Re: Wiring question

["Michel Drainville" <MDrainville@xxxxxxxxxxx>]

On Tue, 12 Mar 2002, Paul & Diane Graveling wrote:

> Can anyone help? I have an '88 E30 325i and I need a wire around
> the dash area which I can use to supply an LED only when the key
> is in the 'Off' position. (Not the 'accessory' position but
> completely off, key removed). I've found a fused +12v line which
> is permanently on, and now I need the switched negative side.


A relay can be installed like someone mentionned earlier but it is not
necessary for that specific application.

What you have to do is locate a relay or light bulb that is activated when
the key is turned to ACC and IGN. On that relay, find the two terminals of
the coil. One terminal will be to ground, the other will receive switched
12v from ACC. This is the one you will need.

On the LED, connect the positive side to fused constant 12v and the
negative side the the switched 12v on the relay coil.(same thing for a
bulb).

When the key is turned to ACC or IGN, there will be 12v on both sides of
the LED so it won't light. When the key is removed, no power is going the
the relay or bulb so the LED will see 12v on the positive side, and ground
on the negative side and will light. The negative side of the LED will see
ground via the relay coil, and since the resistance of the coil or bulb is
very low compared to the LED resistance, it acts like a simple wire.

In case you wonder, yes there will be a voltage drop across the coil or
bulb, but it will be in the order of 0.3v. So no chance of having the relay
or bulb turn on.

Hope this help.

Michel Drainville
91 318ic

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