Multi-information display pixel repair
One of the returning issues with nineties BMW vehicles is fading or missing pixels, columns or lines in the liquid crystal displays (LCD) of instruments like radio and onboard computer. The multi-information display (MID) in the BMW 8 Series is no exception and many owners had to replace the unit at least once already. Repair services do exist, but if you are not shy from a do-it-yourself (DIY) job, this multi-information display pixel repair article provides a step-by-step guide to fix the issue by replacing the LCD flexible flat cable or when necessary the complete display. The required skill level is not high, but caution must be exercised in order to protect the MID against accidental damage.
Contents
Removing the multi-information display
The multi-information display is secured in the center console with retaining tabs – three small ones on top of the unit and one wide tab on the bottom. The bottom tab must be pressed to remove the MID. To gain access to this tab it is necessary to remove the radio. Once the radio is out and disconnected, reach in the opening and firmly press the retaining tab on the bottom of the unit while simultaneously pushing the MID towards you. Usually it slides out without much effort but in some cases the top retaining tabs block it. Just push harder and carefully wrench the unit out. The MID is connected with a single cable. Release the connector's locking lever and unplug it. All connectors behind the center console fit in one direction only and the shape or color makes clear where they should connect to. There is no real need to label the connectors although it never hurts of course.
Opening the multi-information display
Once the multi-information display is out of the car, it is time to open the unit. Always put the MID on a clean and soft towel when working on it – the front scratches very easily. Work very carefully. If something seems to be stuck, check twice if you overlooked something instead of using brute force. To protect the electronics inside the MID against accidental damage, remove any static charges from your body by briefly touching an earthed metal surface, like a radiator or water tap.
Four plastic retaining tabs, one in each corner, secure the backside to the unit. Press the tabs to release the backside and take it off. Sometimes the part number label is pasted over the seam and must be cut or partially peeled off before the backside can be removed. The main printed circuit board (PCB) is held in place by two plastic retaining tabs – one next to the connector and one by the crystal – but it is somewhat difficult to remove because the display support structure is attached to its back and also secured with two retaining tabs; one on each side in white plastic and visible from the outside. Disengage all four retaining tabs while pulling the main PCB out of the enclosure. Everything except for the keyboard PCB should now come out. It may be necessary to play with the three retaining tabs on top of the MID in order to get the black bezel on the display support structure past them. The ribbon cable to the keyboard PCB is stiff and rather short, and may obstruct disassembly. Carefully unfold the ribbon cable to create some extra slack, but try to avoid unnecessary bending or stress where the ribbon cable attaches to the printed circuit board. The ribbon cable is made of solid wire which cannot take much bending before breaking and this is a weak area. Pull the main PCB and display support structure out as far as possible and then continue with the left side first (see picture above). Once the left side is out, the rest is easy. The enclosure remains attached to the main PCB with the ribbon cable to the keyboard PCB, but the ribbon cable can be straightened a bit to get better access to the display.
In one of the next steps the display PCB must be slid sideways to remove it from the display support structure. This is however prevented by the lips of the display PCB that sit in the slots of the main PCB. Locate and undo the two screws that secure the display support structure to the main PCB. The display support structure is still attached to the main PCB with three solder joints and in theory it should be desoldered from the main PCB first, but a shortcut without desoldering is possible: Simply bend the display support structure upwards until the lips of the display PCB are out of their slots in the main PCB. Next unclip and remove the black bezel that secures the liquid crystal display to the display support structure and is held in place with nine tabs; four on the top, three on the bottom and one on each side. The display is now fully exposed.
Fold down the LCD and remove the orange inlay and the two translucent diffuser windows from the display support structure. The display support structure is secured to the display PCB with a retaining tab on the left side and a slot on the other side. Disengage the PCB from the retaining tab (it may require some force to push the tab away far enough), then pull the PCB sideways out of the slot. The display board is now released from the display support structure, but it is still attached to the main board with a stiff ribbon cable. Carefully shape the ribbon cable somewhat to move the display PCB as far away as possible from the display support structure and maximize access to the link between PCB and display.
