Thread: Gen Light
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Old 07-05-2017, 09:03 PM
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Michael, I agree with most of what you say IF you qualify your statements.

For example, with regards to GENERATOR circuits... The GEN light is certainly convenient but it isn't necessary. Many trucks and tractors have NO GEN light. Cars only have them as a convenience, not an essential component for the charging system. I personally LOVE having the GEN light for the reasons you raise.

Ok, let's stick with T-birds because they do have a GEN light. In normal operation;
When you first turn the key, the GEN light shines,
When your engine starts, the GEN light goes out,
When you throw a belt, the GEN light shines.

I would expect these conditions to be true in normal operation, all the time, regardless of whether your car has a generator OR an alternator.

Now, we install a self-regulated solid state alternator, commonly found in most production cars. They require a "sense" wire that turns on with the key switch.

Since the original GEN light is fed from the key switch, it serves as a perfect "sense" wire for an alternator. Generators never require a 'sense' wire. Can an alternator system do without the GEN light? No light is necessary, but the alt still requires a 'sense' signal that turns on with the key. Without that signal, the alt won't charge.

But what happens IF the bulb burns out? There goes the signal to the alt. Not only will the bulb stop working but the alt stops as well. This is bad but it only applies to alternators, NOT GEN's.

A simple small resistor will remedy this potential alternator problem. A good bulb will still work exactly the way you would expect (as outlined above). In addition, if the bulb burns out, the resistor still passes enough current for the alt to 'sense' the key is on. Let me say this another way: The resistor never takes the place of your GEN light but it is a safeguard in case the filament burns open. Again, just in alternator systems.

Another small advantage of the resistor is, it passes just enough power to stop nuisance flickering at idle speeds. The diagram below shows the GEN light connected to the 'I' terminal of the VOLTAGE REGULATOR. It turns the voltage regulator on, which in turn turns on the internal solid state components. Otherwise, if on all the time, the battery would drain.

Just a note about ONE-WIRE alternators... They were never used in production cars. Since they don't have a 'sense' wire, they automatically shut themselves off below ~1,000 rpm. This makes 'remote start' impossible to use because remote start stops cranking your engine when the alt starts putting out 'charging voltage'. One-wire alt's need a 'gas pedal goose' upon startup just to begin charging. - Dave
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