The Alternator Light
 

Hi, All
I have noticed some confusion about this light and thought I should offer a few thoughts.
The light is connected between the 12 volt terminal of the battery and the rotating part of the alternator through one of the brushes to ground.
When the ignition is switched to the first position current flows from the battery through the light and on to ground through the alternator. This current lights up the bulb.
After the car is started the alternator generates about 14 volts, which means that the difference, 2 volts is the voltage across the light, not enough, so it goes out.
Meaning that the alternator is working.
Then, you are driving along and the fan belt breaks or the alternator fails and no longer generates 14 volts. Since the 12 volts is still connected through the light from the battery, the light goes on again and you should stop because if the fan belt is broken the engine will overheat quickly and maybe fail.
So you see, it is not by any means an idiot light.
Ned Monaghan

Ned,

You are correct that it is not an idiot light! And, it must be functional, along with the brushes for the slip rings, for the alternator to be energized by the 60-100 mA 'pre-excitation ' current through the field windings!

There are no heavy permanent magnets in an alternator to generate a magetic field, so that current through the bulb and any parallel resistor present after 1982 (or retrofitted later) are needed before the 14 Volts can be generated! After the 'zero-Ampere' rpm level has been reached, the 'normal' excitation current of 1-4 amps is generated by the alternator, assuming the excitation diodes are functional ...

So the fact that my alternator light in my 87 911 will emit a slight glow which is only noticed at night when all the electrical consumers are on (headlights, foglights, fans) means that corrosion on the bus or ground straps is causing a voltage drop in the system? I had my alternator (paris-rhone) rebuilt, redipped and new regulator installed after noticing that my headlights and interior guage lights would go bright then dim while traveling the beautiful countryside of upstate New York. The rebuild has stabilized that problem, it charges properly, so I am thinking it must be corrosion in the system. What are your thoughts.

Regards,
Tom Dowling
tdowling@juno.com

Tom,

Your alternator was doing the same thing as mine. I hooked a digital VOM up to the car and found out that it was going from normal system voltage of 14.7 or so up to 16! This is why the lights were getting brighter from time to time. Its not good for the lights and I am very surprised that it did not burn up the fuel injection computer.

My regulator was breaking down when it got warm and causing the fluctuations in voltage. I overhauled everything as you did and now the voltage is stable, over two years later.

No idea about the glow on the bulb. Could just be the filiment glowing from the current flow through the circuit. I still call it an idiot light as it cannot tell me anything about my system. I know of people who did not have the light come on, yet the alternator stopped charging. This is why a real volt meter or amp gauge is more important to me to check my electrical systems condition IMHO.

Tom,
When the alternator is charging the output voltage is greater than the battery volts so there is a current flow back to the + of the battery through the alternator light. This causes the light to glow a little.
I recall seeing somewhere that you can do quite a bit of diagnosis of the alternator / charging system using the glow of the bulb. Maybe Warren has these details.
Ned Monaghan

...so any idea why mine does not work? Warren, I didn't make myself clear the other day when I said "I put a good bulb in there and still would not work" What I mean is, I went through trunk, yanked a bulb for the illumination of oil pressure gauge which I know works, swapped it into alternator light socket and watched from in the trunk to see if it would light. It did not when I returned to trunk after turning key to first position.

Does the gauge itself provide a ground for the light like tail light sockets etc? Should I have swapped bulb and then put the socket back into the guage THEN tested or is something simply, wrong!

I know my alternator is working as I have had same battery/strong starts for 2 years. Thanks for the help.

Joe 68L