I appear to have an issue with my 12spd XT8100 shifter. I can use the shorter double sided release lever and it'll shift but If I use the long lever it doesn't always engage to pull the cable. I have given the insides a liberal soaking in GT85 thinking some original grease might have gunked up the insides. And while it is a little better it still misses more shifts than it engages.
Am I likely to get this working again, is it likely that the lubricants are at fault, or is the reality something inside might be broken and I might as well cut my losses and get a new shifter?
I’ve come across that problem before but usually in bikes that haven’t been used very much. The springs on the tiny pawls cannot overcome the stiff grease/dirt/corrosion on the pivot. By carefully removing the cover on a bench the mechanism can be lubricated properly and if the pawl isn’t engaging with the teeth then GENTLY move it backwards and forwards with a small pick or screwdriver until it moves freely. Hope this helps! Edit..There should be no need to completely strip it down, there are far too many tiny, tiny bits!
I’ve come across that problem before but usually in bikes that haven’t been used very much. The springs on the tiny pawls cannot overcome the stiff grease/dirt/corrosion on the pivot
Same as wheelsonfire for me. Cover off, flushed it with some aerosol degreaser to remove the older grease then liberally use of muc-off silicon shine (smells lovely) to get it going again without risking a full dismantle
As above too although I managed to flood with lube without dismantling anything.
I found the problem got particularly bad when very cold ie frosty, and that prompted the lube douche.
Lots of wiggling, I put the chain on the biggest sprocket then changed back down without pedalling to slack off the cable and free up the shifter
I suspect you may find there's a ridiculously small spring, about the thickness of a human hair that has no place in a mountain bike shifter, which has snapped. That was my problem anyway. Without said spring the pawl is basically relying on centrifugal force to engage, sometimes it does, mostly it doesn't.
From memory it's terminal. No spares available, even if the job is doable.
I'm sure you've checked, but make sure the cable is in good condition?
I had the same problem that you describe but the lube flush failed to fix it. It tursn out I has the same issue as @bitmuddytoday which was solved with an SLX shifter which I seem to remember reading does not rely on the tiny spring.
I’ve never successfully rebuilt a shifter once taken apart, but you have nothing to lose trying?
Taking them apart and realizing that you have zero chance of reassembling them does make it obvious that you need to buy a new one though instead of hoping that if you spray enough lube inside it might come back to life.
I took an SLX apart rescently on my son's bike - it was working, just. The cable had come slightly dislodged and was catching on the side of the case. Easy fix, so it's worth opening it up and having a look.
I had something similar to the OPs problem on a 10-spd XT unit. I could shift it up the cassette OK (9th to 1st) but it was thoroughly unpredictable in downshifting. One trigger mostly worked, but the other wasn't so good and it wouldn't drop into 10th at all. Turned out I'd unwittingly got the barrel adjuster fully wound out, slacking everything off and moving the barrel back to the middle position pretty much sorted it.