The liquid crystal display is attached to the printed circuit board with a flexible flat cable – a thin plastic film with conductive carbon traces printed onto. Unlike all other electrical connections in the MID, it is not soldered but glued with a thermal adhesive. With age, vibrations and temperature changes, the bond lets go and microscopic hairline cracks develop in the carbon traces. The result is poor electrical contact and the display shows fading or missing pixels, columns or lines. Sometimes the issue can be repaired by increasing the pressure on the flat cable where it attaches to board and display, but that often doesn't last or does not fix all pixel defects.
Replacing the flexible flat cable
The only proper and lasting repair is the replacement of the flexible flat cable with a new one. In the past this was the territory of professional repair shops, but nowadays the parts and tools can be obtained relatively cheap in online shops. Spare flat cables come with thermal adhesive applied and can be installed with a regular soldering iron and a specially designed T-tip with rubber strip. eBay store German Auto Recycling deserves mention because they not only offer spare flexible flat cables, but also the T-tip with optional soldering iron, and even a kit of a brand new display with flat cable already attached. The latter is interesting to replace broken or scratched displays, or when one isn't confident in attaching the flat cable to the display themselves.
Tear off the flexible flat cable and display from the PCB. The old flat cable cannot be reused so there is no need to be careful with it, but if you will be reusing the display try to avoid stress on the glass. It's thin and breaks easily. Once the display is off, put it on a clean, hard and flat surface with the mat front down. It's best to put a sheet of paper underneath to reduce the risk of scratches.
Spread your fingers over the display and tear off the flat cable from the glass. A lot of residue from the old flat cable is probably left on both display and printed circuit board. Use acetone to clean and degrease the contact pads array on the display PCB. Acetone is great for removing residue from glass, but it is also an aggressive solvent for plastics. Plastic polarizer film is attached to both sides of the glass – the strip where the flexible flat cable attaches to is the only area not covered with polarizer film. Be very careful not to spill acetone onto the polarizer film! When in doubt try to remove most of the residue with a small plastic or wooden scraper (or your nails) before giving it a quick final clean and degrease with acetone. Do not use metal or hard tools! There are nearly invisible metalized traces on the glass substrate and damaging these means end of story for the LCD. Try to avoid finger contact with the contact pads after cleaning.
Before installing the new flat cable a minor modification to it is necessary. On the end with the single thicker trace, a small section must be cut off. Otherwise an edge of the display PCB can put stress on the flat cable or even damage it upon reassembly. Just compare the old flexible flat cable against the new one and you will see it has the cutout. On some spare flat cables the cutout area is outlined with a few tiny dots. Make sure to use a sharp knife and be careful not to damage the nearby trace.
It's best to attach the new flat cable to the liquid crystal display first. That is the most difficult part and not having half the multi-information display attached greatly improves the installation comfort. The traces on the flat cable must line up exactly with the metalized traces on the glass substrate of the display, but the problem is that the latter are nearly invisible. Only under certain viewing angles and light conditions, the traces are reasonably visible. The traces are best visible when looked at into the reflection of a diffuse light source like the sky or a lamp with milk glass.
Add small pieces of tape to both ends of the flat cable to help keep it in position on the display until glued permanently. Apply the tape in such manner that it can be peeled off easily from the inside towards the edges. Otherwise the flat cable might be damaged while removing the tape. Remove the protective paper backing from the flexible flat cable and line up the traces with the display. If properly aligned the flat cable should be very close to being 5 mm away from the left edge and 2 mm from the right edge of the display. Take note of these measurements as reference but line up the flat cable visually – not all flat cables may be cut identically. Secure the flat cable with the tape. Put the display once again on a clean, hard and flat surface with the mat front down. Do not slide the display over the surface to prevent scratches. In the meantime let the soldering iron heat up for at least a few minutes to make sure the T-tip is at temperature. The thermal adhesive on the flexible flat cable needs a working temperature of at least 100 °C (210 °F), but the temperature should not exceed 150 °C (300 °F) otherwise the plastic backing could deform or even melt. Apply mild pressure on the flat cable with the T-tip and start working from the center towards the outsides. Use a large overlap to prevent missing areas and always slightly tilt the display or preferably use an edge as shown in the drawing above to prevent breaking off the contact pads area. The glass substrate of the latter is with a thickness of 0.8 mm (0.031") only half as thick as the rest of the display and breaks very easily! Once you get near the taped ends, peel off the tape but be careful not to tear off the flat cable on the freshly glued parts as that will irreversibly damage it. Continue gluing the ends and then do one last pass over the entire length with a considerable amount of pressure. Do not test the strength of the bond by pulling on the flat cable: The traces on it are easily damaged!
