I had a '59 Chevy Belair that did this same thing.
Since the dash light is wired essentially the same way on both, I will explain my theory.
One end of that light is connected to the +12 volt side of the electrical system and is energized when the key is turned on..
The other end of that light is connected to the regulator circuit in the alternator and supplies the initial "tickling" current to power the regulator and get the charging/regulatin process started in the alternator. Once the car starts the regulator is "self" powered by the alternator and the voltage at the alternator/regulator end of the light goes up to charging voltage. This means that when the alternator is charging properly, the dash light has ABOUT the same voltage at both ends and that is why it goes out when the alternator starts charging.
To be clear it takes a DIFFERENCE of 12 volts to light a bulb. That means 12 volts across the bulb filament (or contacts). 12 volts at one end and 0 volts at the other. When both ends of the filament are at 12 volts, the DIFFERENCE is zero volts and the bulb does not light.
Now the regulator end of the bulb is not connected to the output of the alternator - but to the output of the windings inside the alternator. These windings have a higher voltage than the big terminal on (or the output of) the alternator. It is the regulators job to adjust this higher winding voltage so as to keep the the Big terminal output somewhere between 13.5 and 14.5 volts. The windings inside the alternator will have a slightly higher voltage than that. And it is these windings that the "other end" of the dash light is connected to.
Under a heavy current load, such as is present when the headlights and blower fan are on, the regulator must provide enough current to supply those needs while keeping the battery terminal voltage in the specified voltage range, and that means increasing the voltage at the alternator windings - where the "other end" of the dash light is connected. This higher voltage at the windings pushes the extra current that is needed for the heavier load.
At this point the dash light will have system voltage at one end of the filament and that heavy-load charging voltage at the other end. This can be as much as 3 or 4 volts difference across the alternator dash light. And that is enough to make it start glowing.
Slow down the engine and the charging ability of the alternator is limited and the glowing will diminish. Turn off either or both of the lights and the fan and it should stop glowing.
In other words it's nothing to worry about.
By the way - turning on the headlights (with brights) and fan (full speed) at the same time is how a brand new alternator is tested.
Hope this helps,
Bogy.
"Hardware eventually fails. Software eventually works" - Michael Hartung
From: "Alan Boucher alsthe1@gmail.com [diesel_mercedes]" <diesel_mercedes@yahoogroups.com>
To: diesel_mercedes@yahoogroups.com
Sent: Saturday, August 29, 2015 12:38 PM
Subject: [diesel_mercedes] Charging problems
We've developed an interesting problem on our 560SEL another 126 with a similar layout. While driving with the headlights or AC on at approx 50mph the red warning light on the dash sttatring glowin. The strange thing is the brightness increased as the engine speed increased. I tried one of the plug in testers and it showed normal batter condition and the alternator operating properly. I checked the battery connections and verified the water level in the battery. I haven' checked the terminal block on the fender or the engine grounds yet. Is there anything else we should check?
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Al Boucher
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