Be aware that the switch fitted to the MKIII series of 850 is differs from switches used on the earlier bikes. The number that appears on the back of the US version switch is 149SA, this number is on the white plastic backing plate with the terminal lugs on it. This switch is, in my opinion, more robust than the earlier switches and not as prone to developing problems as they were.
This switch can be disassembled and cleaned and treated with dielectric grease but the disassembly process differs from the older switches.
Remove the lock cylinder first, it will prevent removal of the base plate and contact rotor if it is not removed. Straighten the 3 locking tabs, I used a narrow screwdriver blade sharpened on the end. Pull the white plastic base with the terminal lugs on it from the metal housing with it will come the rotating part of the switch and the triangular contact plate and spring, all captured in one assembly.
While it would be possible to do a half way cleaning job of the contacts without going deeper into the switch you can take it apart further. Look at the back of the switch and you will see the heat staked or upset end of the shaft for the rotating portion of the switch. Carefully scrape away the plastic lump from the end of the shaft until it will snap back through the base.
Now you will be looking at the copper triangular plate and be able to access the contacts with the abrasive of your choice to polish them. I used a Cratex rubber cone in a Foredom tool to polish mine. Coat all contacts with dielectric grease and reassemble and heat stake the shaft. You may have to counter bore the shaft bore to have enough material to effectively heat stake the shaft. A soldering gun or iron is ideal for staking the shaft.
If you lost track of the orientation of the rotating parts the hole in the shaft lines up with the hole in the housing and the single contact dimple end of the copper triangle points away from the hole in the housing. (the contact piece has a rectangular hole in it and it can only go in one of two ways, the correct orientation has the single dimple pointed to 6 o'clock if you have the hole in the shaft and housing pointed to 12 o'clock) Would post pictures if I had the appropriate camera but if you got this far without breaking anything I am sure you will figure out what I am attempting to say about the orientation of this piece.
Hpshooter
T W Hudson
INOA 2036