Ignition Light/Bulb Needed for Charging?

[DaveBoettcher]

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 is it true?

[ColoradoSteve]

Don't know about that, I doubt it. Would be simple enough to check, though. Put your volt meter on the battery with the engine off. Check again with the engine running. If your alternator is shot, you'll know it by your reading. Also check your belt, that may be the problem. And if you have no juice in the battery, then none of your lights will come on.

[DaveBoettcher]

Hi Steve,

Thanks for that but my car is fine, it was just a hypothetical query because I couldn't believe that anyone would design a charging system that relied on a bulb not blowing - especially as the bulb goes out when the system is actually charging - sounded crazy to me.

[kdjones2000]

At least on the early cars, I have heard also that the light bulb needs to be good or the car will not charge the battery. The bulb is an important part of the circuit as I understand it.

I guess one way to check is to look at the schematics

[matthewb]

I am not sure what ignition light you are talking about but on the early cars at least, the battery light needs to be good in order for it to cahrge. The light is in line with the "on"wire for the alternator. if the alternator dies or is not producing power then the alternator end of that wire sees ground and the light will light up because it has 12 volts on the other end.

[DaveBoettcher]

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.

[kdjones2000]

Dave, 2 people just told you that it needs to be good....

[bearone2]

Originally Posted by DaveBoettcher

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's been mentioned earlier in the thread.
on the older cars the alt bulb was part of the charging circuit, like series xmas tree lites but it's not an issue on the later cars.

i had a '73 volvo with the same type bosch setup on the lite and it failed once and i used the hi-beam light bulb, which was the same size.

[kwikt 911]

Hey, does this apply to a 968??

[bearone2]

i had it occur with the '84944na but it doesn't affect later cars and can be seen in the late 944 wiring diagrams.

[Lapponia]

Hi Dave, good to see you here again! To understand this, go to this very informative article: "Boch Charging System, as used on some BMW motorcycles and Volksvagen Cars".
www.ibmwr.org/airheads/charge.shtml. It will explain the charging funktion very well. Cheers.
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Lapponia 1984/944 NA