Saturday, August 25, 2012
OBC Bulb Failure and A Peek Inside the OBC
During the last few months the backlighting on the main display of the OBC has occasionally failed to illuminate. When the malfunction occurs the text continues to change appropriately when I press the various buttons so I know the OBC is functioning and the problem is limited in scope to the display backlighting circuitry.
Bimmerforums is host to a long-running thread in which the author is developing a replacement processing core for the OBC. Since I figured he had a lot more experience with the circuitry of the OBC I explained what I was seeing and asked him for ideas. He pointed out that the displays connect to the PCBs via ribbon cables and suggested that one of these may be at fault.
I had absolutely no incentive to pull the unit during the recent front suspension overhaul as it worked the vast majority of the time, but when one of the backlighting bulbs failed this week I decided to pull the unit today and crack it open to see if there was anything obvious amiss that I could fix.
 After I pulled the back of the OBC off this is what I saw...the main processor board mated with the front panel. |
 The LCD display and PCB pulled out of the top of the OBC bezel. While the ribbon cables appear to use connectors, they are just strain reliefs. The conductors are soldered to each board. |
 A closeup of the display unit PCB. So, does the manufacturer use screws so the PCB can be removed from the display for servicing? Hahahahahahahaha. No. |
 The " BC" on the part label refers to BMW's official name for the part: Board Computer. The country code "US/GB" likely denotes the language of the key legends. |
 See that ridged, raised portion of the black shell? That's what you have to push on through the hole in roof of the sunglass pocket to get the OBC to pop out. Not as easy as it sounds, but it works. |
 The display module is back in the bezel. Incidentally, the processor PCB is held to the unit by the vertical black retaining clips positioned a couple inches from the left and right sides. |
I had hoped that the ribbon cables utilized connectors I could re-seat but as you can see cables were all soldered to the boards. A brief examination of the cables and the solder joints revealed no obvious problems. I also took a close look at all the components, paying particular attention to the electrolytic caps but noticed no bulging or other telltale signs of failure, imminent or otherwise. There wasn't much else to at that point except replace all the bulbs and reinstall the unit in the car.
These units are now $725 from BMW, which is a bit disturbing, but at least they are still available.
Mileage: 224200