ignition light stays on WHY

[craigdouglaswright]

my batter ignition light stay on when running , alternater is charging and need help sourcing the root cause.

 

[The Mad Hat Man]

How can the alternator be charging if the light is on?
That us the trigger wire to enable the alternator field coils.

 

[Browny90]

Check the small wire hasn't come off the back of the alternator, or the wire isn't broken elsewhere, this gives a signal to turn off the light.

 

[Browny90]

How can the alternator be charging if the light is on?
That us the trigger wire to enable the alternator field coils.

The main wire goes to the starter and the batter but if the small whire that gives a signal to the dash to turn off the light is broken or faulty it will charge but leave the light on..

also, if the alternator has removable brushes then these can be worn enough to turn on the light but still allow the battery to charge.

 

[The Mad Hat Man]

How?
If that wire is broken, how are the field coils energised?

It doesn't supply a signal, it receives a voltage via the light!

 

[ObiWanKenobi]

My ignition light stays on. I've installed a brand new alternator. I'm thinking of doing these three tests:
Engine not running, but ignition switch on: Remove small wire from alternator and see it Ignition light stays on. It should go off.

Engine not running, but ignition switch on: Ground the small wire to the engine block, and see if ignition light comes back on. It should.

With car running, see what voltage is coming out of small wire itself while attached to the alternator using a multimeter. Should be 12V.

What else?
Thanks,
Mike

 

[The Mad Hat Man]

My ignition light stays on. I've installed a brand new alternator. I'm thinking of doing these three tests:
Engine not running, but ignition switch on: Remove small wire from alternator and see it Ignition light stays on. It should go off.

Engine not running, but ignition switch on: Ground the small wire to the engine block, and see if ignition light comes back on. It should.

With car running, see what voltage is coming out of small wire itself while attached to the alternator using a multimeter. Should be 12V.

What else?
Thanks,
Mike

totally wrong!
wire disconnected - light on.
wire earthed - light off
voltahe will be lower than 12v as it is the earth side of the bulb.

 

[ObiWanKenobi]

Hey Mad Hat Man -
Thanks for the quick reply. I'm new to this, though love learning how it works.
I thought that the ignition light gets power from the ignition switch - positive - and the bulb is grounded inside the alternator. (I am thinking of it like the oil pressure switch and oil pressure warning light.)
Then, when the alternator is running because the engine is running the belt, the alternator feeds positive current back at the bulb through that same little wire, and that cancels out the current from the ignition switch - turning off the bulb.
So I guess I'm looking for clarity on what's really happening, so I know how to think about the ignition light when it is on or off.
Thanks!
Mike

 

[The Mad Hat Man]

Hey Mad Hat Man -
Thanks for the quick reply. I'm new to this, though love learning how it works.
I thought that the ignition light gets power from the ignition switch - positive - and the bulb is grounded inside the alternator. (I am thinking of it like the oil pressure switch and oil pressure warning light.)
Then, when the alternator is running because the engine is running the belt, the alternator feeds positive current back at the bulb through that same little wire, and that cancels out the current from the ignition switch - turning off the bulb.
So I guess I'm looking for clarity on what's really happening, so I know how to think about the ignition light when it is on or off.
Thanks!
Mike

That is exactly how it works

 

[ObiWanKenobi]

Okay, Thanks! Will test tonight!
Thanks!