Originally Posted by bigbossman

I've had a 100% success rate getting trigger shifters functioning again.

What happens is this - the little spring loaded pawl gets gummed up as the old grease hardens, and stops engaging the ratchet teeth. So, when you shift up and put the shift cable under load, the pawl doesn't catch and keep it from shifting back down.

What you do is this - remove the shifter from the bike, and remove the cover (usually one Phillips screw in the middle). Work the levers and look for the "stuck open" pawl. Hose it with WD-40 and work it back and forth with a small screwdriver or dental pic.

When the old grease gets dissolved and comes out, the spring will snap the pawl back in place and you're done. Visually check for function and re-assemble. It's easy to do, and you'll never have to scrounge trigger shifter pods again.