BMW Repair: Speedometer for 1998 318ti, metal shavings, salvage yard

Question
My repair shop just inform me that the reason my speedometer quit is because of the metal shavings attaching to the speed sensor and also that the differiential was going out the reason for the metal shaving.Prior to this I have not heard any indication there might be a problem.This vehicle gets out pretty strong and operates at high RPM,s 65 MPH @ 3500 to 4000 RPM,S in 5th gear. Any advice on this would be greatly appreciated.
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Followup To

Question -
Apoligize for the lack of info; My speedodometer on my 1998 318ti started acting erratic with a bouncing motion and then preceeded to quit. The odometer and cruise control also has quit operating
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Followup To

Question -
My speedometer for my 318ti has completely stop working My rpm hand still works the other gauges seem to be working but the speedometer has completely stop working?

Answer -
Many reasons this could happen.The speedometer is electronic.The sensor is located in the rear end.I would be looking for a wire that is broken or green inside.You'll see the wire swollen.

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With the needle bouncing it makes me believe it more the head unit.Check the gears in the speedometer

Answer
Having metal filings in the reaend is not good,especially if it was enough to affect the speedometer.Have you check for another unit in a salvage yard?If it is only bearings worn it may be possible to replace only them.The problem..money ,it depends on how much you want to spend