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Originally Posted by greg
The mileage memory isn't in the speedo cluster. My blackface cluster was replaced - it didn't affect the mileage. But it is rememered SOMEWHERE, in the data for one of the car's many computers. It is conceivable that if that computer (or its memory) were replaced the mileage would be lost. It is also conceivable if not likely that there is some form of backup where the info is stored in more than one place specifically for this reason.
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Here's what I've heard.......
The odometer and VIN are both stored in the speedo cluster. BMW tech retrieves & uploads this info into the new speedo assembly when they replace it. Anybody who tries to install a new unprogrammed BMW cluster or used ebay or salvage cluster will learn this the hard way. I researched this subject thoroughly when my pixels failed before deciding to use the BMWNA warranty route (dealer repair). I avoid dealers & Techs whenever possible, so I tried to find a plan B at reasonable cost.
The odometer reading is also stored in the light module, DME and key remote. Software runs across the common data buss that compares the odometer values in each storage area. If they are not synchronized within a certain # of miles (counts), the red antitamper LED illuminates nest to the odometer reading that can not be "easily" reset. Re-synchonization may be possible with legal tools, but the dealer may not agree to assist you in this effort. A bunch or 5er owners who installed new speedos themselves found this out the hard way. Their cars displayed 0 miles & they now have tamper lights lit and resale value is affected. A new speedo assy will not "learn" the existing odometer reading unless programmed.
This strategy thwarts odometer tamper efforts & it also thwarts honest folks trying to troubleshoot electonics problems by swapping out modules.
Unfortunately a dishonest used car salesman can alter odometer readings of any car using hand-held universal reprogramming tools sold (& legal) in many areas of Eurpoe and Canada. This basically means the antitamper scheme really only penalizes the honest owner who wants to troubleshoot lighting, speedo and engine controls using salvage yard spares.