Ignition light staying on.

Not an R but an early Ford Puma and I want to try and understand what's going on.
As said, the ignition/battery light comes on with the ignition but stays on when the engine starts. With the engine ticking over I'm getting 14.2 volts across the battery, so it is charging ok....
Just to eliminate things I changed the battery and then changed the alternator but the light still stays on and the battery charges. My local garage says must be broken or shorted wire between light and alternator. Trouble is I can't see how either can be right. The alternator needs the 12 volts through the lamp supply to the field coils or it won't start up so if the wire is open or short circuit the alternator won't work. That's if I understand how it works right.
Probably haven't explained it very well but anyone got any thoughts???

How many wires on the plug john ?

Have you got a wiring diagram for it then John. Your right in the fact that the alternator requires a 12v feed from the ignition which it must be getting for it to be charging the battery. Could be an earth fault on the light?

On fords its a blue wire once started unplugging it should turn it off I think it works as a earth but also a live to energise it

There's 3 wires, thick red one on its own and goes off to the starter and 2 others in a plug. It hasn't got the Ford smart charge system on it.

I've got a Fiesta wiring diagram which I think is the same and it shows a blue/black wire direct from light to alternator. I've not yet tried unplugging it when running. Can give that a go tomorrow.

As you say Johnny, it's live to energise it but once the alternator is running it generates it's own 12v across the field coils so both sides of the light should see 12v - and the light goes out....

Yes that's the way I see it my mate is into his fords

12v+ from light, once alternator running the input that went to earth is 12v+ so light goes out. You want take this light wire off alternator & see if light goes out.
If it don't you got a leak (resistive short) on the bulb circuit. Also measure voltage prior to start from the bulb wire at alternator plug ... check plug for potential source of leak too ...

Checked all the connections I can get to, including behind the dials and can't see anything wrong.Voltages at the alternator are:
Ignition on/engine off  0.5v
Engine on                10.5v
Unplug wire from alternator:
Ignition on 8.5v
Engine on 10v
With the wire unplugged I would have expected 12v, so as you say looks like a resistive short 'somewhere'
The strange thing is that with that wire unplugged I started the engine and the alternator was still charging (14.2v) and the light was still on.. (Residual magnetism in the field coils enough to get it started??)

Prob easier said than done but bulkhead area chaffs the wires and check the live red I think under steering colum for burning to ignition

johnboy wrote:Checked all the connections I can get to, including behind the dials and can't see anything wrong.Voltages at the alternator are:
Ignition on/engine off  0.5v
Engine on                10.5v
Unplug wire from alternator:
Ignition on 8.5v
Engine on 10v
With the wire unplugged I would have expected 12v, so as you say looks like a resistive short 'somewhere'
The strange thing is that with that wire unplugged I started the engine and the alternator was still charging (14.2v) and the light was still on.. (Residual magnetism in the field coils enough to get it started??)

With 8.5v at alternator plug you know you likely have a resistive short, would of expected 10.8v-up . If can't get at the harness wiring for testing it might be easier run a new cable to join into alternator harness. Most alternators will work without the bulb feeding the core field as core magnetised to some level like you say.

Cheers Mr B, will have a go at that over the weekend and see what I can Find. First I'll cut the wire close to the instrument cluster and see what happens. Just to be sure it's not a problem on the pcb behind the instruments..

^could indeed be instruments if that light all on a pcb & being ford quite likely as they awful with electrics :-S may luck be with you at the weekend.