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This bike is over 17 years old and not particularly high quality, but it does not make sense to replace the whole thing over one shifter. So I am looking to repair it if possible. I'm confident with most aspects of maintenance, but I've no idea what goes on inside these things.

Side view

When changing 'down' to an easier gear with the thumb, the lever slips and the gears do not change. The lever moves further than is expected, with no resistance. By simultaneously applying lots of upwards pressure with the thumb, you can sometimes get it to shift. Once it does shift, the rear functions happily. I've had no other derallier issues.

Top view

There's an allen key fitting on the top, and a screw on the bottom. I have not opened it up, worrying that springs will ping out and all will be lost!

Underside

I've no idea if it needs more lubrication... or all the gunk cleaning out. Go tighter... or looser?

Thank you.

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    You may be able to take the covers off and clean out the old grease before relubricating. Taking the covers off shouldn't make it go ping. I think we've discussed this before hence this is only a comment; I'll try to track it down, but roughly: clean out what you can, flood with light oil, wait, clean again. Found it. Different type of shifter, same issue
    – Chris H
    May 10 '19 at 16:32
  • Thanks. So I need to clean out the gunk and then use a not-WD40 lube on the pawls. I did search on this website before posting but didn't find that question, so thanks for linking. Will report back... May 10 '19 at 16:54
  • That's right. It's worth a try anyway. I used WD40 as part of the cleaning process, to soften the old grease.
    – Chris H
    May 10 '19 at 18:52
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    The usual cause of problems with these is that the lube inside has become hardened and is causing sticking of the "pawls" that make the shifter ratchet. The best thing to try first, before attempting disassembly, is to blast some spray lube into the unit and work it back and forth a bit. The spray lube dissolves the old grease. May 10 '19 at 21:11
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    Unrelated - your brake levers cable adjust seems quite extended and probably almost out of thread. Consider tightening it all the way in and taking the slack out at the pinch bolt down on the brakes.
    – Criggie
    May 11 '19 at 0:27
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These shifters are not really supposed to be repaired. Sometimes contamination interferes with the mechanism and can be cleaned out, but if the ratchets have worn out you just need to replace them.

Shimano makes 8 speed shifters that are compatible with older derailleurs. The current Acera/Altus range includes 3x8 and 2x8 shifters for 2 and 3 front chainrings.

Here's the Shimano range line-up page for Acera/Altus: https://productinfo.shimano.com/#/lc/acera_altus/3x8

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  • There's little danger if OP has a crack at fixing, or at least improving the shifters. Replacement is a valid solution if the suggested cleaning doesn't work. I've never seen a worn out ratchet, but a gummed up one.
    – Criggie
    May 17 '19 at 14:10

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