By: Bob Wilson
Should the digital clock on your BMW K100 begin to suffer from missing digital segments, there is a good possibility that you don't have to get ripped off to the tune of $200 to buy a new one (especially painful since there is less than $5.00 worth of components in the thing!). Here is the procedure to try before your BMW parts man relieves you of your wallet.Requirements:
Remove the 4 Allen bolts from the back of the instrument nacelle. Remove the connector cover from the rear (once the unit is free from its mounting). It is held in place with a single phillips screw. If your bike has a fairing, you will probably need a stubby phillips to get in there, since the cables leave little free length. Once the cover is removed, unplug the 2 connectors, taking note of which is right, and which is left (they are otherwise identical).
With the nacelle on the bench, remove the multiple small phillips screws from the rear of the unit. Do not remove the 2 larger countersunk screws near the center area of the back. Pull the back off the nacelle.
Inside, you will find 4 or 6 phillips screws that hold the "guts" into the front part of the nacelle. Remove them, then remove the internal assembly from the front housing in one piece. Be sure to pull the trip odometer reset knob out sideways against its spring as far as you can, to allow the internal assembly to be pulled out.
The Speedo and the Tach assemblies must now be separated from the main internal assembly. They are held in with a few small slot-head screws, and you have to unplug a few strips of flexible interconnect cable.
Now the square bezel around the clock module can be pulled upward and off, and the clock module can be pulled free together with its clear acrylic mount.
Fixing the clock:
There are two common reasons for missing segments (actual electronic component failure is not very common). One is that the circuit-board-to-LCD connection is faulty (oxidized), and the other is that one or more of the the multiple pins of the clock driver IC has a bad solder joint.
Remove the clock circuit board from the acrylic housing by flexing the mounting "lances" back to release the board. Be careful not to lose the two "Zebra Strip" connectors when the board is pulled out. These are small, rubbery strips that have near-microscopic stripes of colour on them. They fit into two slots in the acrylic housing, and connect the circuit board to the glass LCD. Once installed, they are compressed between the LCD's glass contacts and "pads" on the board.
Pull the LCD out the side. NOTE THE ORIENTATION! There is a very small "bump" of clear silicone on one end. When reassembled, this end goes in first, with the thicker of the two bonded glass plates of the LCD facing AWAY from the circuit board.
Inspect the circuit board carefully (use of a magnifier is recommended). DO NOT stick a screwdriver into the variable capacitor (on the board) and try turning it! This is the accuracy adjustment for the clock's crystal oscillator. Using some fine ScotchBrite (NOT SANDPAPER!!), lightly AND CAREFULLY buff the 2 rows of pads on the back of the board (that the Zebra Strips rest against). An oxide layer here can cause the missing segment problem.
Next, resolder any obviously bad solder connections. In particular, the clock IC leads are just surface mounted (they only are soldered to pads on the surface of the board), and thus the solder connection can easily crack. Resolder each lead by applying a small amount of solder, and "wipe" the soldering iron down the lead, away from the side if the IC, and onto the pad. If you create a big blob of solder in the process, don't panic! Just use the solder wick (placed between the blob and the iron) to suck up the excess, and try again. The trick is to use a clean iron (wipe the tip over a wet sponge just before each use), and use VERY LITTLE solder. A correctly soldered lead will have a small shiny fillet of solder at the end of the lead and smoothly joining to the pad.
Carefully inspect your handiwork and reassemble everything. Two more points, though. First, you can try out the clock before assembly by connecting a 6V battery (any size) to the two tall rectangular pin sockets on the board. Be SURE to connect the (+) wire from the battery to the socket marked (+), and (-) battery wire to the socket marked (-)! DO NOT REVERSE POLARITY OR YOUR CLOCK IS TOAST! Do not use a 12V battery!
Second, when you are ready to put the back on the nacelle again, take the O-Ring that seals the back and grease it with a thin film of silicone grease (or vaseline, if that's all you have). DO NOT USE LUBRICATING GREASE!. Pull the O-Ring through your fingers to distribute the grease evenly. Ensure the mating plastic halves are clean, install the O-Ring onto the lip on the rear cover, and assemble the thing.
One other thing. Be sure NOT to allow any oil or grease to contact the GoreTex breather discs in the rear cover. These things allow air to vent in and out, but block water. If oil or grease contacts the material, it is no longer hydrophobic, and will not block water entry. If you are unlucky enough to get the GoreTex contaminated, wash the entire back cover with a strong detergent to remove the contamination, then rinse thoroughly with clean water. Goretex is just porous teflon, and will completely withstand anything you can throw at it.