I'm trying to fix my wife's bicycle which sat out all winter. I can't get the rear derailleur to downshift from the twist grip shifter. I have inspected and cleaned and lubricated the cables, nothing is frayed or stuck. I can move the derailleur with my thumb quite easily, and if I hold it I can move the shifter. I can also upshift, but I can't get the derailleur to downshift from the shifter. Any ideas? It's a Shimano system.
I solved a similar problem by dousing the inside of the shifter liberally with WD40 and leaving it overnight, I mean really soak it until it's dripping. I had to do this twice but the shifting was perfect afterwards.
When downshifting you are increasing the tension of the cable and moving the chain to the gears with more teeth. You've said there is no friction along the cable path and that the derailleur can move freely, so if you can't increase tension, I would say it's the shifter.
The derailleur has a spring pulling it against the cable because the cable can only pull, not push. Is that spring intact? If not, it's probably best to replace the derailleur.
Sounds like your shifter is jammed. You might be able to repair it, but I'd just get a new one. To confirm this diagnosis, remove the cable. If you still can't move the shifter, then you know it's bad.
And next time, don't leave your bike out all winter.
The rubber-hose-like cable housings used to be made of lower quality materials such as plastic. This made the amount of friction within the lines to be high enough that lubrication was needed, usually a heavy grease or oil.
On modern lines with the flexible, soft rubber-like material, lubrication has an adverse effect on your cables, making them feel kind of gunky, slow, etc.
As for the gearing problem, I would first try to maneuver the barrel adjuster to see if that solves the problem.
- loosen the gear cable just a tad, (maybe a quarter inch)
- re-index your gears
From what you say your problem is, I would think that one of these methods should work. If not, however, I would look into replacing the cable altogether. If nothing else, you could also just take it to your LBS and have them take a peek.
Since you mentioned the cables and derailleur are both freely moving, it's most likely the shifter. Now I had a jammed shifter before but it wasn't due to corrosion. Cables need to be very freely moving, otherwise what can happen is the head of the cable inside the shifter can become dislodged and jam, preventing the shifter from moving. I had this issue personally on a 24 mile ride, causing me to be stuck in one gear for a majority of the ride.
Considering the bike was left outside, corrosion can be the culprit, especially for cables. Sometimes a cable can look and feel fine but in fact it is too restrictive for proper shifting. The easiest fix would be to replace the shifter and cable. It's not a difficult nor expensive repair, just make sure you get a replacement shifter with the correct ratio (1:1 for SRAM derailleurs, 2:1 for Shimano derailleurs). Most shifters require you to snip the cable anyways before taking it off. If you must leave the bike outside, consider going with a full length housing.