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Old 03-10-2022, 06:06   #1
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Alternator Idiot Light

Does the Alternator Idiot light indicate excitation has occurred or that there is actual B+ current output flowing ? I have just discovered that one of my alternator’s B+ Output cable is open to the DC panel however when the engine was last operating the “Amp” idiot light extinguished as normal…I currently have no voltage or amperage gauges on the system (which will be rectified soon) and due to other repairs underway cannot start this engine..
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Old 13-10-2022, 02:33   #2
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Re: Alternator Idiot Light

Should I have posted “Alternator Idiot Light” to a different forum ? 2 weeks without a single response…
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Old 13-10-2022, 02:37   #3
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Alternator Idiot Light

Does the Alternator Idiot light indicate excitation has occurred or that there is actual B+ current output flowing ? I have just discovered that one of my alternator’s B+ Output cable is open to the DC panel however when the engine was last operating the “Amp” idiot light extinguished as normal…I currently have no voltage or amperage gauges on the system (which will be rectified soon) and due to other repairs underway cannot start this engine..
https://www.cruisersforum.com/forums....php?p=3687236
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Old 10-01-2023, 10:30   #4
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Re: Alternator Idiot Light

I'm not sure if it will indicate OK (not glow) from just excitation and no output. But measure your voltage as soon as you get it running and you'll know. It should be maybe 14.3 V or more if charging.
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Old 10-01-2023, 10:37   #5
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Re: Alternator Idiot Light

The alternator "idiot light" supplies current to the regulator when the ignition key is turned to on to allow excitation to occur. It goes on because current is flowing to the regulator from the battery.

When the alternator starts producing current to the B+ terminal at a voltage higher than the battery voltage a circuit within the alternator takes over the supply of current to the regulator and there then being no current to the regulator via the "idiot light" circuit, it goes off.
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Old 10-01-2023, 11:44   #6
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Re: Alternator Idiot Light

Quote:
Originally Posted by Gumshoe View Post
Does the Alternator Idiot light indicate excitation has occurred or that there is actual B+ current output flowing ? ...
Yes BUT the amount of energy can be as low as .1 amp or less. The light is typically wired into D+ or I circuit. It energizes the voltage regulator but that's a very general statement. There are different ways of doing the indicator light
As an example you can have brushes worn down and limiting the alternator output to say 5 amps. It's enough to turn the indicator lamp out but not enough to charge any load more than around 5 amp. A very loose belt would do the same thing.

If you want to accurately measure output of B+ a shunt and ammeter is a better way to do it.
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Old 10-01-2023, 12:55   #7
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Re: Alternator Idiot Light

As already indicated upthread, the function of the alternator idiot light is twofold.

1. It provides the initial current for field excitation; once the alternator rpm is high enough (~ few hundred rpm), the alternator becomes self exciting and the field current is provided internally. This is the point where the idiot light is extinguished.

2. It indicates the alternator output voltage is the same or higher than the battery voltage. It does not indicate anything about the alternator output current.
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Old 15-01-2023, 11:03   #8
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Re: Alternator Idiot Light

Thank you wotname….as I mentioned upthread I will add amp and voltage gauges to this system…..
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