Ignition Light/Bulb Needed for Charging?

[DavidBoettcher]

A friend of mine said that if the ignition light bulb is blown then the alternator won't charge the battery. This sounds ridiculous; but I don't know enough about auto electrics to know whether it true, so I am hoping that someone familiar with this aspect of vehicle electrics can give me chapter and verse.

I'm talking about the light that comes on when I turn the ignition switch to on but before the engine is running. I know that this comes on if the alternator is not producing any output, but the suggestion from my friend is that if this bulb blows the alternator is no longer able to charge the battery. This seems ridiculous to me.

Clearly the charging current doesn't go through the bulb, because it is dark when charging. The wiring diagram doesn't show the internals of the alternator so I can't check if the ignition light has some controlling function on the alternator, but to me it would seem unlikely - I wouldn't want the important function of battery charging to be hanging off a bulb that might blow at any time.

It has been suggested to me that on older systems with dynamos and make-or-break controllers that the ignition light bulb did have some controlling effect, but that this isn't the case on more modern alternator based systems.

 

[LionelHutz]

No, it's not true.

The light circuit is often used to provide some initial field current to get the alternator to begin working. The light was also typically paralleled with a resistor to increase the current and protect against just such a failure from happening. Also, most alternators will hold enough residual magnetism that a rev of the engine will still turn them on.

 

[HOLDAFLAT]

I disagree LionelHutz.

It is very possible for the alternator to fail to charge if the charged indicator bulb is blown. It just depends on the alternator in question.

As for a bit of an explaination as to the way the light works;
1) Battery voltage is supplied to one side of the bulb when ignition is turned on.
2) current flows through the bulb to the W or D+ terminal of your alternator.
3) inside the alternator that current is passed through circuitry to earth - lighting the bulb.
4) once the engine is started the alternator uses this current to provide some startup magnetism and get things going.
5) once the alternator has reached battery voltage, it is also supplied in turn to the W or D+ terminal.
6) as the bulb now has battery voltage on both sides, the net voltage across the bulb is zero and it goes out.

all in all mate, if your bulb is blown, and your alternator wont charge - just replace the bulb. its alot cheaper than ginning about with new alternators.
also - the charge indicator is red. and that means its important.

 

[LionelHutz]

Well, if you must disagree then you must know which manufacturers do not include a parallel resistor?

In the vehicles I've actually looked into, the manufacturer has always installed a parallel resistor so good bulb or not the circuit will work.

 

[HOLDAFLAT]

Oillift stationary engine packs for one.

 

[patprimmer]

DavidBoettcher

In the few instances I have seen an alternator with a blown charge warning bulb, the alternator has still worked.

If the bulb is suspect and the alternator is not charging, replacing the bulb as a first move seems a VERY simple almost no cost test. If replacing the bulb with a working bulb still does not fix the problem, repair or replace the alternator.

By following that procedure, the above argument becomes irrelevant, well at least it does to you.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules