Automatic Headlight Control Upgrade

It was always annoying to me that BMW did not have Automatic Headlight Control until the very recent model years. This was a shock to me when I bought my E32 750iL. My 740 does not have it either. I decided to upgrade. I bought an after market AHC module so that the lights will come on when dark and turn off when light automatically. This module activates when the ignition is switched on and will leave the lights on for one minute after the car is turned off.

One of the most challenging things was to figure out how the BMW light switch is wired. As it turns out, when the light switch is in the "off" position, there is a constant ground going to the LCM (light control module). When the light switch is turned to on, the ground signal is interrupted. Once the LCM sees the interrpted ground signal, it turns on the lights. I simply used a relay to make this work (since the AHC triggers on positive).

Below: I used the blank plate next to the instrument light dimmer. The switch will activates the module and the LED will turn on when the module has turned the lights on. These are enhancements I made. The AHC is designed to be working all of the time with no indicator. I also wired it as such that I can always override and active the lights manually. I also added a linear stepped potentiometer to adjust the sensitivity of the photocell (placed in parallel with the photocell).

Here you can see where I placed the photocell. It blends with the upper curve of the dash vent very well.

This is where I placed the potentiometer. It is a linear pot with ten steps. It allows me to tune the sensitivity of the AHC. It took some trial and error to determine the best pot value. The glow around the knob is from the flash light helping my digicam focus.