My JDM mirror started juddering and making a grinding noise when the fold button was pressed. It would fold in if you gave it a little nudge but was not ideal.
I fixed this last week and thought I'd write down how to do it as looking at the way it was caused I think it will happen to all folding mirrors given time.
I have a spare mirror assembly about somewhere so will try to get some pictures in future, but for now here is a wall of text:
The way the folding assembly works, is that you have a small DC motor which uses a worm drive to drive a plastic gear, which in turn drives 3 small identical brass reducing gears. The primary reducing gears (3 of, mounted on 2 steel shafts) are where this fault occurs. The gears are free to rotate on the 3mm thick steel shafts which they are mounted on, and these shafts are held loosely in place by recesses in the black plastic casing of, for want of a better description, the 'folding assembly gearbox'
The problem occurs in how these shafts are held in place. The fact that the way it is assembled/the tolerances used from the factory allows these shafts to have a couple of mm axial float in the palstic recesses, combined with the relatively high torque exerted on them by the 3rd reduction gear, means that instead of the gear freely rotating on the shaft, the shaft rotates binds against the gear and in turn rotates against the plastic recess, wearing it away.
On my mirror assembly the shaft had worn the upper 3/4 of the lower plastic recess oval, allowing the lower gear to move about when under a high torque condition (such as when the mirror first starts to fold in), disengage, and cause that awful sound of grinding gears.
The fix is simple - brace the shaft in the housing so it cannot move up/down, so it is fixed in place and the gears rotate about it as intended.
How I did this: I removed the couple of mm float that the shaft has in its mountings by placing a ~2mm length of 3mm thick steel bar in the upper plastic shaft recess. This was just enough that when the gearbox cover is screwed down, the shaft is placed under tension, forced down into it's original lower recess and is therefore no longer free to move about, or raise up into the worn area of the lower recess. With this in place there is still adequate plastic recess available in the upper case to keep the shaft located radially.
Being sure to grease the shaft and the gears well as they will actually be rotating about the shaft now, I now have a nice quiet folding action once more.
Hope this helps someone sometime. I appreciate it's hard to explain in words so will endeavour to grab some pictures
EDIT - forgot to add one more thing - In order to access the folding gearbox, you must:
1) remove mirror from car
2) remove mirror up/down/left/right module with 3 screws, remove plastic bodycoloured mirror casing with 4x screws
3) remove the 3 screws from under the mirror, which are loctited in place
4) separate the triangular rubber seal from the body portion of mirror housing by cutting the electrical tape holding it to loom.
5) push this wire through the body portion to give you slack to work with on the mirror assembly
6) unclip the white plastic cable stay thing which clips into the top of the folding gearbox
7) unplug the loom to the folding motor, and unscrew the 4 small screws holding the upper gearbox case on, and slowly pull apart.