[No HA content] Car "alternator warning light"

[Geoff <Geoff@xxxxxxx>]

We are having a discussion on one of the uk car NGs about car ignition
warning lights and *exactly* what they mean so I thought I'd ask you
guys as Electronics Consultants   :-)

Several of us have had cars where the ignition/alternator/
call-it-what-you-will light on the dashboard glows dimly all the time
the engine is running.  It's not mechanical - e.g. slipping belt - and
the alternator seems fine and the battery doesn't run flat.  Battery
voltage is 12.6-13.0 at rest and 14.2 when engine running and
on-charge.

Change either the battery or the alternator and the problem sometimes
goes away.

Increasing the load - lights, heater blower, etc - makes little or no
difference to battery state or intensity of the glowing light.

So what does this mean?   How do these lights work?  Someone suggested
"There are all sorts of types of alternator warning light
arrangements, although I *think* the most common sort these days picks
up its (+) from the ignition switch, and its (-) from the
regulator/field diode connection. I'd measure the volts at the warning
light output from the alternator - this should tell whether the fault
is in the alternator or the lamp wiring."

Any thoughts on what these warning lights *actually* mean?   And how
you'd decide whether it was (a) alternator (b) battery (c) duff
wiring?

TIA
Geoff


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