Battery Light Supply

Ok I am struggling with this but I know is so simple

1st job...I Need to light up a external light on the vehicle to warn me the engine is running
2nd job...kill a earth supply to a additional relay when the engine is running

Both can be done via the battery light but can't see how they are switching it


..................ignition on.....running
....................light on.......light off
Pin 2...............12v.............12v
Pin 5.................................12v
Thought they are killing the earth but No

Sounds like a sinking and sourcing kind of circuit.

If a light bulb gets 12v one side and 12v the other, then there is no potential difference (the real name for voltage). Make either side ground then the light will come on.

The one that stays 12v probably comes straight from the battery, while the one that goes to 12v when running probably comes from the alternator.

You could do what you want with some kind of double pin, double pole relay - as you say, straight off the battery light, as long as there is enough current there at the battery light to make the relay activate.

Just wire the relays activation pins it in parallel with the bulb.

You might also be able to use the fuel pump relay some how.

I see where you are coming from, they do use a diode in the ciruit on in the back of the clocks too, will have to sleep on it to get my head round it, can't use the fuel pump as that is live without the engine running
Got all the runlock system fitted, I can stop and start it remotely with no keys in the ignition but need a failsafe way of not trying to start it with the engine running

That diode will be there to stop the battery draining away into the alternator or to stop the alternator from charging the battery via the charge light (or something light that anyway! - i'd have to examine the alternators internals to say exactly what it does).....

But here is a little cut out from the wiring diagram to help you visualise the crack:-

HighlyJetted wrote:Sounds like a sinking and sourcing kind of circuit.

If a light bulb gets 12v one side and 12v the other, then there is no potential difference (the real name for voltage). Make either side ground then the light will come on.

Thanks Matt, think you have hit the nail on the head, will try the weekend

Dino