Classic cars forum & vehicle restoration.

[Ray White]

I am confused with the way this works.
1) is the ignition warning light earthed?

2) I have two wires to the light... one from the alternator ...and one from the ignition switch. Does one side go to live and the other to earth?

Please help. This is driving me crazy.

[MikeEdwards]

Consider that to light a bulb, you require a "potential difference". The light comes on when the voltage coming off your alternator is not the same as the voltage on your ignition switch (which in theory is the same as your battery voltage) - in either direction.

If your alternator is outputting nothing, but your battery has voltage, then you have a potential difference and the bulb lights up - in this situation, the alternator is effectively your "earth" because the other side is positive in relation to it. If your alternator is outputting 12v but your battery is down, again you have a potential difference but in this case the battery is the lower side of the equation.

This is why you can't just stick an LED in your warning light. Neither side is actually an earth, but one or other side might be zero voltage. When the two voltages are the same, the bulb goes out.

Or at least it's something like that. I'm sure someone will correct anything I've got wrong here. I'm playing with alternators myself at the moment, trying to get rid of an occasional faint warning light.
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1976 Vauxhall HP Firenza, 1976 Vauxhall Sportshatch (x2), 1986 Audi coupe quattro, 2000 Audi TT

[Ray White]

Thanks for that, Mike. So the answer is yes and no or ...not exactly.

In other words, there is no black wire from the earth terminal in addition to the yellow wires from the alternator and the ignition switch.?

[MikeEdwards]

Correct as I understand it - the "earth" is whichever has the lowest voltage, either the alternator, or the battery. So for your lamp, both sides go to live, but if the light is on, it just means that one is more live than the other.

It's not quite as I said above - not just that one side will be zero voltage - it may be that the battery is at 13v but the alternator is faulty and only putting out 11v, hence a dim warning light because you've only got a potential difference of 2v. When you turn on the ignition but don't start the car, you've got your full battery voltage on one side, and nothing coming from the alternator on the other, hence a bright light because of the difference between the two.

Apologies for my poor use of terminology, it's been ages since I did stuff about potential difference in O-level physics and I suspect I shouldn't be using "voltage" and "earth" in the way I do, strictly speaking.
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1976 Vauxhall HP Firenza, 1976 Vauxhall Sportshatch (x2), 1986 Audi coupe quattro, 2000 Audi TT

[Ray White]

Thank you Mike. I didn't do 'O' level physics... and failed maths twice before giving up.

(Surprising really as I qualified as a classical pianist and there is maths in music theory. )

I am doing a lot of work in the hope that it turns out O.K.; which is a bit frustrating.