Disco 2 Can I get an alternator signal anywhere?

Presumably there's a signal from the alternator that's isolated from the battery which lets the ECU know that it's working so that the dashboard light goes off. Is there a sensible way I can get at some part of that to tap off it?

The Disco charging system has nothing to do with an ECU. Honestly, it's quite an old fashioned circuit in which the dashboard indicator light is wired into the alternator energising circuit.
To find an electrical signal to show when the engine is running isn't particularly easy, I've even looked at a vacuum operated switch on the vac line to the brake servo to try to make such a signal.

From that it looks as though splicing into fuse 27...? green wire to yellow wire bottom left should get it, doesn't it?

I think you'll find that all you've got at that point is +12 volts; the battery voltage. It won't indicate the alternator action or condition.

Oh, really? No sign of that on the diagram lol.

It's pretty clear on the diagram that F27 is an ignition live which goes to the instrument pack, the alternator's signal can be picked up on the brown/yellow wire from the alternator's plug or C0448-2 in the engine bay or C0047-7 under the dash... see RAVE -Electrical library- connector views

what usually happens with alt lights.....which this seems to be the same.....12v comes from ign goes through the alternator light and earths through the alternator when its not turning.....the light lights....the brown/yellow wire will be 0v. start the engine the alternator starts charging and turns the brown/yellow wire in to charging voltage...we'll say 12v for easyness.....the alt light cant get an earth now as its + at both sides so it goes out.

so brown/yellow wire will be +12 when alternator charging 0v when not charging......this you could use to fire a relay for say split charging

sierrafery said:

It's pretty clear on the diagram that F27 is an ignition live which goes to the instrument pack, the alternator's signal can be picked up on the brown/yellow wire from the alternator's plug or C0448-2 in the engine bay or C0047-7 under the dash... see RAVE -Electrical library- connector views

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Tried this and couldn't find a brown and yellow wire in the C0047 connector. Counted out to find wire number 7, which is black and white, but that doesn't seem to get 12V when the car starts.

That's because you confused C0047 with C0067 cos that's the one with BW on pin 7, there are cases when those two plugs which are near each other to be positioned the other way around than it's shown in the picture so 0047 to be in the middle near the white one and 0067 one side

sierrafery said:

That's because you confused C0047 with C0067 cos that's the one with BW on pin 7, there are cases when those two plugs which are near each other to be positioned the other way around than it's shown in the picture so 0047 to be in the middle near the white one and 0067 one side

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Ah, that makes more sense. So I've found that signal, and I get 12V once the engine starts; but when I attach a relay instead the voltage drops to 5-6V. Do I just need a relay with a higher resistance?

Naf623 said:

.....when I attach a relay instead the voltage drops to 5-6V.....

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What do you mean by ''relay instead" if you want to trigger a relay with that signal you have to splice into that wire with one side of the coil and the other side to earth... anyway what kind of relay do you want to use there? that signal is not a power feed

sierrafery said:

What do you mean by ''relay instead" if you want to trigger a relay with that signal you have to splice into that wire with one side of the coil and the other side to earth

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Yes, and when I do that the voltage that the relay sees is only 5-6V, whereas when I attach the meter across from the signal to ground I get the full 12V. I'm not trying to power anything from it, I just want it to trip a relay - that will turn the power on.

i dont really understand what you are doing

what kind of relay, a simple automotive one? , make sure it's not one with diode on the coil... anyway, as long as the relay is closing and you dont get the battery warning you are OK

Yep, a straightforward 12V relay, but yeah it does have an LED now that you mention it, good point. I have some solid state ones coming anyway, will see if they fare any better.