> Why would the readings be similar with engine on and no extra load, but
> different when load is added? Also what about the no load discrepancy at
> low idle? Not arguing, just trying to understand.
I don't have a good explanation for that, Ian. In fact, I read your post
too hastily and missed the fact that you were comparing the "car voltmeter"
to your multimeter. You might want to repeat your tests using the same
meter for both measurements, so meter accuracy doesn't obfuscate the
results. The original Lucas voltmeter from my Stag was so loose inside that
even lightly rapping it on a wooden bench would change its reading.
The light is basically comparing the alternator output voltage (in most
cases through a separate set of diodes from the AC windings) to the voltage
in the ignition circuit. If they are different, the light glows.
If there is a high resistance connection somewhere along the path from the
starter solenoid to where the warning light is attached to the ignition
circuit, then more load would cause more voltage drop across the resistance
and hence a higher differential. As long as the alternator is matching the
battery voltage (eg idle), it doesn't matter if the current is coming from
the alternator or the battery, it will still cause a voltage drop. But that
would not explain the change between 600 rpm and >1000 rpm.
Of course, one solution is to add a diode in series with the light, so it
only comes on when the ignition circuit voltage is higher than the
alternator output voltage. But your comment about "slowly dropping" says to
me that something is likely getting hot (possibly hot enough to eventually
let the smoke out), so I would want to hunt down and fix the problem.
Also, I assume you've assured yourself that it's really the ignition lamp
that is glowing. That plastic tube can allow other lights to show through
it; mine sometimes appears to glow a bit if one of the instrument lights
isn't seated in it's holder.
Randall
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