Not sure if anyone's had a similar problem been looking at a friends 650 Twin (800) 2008 approx 20k. Left overnight/from cold the starter struggles but ok soon as it's been running a short while. The voltage at the starter drops as low as 5/6 volts on the cold start (new battery fitted & old battery fine on my bike so eliminated that) stripped the starter checked brushes/bearings/armature/magnets & continuity etc all according to the surprisingly detailed Haynes manual all seems good. All electrical connections checked and cleaned I guess the next/only thing to try is a new starter but thought I'd ask here just in case.
Regards AL
There is a history of alternator failures on the older F800 twins, where the stator windings overheat and fail. I had to change my stator, which was cooked, on my F800 GS. BMW eventually sorted the problem with an updated stator, fitted to new bikes around 2012.
I would check out and confirm the charging circuit is OK before paying out for a new starter, I’m not aware of any starter issues.
On a healthy alternator the DC voltage should be around 14.5 volts measured across the battery, with engine running.
The three phase AC voltage directly from the alternator (before it goes through the voltage rectifier should be around 50 to 60 volts across the three yellow wires at 3,000 rpm. If it is the stator, be aware that BMW only sell the complete alternator assembly, which is pricy. You can buy updated stator only from a company in Germany, via eBay and it’s fairly easy to change.
BMW also recommend that you change the voltage rectifier, I fitted a new one, however the old was fine, and is now a good spare.
Hope this helps and good luck
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