I have 6600 shifters on my bike with 20,000 miles.

The rear der is shifting erratically. Have replaced the shifter cable, and chain. It has the problem with multiple wheels with different cassettes. Have tried cable tension adj. and limit adj. I have cleaned out the shifter with WD40 followed by good lube.

The problem appears to only appear on shifting to larger cogs. When in the 12 cog, shifting to the 13 fails entirely, then it is erratic going to bigger cogs from there, probably from the bad start.

I MARKED THE CABLE BY THE REAR DER AND I CAN SEE ON THE FIRST SHIFT FROM THE 12 COG TO THE 13 THE CABLE MOVES A VERY SMALL AMOUNT, NOT NEARLY AS MUCH ON OTHER SHIFTS. And of course no shift occurs

My only thought is that the shifter is worn out? Is it possible for the first shift to be bad, and the rest OK?

Thanks in advance for any assistance.

You've replaced some parts but did you take the time to properly set up the rear derailler? It sounds like a simple tension issue based on your description but given the mileage on the bike and the fact that you've fiddled with the adjustments already, I would suggest starting from scratch on the set up. If you still have a problem after that, then start looking at replacing components. www.parktool.com has a great rear derailler set up procedure. Follow it step by step.

Question for the OP:

When you use the shifter, it's not indexing correctly. How about when you pull on the shifting cable by hand?

Went through entire Park rear der adjust. No help. I really think the shifter is kaput because of the non movement on the first shift from cog 12 to 13. Next stop, the LBS?

My friend's 6600 was doing the same thing. I added some tension by turning the barrel adjuster one full turn counter-clockwise.
Fixed.
Be sure that your cable is properly routed at the derailleur and that the cable is not slipping at the fixing bolt.

Also be sure that you let all of the cable out of the shifter before attaching the cable to the derailler.

PROBLEM FOUND!!!

Where the cable entered the casing at the rear der is where there was little or no movement of cable. Where the cable entered the casing that goes to the shifter the cable was moving correctly. Ummmm! there is nothing between those points but the cable itself which is good, AND THAT CABLE GUIDE UNDER THE BOTTOM BRACKET!

SOLVED, REPLACE CABLE GUIDE, THAT IS GROVED AND BEAT UP.


Thanks for all the assistance.