Taking a huge lighter to a BMW

First Post! Wow!

Car: 2009 BMW 135i M Sport

Idea: Wire Dré's radar detector to a switch in the dash to avoid the need to use the cigarette lighter sock 

Approach: The US spec model of the 135i with the Sport and M Sport package has 2 non-functional buttons just to the right of the headlight knob. It seems as though buttons are blanks but if you take the switch apart you can cut the end off of a pen and stuff it in the back of the button to make it fully functional again. We were thinking we could find the power wire associated with this button and piggy back off of it to power the radar detector, using a screw close by for a ground.

Process: Dré removed the panels to get to the switch housing. Most of these have no screws. There is 1 Torx T-20 screw under the headlight switch, maybe 2. Everything else just needs to be pried off. As the plastic flexes and creaks under pressure you'll swear it is about to snap in half. Usually at about that point it breaks loose loudly causing an immediate reaction of frantically looking for broken plastic clips. If you're lucky there won't be any. In our case there was a couple heavily deformed plastic pieces but nothing was totally snapped off. That's just a byproduct of working on a Snap Tite car. Remember those car models they used to sell at Wal-Mart? Those were the business when I was a kid. Follow the jump to read more
Once everything was apart we examined the wiring bundle to see if we could identify the hot wire from the fugazy foglight switch. There were 9 wires with no way to tell what goes where for what purpose. We did some Google searching for the part number on the switch with no real results. We found some old wiring diagrams for Euro spec 1 series at bmw-planet.com but nothing on there for these buttons. We took apart the switch even more down to the circuit board. Even looking at the circuitry it wasn't apparent which was the money shot. We tried the one that we though it was and it didn't work.

About this time Dré decided plan B should be initiated. We didn't have a voltage tester on us at the time so we were going to test each one by unsheathing a bit of the insulation from each wire. On the second wire we began to get excited. We were able to power on the Radar detector in the accessory ignition setting. Once the car was turned on the power was lost though. On to the next wire. About this time a spider tried attacking Dré. I chased it under the seat and a lengthy battle ensued. Eventually the spider succumbed to the power of Dré's yellow crack spackler that you can see in the first pic.

After the sixth wire we had an epiphany. This was never going to work :(. For one thing we realized that none of the fog light wires have enough voltage to power the radar detector. Everything is controlled by the ECU. These buttons only send micro voltage signals to the ECU and it does everything else. We were on the wrong end of the wire. We should have been digging in the front bumper for the wires that power the lights. Also the button could never have offered continuous power. When you press it it makes contact at 2 points on the circuit board. When released the contact is broken so if there was power it would only last while the button was held in.

Result: Fail. Unfortunately not our best project idea but we definitely learned something. There was no permanent damage done. We just wrapped up the exposed wires and bundled everything back up neatly. Everything is back together and the headlights even still work.

Lost...6 pieces of electrical tape, 1 ml lighter fluid, 2 hours of life
Gained....Electrical knowledge, found the location of the primary grommet in the firewall for the wiring harness, found a Web site containing wiring diagrams for BMW's.

da end,
Loesch