Lining up the traces on the flexible flat cable with the contact pads on the printed circuit board is easy, but it may be a bit frustrating to handle with the main PCB and enclosure attached. One way is to tape the display PCB to a table and have the main PCB and enclosure dangle over the edge. Once again use small pieces of tape to fixate the end of the flat cable to the PCB in such manner that you can easily peel off the tape without risking damage to the flat cable. It's best to make the flat cable not cover the contact pads completely, but leave some copper visible. The flexible flat cable is on the short side and this way the display is a bit further away from the PCB which will make reassembly easier later on. Use the same technique to glue the flat cable to the board as to the display. The PCB does not break that easily, so it is safe to apply a good amount of pressure with the T-tip to make the best bond.
Once the flat cable is solidly attached, it is time to put the display support structure back in place. Note that the LCD is now dangling from the PCB, so try to support it while handing the MID to minimize the stress on the new flexible flat cable. Guide the display PCB into the slot of the display support structure until fully seated, then push hard to secure it into the retaining tab on the other side. Lay the display one last time on a clean, hard and flat surface with the mat front down and carefully clean the backside of the display with a damp cloth to remove any finger prints. If the LCD was replaced with a new one as well, don't forget to remove the factory protective tape on front and back first. Next insert both diffuser windows and the orange inlay into the display support structure, and carefully fold the display into its normal position. Give the display one last clean and clip the black bezel back over the display.
If you are unsure about the outcome of the repair, now is a good moment to perform a quick test even though the multi-information display is not yet fully reassembled. Put a towel over the center console to protect it against scratches from the sharp edges from the printed circuit boards and solder joints and make sure the exposed solder joints and components cannot touch conductive metal objects. Refer to the next chapter for testing instructions. If all is well, continue reassembly.
Push the lips of the display PCB into the slots of the main PCB and bend the display support structure down to its original position. Secure the display support structure to the main PCB with the two screws and insert the whole back into the enclosure starting with the right side. Push forward until the display is flush with the front and the display support structure and main PCB retaining tabs engage. Finally clip on the backside and the MID is back completely reassembled.
Testing
The multi-information display has a hidden service menu that contains a display test. This test will activate all display elements making it easy to judge the result of the repair.
Reseat the plug on the back of the MID and engage the connector's locking lever. Loosely put the MID into its opening in the center console or hold it in your hand. Turn the ignition key to position I to turn on the MID and press one of the information buttons to get a quick impression of the display. If the display shows only garbage, the sad news is that the flexible flat cable was not aligned correctly. You will have to perform the repair all over again with a new flat cable or hope that one of the repair services accepts to finish the job for you.
If the display looks fine at first sight, press buttons 1000 and 10 simultaneously to show the service menu function selection screen (TEST_NR: --). Press button 1 once to enter function 01, the display test, and confirm with the SET/RES button. During the test all display elements are activated and missing elements are easy to spot. Use the picture below as reference. If a few faded or missing elements remain, perhaps the T-tip was not hot enough or not enough pressure was applied. Partially disassemble the MID again, and do another pass over the flexible flat cable with the hot T-tip.
Installation
Once confirmed all pixels work again, it is time to install the multi-information display back into the car. The installation is straightforward; insert the plug on the back and engage its locking lever, then push the MID down into its opening until it is flush with the center console. Lastly reinsert the radio.
