Repost: Ignition Controlled Aux 12V power supply (has pictures) - Bimmerfest - BMW Forums

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Old 02-14-2010, 05:34 PM
joekahng joekahng is offline
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Location: Los Angeles, CA
Join Date: Jul 2009
Posts: 31
Mein Auto: 03' 745i & 06' Cayenne S
Repost: Ignition Controlled Aux 12V power supply (has pictures)

Hi guys, I will try to keep this simple and short.
I also got sick of using the cigar lighter jack for aux power so I followed this other members thread,
and tried it and it works very well.

Sort of a repost but I hope this helps for those who wants to hook up their Radar detectors, or any other gadgets that require 12V power supply.

1. remove the driver side's under panel. you need to remove few screws, open it up, remove the dome light connection, and remove the entire panel.

2. get under there and find the brake light switch. it has a 4 thin wires, the green/black wire is the 12V switch wire. This wire goes to the relay trigger slot on the relay. (I used a wire peeler to just remove little bit in the middle and tap in, the wire is too thin to splice into)

3. on the left portion of the brake light switch, there is a big bundle of wires, you will find a Red wire with Yellow dots. This is the thickest wire in the bundle, you cannot miss it. use a slicer for tap in.

4. if you look at the pic of the relay i bought from radioshack, it requires female "spade" connecters to hook up the wires. very simple. just connect all the wire as mentioned in the picture and ground the ground wire to any screw.

5. after connecting everything, I zip tied everything (the hardwire kit, relay) so it would be clean, and tucked it nicely, and replaced the panel back.

Now I get 12V Auxilary Power supply, ignition controlled, no more waiting for it to go on the sleep mode, which is total BS in my opinion.

hope this helps.
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Old 02-22-2010, 10:08 AM
bobcat99 bobcat99 is offline
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Location: Dallas
Join Date: Feb 2010
Posts: 1
Mein Auto: 2007 750Li
Didn't quite work on my 2007 750Li

I tried to use this solution to power a Valentine 1. There is a green/black wire on the brake switch and it does measure 12v with the ignition on. However, when it is connected to the relay coil, the voltage across the coil pulls down to between 3 and 4 volts, and will not engage the relay.

I found switched power at the diagnostic plug near the hood release. There was a wire, green with yellow, as I recall, near the bottom of the connector. This seems to work. To access the wire, remove the screw attaching the hood release, and the screw under it. Remove the left-hand foot pad (velcroed), and pull out the panel.
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