different fault on another. Fine if you worked on several though.

  • uh, no the classic warning light is between ignition positive and alternator input. it comes on when the alternator can't make enough current to power the field windings.
    – Jasen
    Commented Feb 26, 2021 at 4:47
  • @Jasen ignition positive is battery positive - there is no voltage reducer for that part of the circuit. Field windings? I think you mean stator windings which are 3 phase either star or delta wound.
    – Solar Mike
    Commented Feb 26, 2021 at 5:03
  • Not the same, there's an ignition switch between. Voltage reducer means what? well the same point is connected to the rectifiers that provide current for the field, but they don't pass current that lights the lamp. what is connected to battery positive it the actual altenator output.
    – Jasen
    Commented Feb 26, 2021 at 12:23
  • @Jasen depends on the make of alternator, some only have the 3 warning light diodes feed the warning light and the feed for the rotor circuit comes from a different point on the rectifier pack - the other 6 diodes. Worth checking out various makes of alternator especially the differences with those with internal and external regulation and also those with selectable low, mid and high charge rates (used on machines where the engines run at constant rpm unlike cars).
    – Solar Mike
    Commented Feb 26, 2021 at 12:27
  • 1
    @Jasen voltage reducers were used to control the behaviour of fuel and temperature gauges, but that was a different issue.
    – Solar Mike
    Commented Feb 26, 2021 at 12:29

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .