Ex MOD 90 Ignition light doesn't (but alternator still charges)

As per title, the battery / ignition light has never worked on my ex mod 90, certainly since I fitted the 300tdi engine (cant remember if it worked when the 2.5NA was fitted).
My understanding is that if the light doesn't work, the alternator shouldn't charge. I get a solid 14.4v at the battery when the engine is running so I'm not really worried about anything but I am curious as to why the alternator works when the warning light doesn't??

As an aside, despite googling numerous search terms, I can't find out what type the warning light bulbs are?
The 90 is a 1985(ish)

Cheers,

Rich

I'm sort of on the cusp of understanding this ie- I think the bulb has a + and - to light up (obviously) the bulb is fed + from ign live and - from the alternator. When the alternator is charging the - from the alternator turns to + so the bulb goes out.
Guess you have no ign switched + or bulb is blown?

Have a look at this ... https://www.howacarworks.com/ignition-system/troubleshooting-the-ignition-warning-light

Ooh, i have this problem too! '85 110 with a disco 209tdi conversion. The only time the light comes on is when the engine runs on after i switch off the ignition. This latter issue is just a fuel solenoid issue, nothing serious, but it's interesting that this is the only time the ignition light ever comes on - i assume its the warning light being reverse driven by through the alternator field windings.

One way to check your warning light wiring is to turn on your ignition but dont start the engine, and remove the little wire on the back of the alternator and short it to the metal body (or any convenient earth). The ignition warning light should come on.

Thank you all for the replies! Will investigate further when it’s warm enough for my fingers to work!
It’s never bothered me before but after the alternator failed (seized and left me stranded!) I would like to have a working warning light (although I know it wouldn’t have been of any use whatsoever in that particular situation!)

Cheers
Rich

I have an led battery tester permently wired in and glued to the dash on my 90,Lets me see the battery volts at a glance when engine off and if charging when running.

There's a resistor in the circuit that allows current to flow in the alternator exciter circuit if the bulb blows.