How does the ignition light work

[cleverusername]

I ask because the one on mine behaves strangely. On dimly at idle but disappears wen reved. So bad alterntor? Except a multimeter reads 14.4v at idle. Plus I can get the ignition light to go out at idle by turning on the heater blower, which makes me suspect that something is crosswired.

[soe8m]

The ignition light, around 2 watt, has an ignition positive (white wire) on one side and a brown/yellow on the otherside that goes throught he field coils of the alternator and is grounded there. The 2 watt of the light makes a current running through the alternator coils what does "activate" some magnetism for the alternator to start charging. When that does happen when the alternator start rotating a voltage is generated in the coils and now the ignition light has a positive voltage on both sides and goes out.

When the alternator does produce less voltage than the battery, the light goes on because one side of the ignition light has battery voltage and the other side a lower than battery voltage because of a faulty alternator.

The other way around does happen rarely because when the alternator does put out 17 volts the same current is also on the battery so 17 volts on both sides of the ignition light.

But when the alternator puts out a healthy 14,4 volts and the ignition light is on that can be caused by bad wiring. When the alternator has 14,4 volts the battery has also and the ignition light should have also. Bad wiring causing for example only 12 volts on the ignition light white wire caused by corroded connectors or a wacky ignition switch can make the light go on.

The ignition light is an integral system of the charging system. When this little bulb is broken the alternator won't charge. This bulb can also not be replaced by a led as the 2 watt is needed for the current through the alternator's coils.


Jeroen

[cleverusername]

So that means that if the bulb fails, the car doesn't charge? I can see that causing problems, especially with the difficulty of getting those bulbs.

[xvivalve]

It could also be dim at idle simply due to a slack belt, but 14.4v indicates it is not slack...

[SprintV8]

So that means that if the bulb fails, the car doesn't charge? I can see that causing problems, especially with the difficulty of getting those bulbs.

Correct.
At also if you change the bulb to a LED it may also not charge.

[soe8m]

So that means that if the bulb fails, the car doesn't charge? I can see that causing problems, especially with the difficulty of getting those bulbs.

Yes as stated in the post above. The wattage of the bulb is on most cars 2watt to have the alternator "activated". In a dolomite the ignition bulb is also slightly larger than the rest of the bulbs in the cluster and that is not for better warning...

Jeroen

[Bumpa]

I have a question that is not unrelated to the original query. On my 1850HL the ignition light won't go out until the engine has been revved to about 2000 rpm. Then it is fine and the alternator charges properly with the gauge reading around 14 volts even when idling. Why does it need a "kick-start" like that? Is there anything I can add to make it kick in immediately?

[dursley92]

I have a question that is not unrelated to the original query. On my 1850HL the ignition light won't go out until the engine has been revved to about 2000 rpm. Then it is fine and the alternator charges properly with the gauge reading around 14 volts even when idling. Why does it need a "kick-start" like that? Is there anything I can add to make it kick in immediately?

Funny thing I had a similar problem of no/little charge until revved with a new Lucas A127 alternator.
Once it is "switched on" it works fine.

[soe8m]

I have a question that is not unrelated to the original query. On my 1850HL the ignition light won't go out until the engine has been revved to about 2000 rpm. Then it is fine and the alternator charges properly with the gauge reading around 14 volts even when idling. Why does it need a "kick-start" like that? Is there anything I can add to make it kick in immediately?

The correct bulb?

Jeroen

[cleverusername]

I have a question that is not unrelated to the original query. On my 1850HL the ignition light won't go out until the engine has been revved to about 2000 rpm. Then it is fine and the alternator charges properly with the gauge reading around 14 volts even when idling. Why does it need a "kick-start" like that? Is there anything I can add to make it kick in immediately?

The correct bulb?

Jeroen

My money is on a worn alternative. I have had a Rover alternator do this and it we down to a duff voltage regulator module. It didn't supply enough voltage to energise the coils unless you reved the car.