Re: headlights & OBC questions (archive)

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: greetings,
: i have owned my 93 850ci now for 3 weeks and am fixing minor problems like seat cables,missing plastic nut caps,etc. but am looking for some help on brighter headlights as aiming them seems to help a little bit but am looking for suggestings on higher output bulbs. also the
: check system on the on OBC says R/foglight failure and License plate light failure. this vehicle does not have rear foglights and the license plate lights are both working, any suggestions on these faults?
: TIA,
: Tedster

Welcome to the wonderful world of multiplexed electrical system gremlins. As background, the LCM (Light control module) performs a self test when you fire the car up. It basically reads resistance in all of the light circuits (whether the lights are off or on) and looks for higher (than some set level) values. A burned out bulb-which is in essence an open circuit-has infinite resistance. Where it sees higher resistance, it assumes the bulb is out, and gives you the check warning. As the cars age, corrosion tends to set in on connectors, sockets, grounds, etc., and resistance increases to the point where it can trip the LCM warning, but the bulb/circuit is still good. I had the same situation with the license plate light and L tail-light on my car.(BTW I would ASSUME the warning is for RIGHT foglight-not rear).
Here's MY solution. First (optional) I went to the dealer and had them hook up the DIS computer (now MODIC)-this can read the numbers and types of faults out of the various modules including the LCM. Mine said everything worked with no faults, but that the license lamp had been out like 25-30 times. Same with the tail-lamps. What I ended up doing is replace the bulbs with new even though they worked-remember as an incandescent bulb burns, the heat eventually vaporizes the filament to the point of failure, so the resistance increases as the bulbs age. I then pulled all the trunk panels and cleaned all the connectors I could get to with contact cleaner, and removed the grounds and sanded and reattached them to decrease resistance. AND after that the problem never reappeared. Sorry for the long post--good luck with your problem.

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