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Old 01-09-2006, 11:05 AM
UCrewX5's Avatar
Join Date: Mar 2005
Location: Hockeytown, USA
Posts: 9,737
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Using Rear Fog Light Switch to Operate Angel Eyes

Contributed by: UCrewX5

Rear Fog Light Switch to Trigger Angel Eyes

Objective: To retrofit the BMW European rear fog light switch into a U.S. spec vehicle to serve as the on/off switch for the Angel Eyes installed in a 2001 X5.

The first step in the project was to obtain the BMW light switch module that has the rear fog light switch. The switch for my particular application is part number 61 31 8 380 255 (note that if your X5 is a newer model that has automatic lighting then your part number will be different). Unfortunately, this is not a simple plug-and-play swap of the old switch because the wiring harness and LCM on the U.S. spec vehicles are not wired or pinned to accommodate the rear fog lights.

Figuring this out took a little bit of time and reverse engineering with a digital multi-meter. The end result of this analysis determined that the fog light switches use a pulsed negative trigger in order to signal the LCM. On the original switch the front fogs uses Pin 9. For the new switch, I determined that the rear fogs use Pin 8.

Pin 7 - Common / Ground Pin 8 - Rear Fog Trigger Pin 9 - Front Fog Trigger

Unfortunately for us in the U.S., Pin 8 is not “pinned” in the wiring harness. Therefore I had to insert a pin in the wiring harness in order to have access to the pulsed switch output.

Now that I had a known signal output from the rear fog switch in the form of a negative pulse output, the next step was figuring out how to transform that signal into something that could be used to operate the Angel Eyes. The solution I used is the PTR7 from Peripheral Electronics - http://peripheralelectronics.com/periph_products/cd_changer_autos/instructions/ptr7.pdf which is a programmable trigger output module. I used mode 3, which takes a pulsed input and sends a latched output to the relay that I use to power the Angel Eyes. For this application I used the following wiring assignments:


Red wire - Switched 12V power
Black wire - Chassis ground
Brown wire - to pin 8 on the BMW rear fog switch
White wire - to the relay used to power the Angel Eyes (negative trigger)

The final result of this project is the Angel Eyes are now controlled by the switch in the dash that in Europe would be used to trigger the rear fogs. When the vehicle is turned off the AE’s turn off, and the switch must be pulsed again when the switched power is turned back on in order to turn the AE’s back on. This is exactly how I wanted them to operate!

Here is a picture of the new switch installed in the X5. The fog switch on the left controls the front fogs and the one on the right controls the AE’s:

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