When I cold start my 2004 745i and the temp is below freezing, the LCD brightness is normal. Within 5 minutes the brightness fades until the display is black. It takes about a half hour until the car warms before it is again legible. My local dealer has no idea where to start. Any thoughts about what controls the backlight? Is the bulb itself actually behind the display in the dash?
I'm not familiar with your particular vehicle, but a true LCD screen has no "bulb". The display is self-contained, and I suspect you have a failing display screen.
That's not entirely true. The E32 and E34 LCD DOES use a bulb for backlighting. I should know because I had to pull my steering wheel to remove the cluster in order to change mine. You could only see the numbers on the odometer with a flashlight. Replacing the bulb (as well as all the other bulbs) BEHIND the LCD display resolved this issue. You seem to be confusing an LED (Light Emitting Diode), a component that emits its own light, with an LCD (Liquid Crystal Display) which does not emit light on its own but relies on an external light source (either ambient lighting, as with a calculator or watch display) or a bulb to provide backlighting. Some laptops even use a large flourescent backlight to provide lighting behind the display on a laptop. I suspect that the OP has a failing bulb. If he's feeling sufficiently brave, if he were to disconnect the battery and remove the cluster, then remove the display, he may indeed find one or more bulbs BEHIND the display unit. I recommend purchasing a service manual if you're going to attempt this as a DIY, since there may be other systems affected by disconnecting the battery and removing the airbag steering wheel.
While I do understand the difference between LCD and LED displays, I would never have guessed any automaker would make it possible to replace the backlight source in a display. In other words, I made an assumption that it would be a sealed unit, only service-able by total unit replacement. Just goes to show what I DON'T know....
Not a problem. The dealer tried to persuade me to spend $700 on a new cluster I was having other problems with my cluster also. Due to three of my five capacitors being bad, the gauges would not turn on unless the car warmed up for five minutes, and the trip odometer would reset to 0 every time I turned the car off. I wound up buying 13 bulbs for a total of $11 from the dealer, five capacitors for 30 cents each, and completely repaired my cluster for less than $15. Links below refer to replacing bulbs and capacitors on an E32 or E34 cluster http://bmwe32.masscom.net/sean750/dash_bulbs/changingClusterBulbs.htm http://bmwe32.masscom.net/johan/dash_caps/dash_caps.html
That's all well and good but on a 2004 745i--an E65/66, not an E32--I'm willing to bet that the "LCD display" referred to by the OP is the iDrive control display, not the dinky OBC panel in the instrument cluster. That boils down to diagnosis and repair of a faulty flat-panel computer monitor. Dealer techs with those skills could be rarer than hens' teeth since the display, like the cluster, is normally replaced as a unit (at $3000+ for the part alone). It is undoubtedly repairable but the hardest part may be getting information on its innards. Perhaps try to identify and contact the supplier of the display units, to see whether they can provide any help. Armed with that knowledge the dealer techs might be willing to tear into it (for a price...). Otherwise, might be time to find an independent shop with a reputation for really good electronics work.
who you calling "dinky"??? LOL Actually, the E32/E34 LCD's, while being far less sophisticated than the LCD modules that went into the E38/E39/E53 models, were by reputation, a LOT MORE reliable. There are actually at least two LCD modules the OP could be referring to -- the one inside the cluster, and the I-Drive display at the top of the center stack. Unfortunately he didn't specify which he was referring to. Either way, if the OP is feeling adventurous and knows someone with some electronic repair experience, buying a Memory Saver from Bavauto for $10 (to preserve the memory settings in the car), disconnecting the battery, then pulling the SRS airbag then the cluster, MIGHT be worthwhile.
The display is indeed the i-drive display. The problem is that even if I was to spend $3K+ on a new unit, there's no guarantee that the problem is located there. For example, I suspect that the ambient lighting sensor adjusts the display intensity. The problem could be in that circuitry, but again, there's no way to know where that circuitry is located. Also, other damage might be incurred by the disassembly. One would think that illumination sources could be located in a convenient location and routed to the devices by light guides. I happen to be an electrical engineer and would be delighted to find an information source that would explain the logic in these circuits and relate them to modules. Anyone know of a good source? My local dealer isn't going to be helpful.
I've found that purchasing a Bentley service manual is infinitely helpful and useful It's a DIY service manual. It doesn't really get into the nitty gritty of voltmeter level diagnosis, but at least it will help you narrow down the cause of the problem without further increasing your potential service bill. I will say this for the Bentley manual -- it's not the solution for ALL of your BMW problems, but it's a heck of a lot cheaper than a dealer would charge you and STILL leave you with unanswered questions. http://www.bentleypublishing.com Interestingly enough, there's no manual for your E65 7-series, but there is one for a functionally similar E60 5-series. http://www.bentleypublishing.com/bmw/repair-information/bmw-5-series-e60-e61-repair-manual-2004-2010.html
