Fixing the GRX or Road Shifter Won't Catch / Engage problem

The Problem

A quick writeup on a frustrating issue that started after I replaced my bike’s inner gear cable - and the unexpected culprit behind it.

Symptoms

After replacing an inner shift cable, I noticed that my downshift lever wasn’t engaging, and thus wouldn’t shift. By failing to catch the pawl (the mechanism that produces that satisfying click), the shifter would swing out all the way, rather than shifting gears as expected.

(Mis)diagnoses

• I initially thought the old cable may have frayed and left some stray wires in the mechanism. This has happened to me before, and generally wreaked havoc on the entire apparatus. However, I couldn’t see any evidence of that.
• I thought that the spring mechanism might have come loose somehow? Or was it just worn out? Seems odd that it would suddenly start happening out of nowhere, especially after cable replacement.

Taking a closer look

After a decent period of time, I ended up taking off the shifter hood and unscrewing the bottom plastic plate/cover to expose the shifter internals so I could take a better look. The below images show the bottom plate before and after removal of the shifter hood and bottom plate. The bottom plate is exposed once the shifter hood is removed.

The Culprit

Once I took off the bottom plate, a dried up and dead beetle fell out of the shifter! Unfortunately I don’t have a photo here, as it was quite small and all that was left was the bug’s exoskeleton, but it was obviously large enough to do some damage. I can only presume that by replacing the shift cable I had somehow knocked it out of its original spot and into a place where it gummed up the shift mechanism.

What You’ll Need

• Phillips head screwdriver
• Degreaser (WD40 or workshop degreaser)
• PTFE dry lubricant (Triflow, DriSlide, or alternatives)
• Hairdryer (optional: to tighten up the hoods)
• Plastic card or dull knife (optional: to tighten up the hoods)

Repair

With the blockage identified, the fix was straightforward: flush out the debris and relubricate the mechanism.

RJ The Bike Guy has a video on YouTube detailing how he fixes this problem. I did something very similar: I used degreaser to blast out all the old bug parts and kept working the shifting mechanism until the downshifter would catch.

Relubrication

To relubricate, RJ recommends Triflow, which appears to be a light PTFE lubricant spray. According to this Reddit comment, DriSlide Bike Aid is another good option.

Unfortunately I had neither on hand, and they didn’t seem to be available near me, so I used a low viscosity grease and tried to really work it into the shifter mechanism. In retrospect, Supercheap Auto sells a PTFE Dry Film Lubricant which probably would have done a better job.

Reassembly

Putting back on the shifter hood was the most annoying part of the process. What makes it troublesome is that the hood tends to stretch after you’ve removed and then put it back on. A trick for this is to use a hairdryer to heat up the hood, and then you can smooth it back into place with a butter knife, card or similar. A before and after is pictured below.