How to tell if Bafang gear shift sensor is working

oldorchard

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Aug 14, 2022
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How can I tell if a gear shift sensor is working? I picked up a great BBSHD mountain bike build secondhand. It has a gearsensor dot com brand gear shift sensor installed and a Bafang 500C display. The display does have a brake cut-out indicator light--it's yellow in the lower left-hand corner--that works well whenever I brake. Should that also light up and cut power if the gear sensor is working? Or is there no gear sensor indicator light on the Bafang 500C display?

Bit of background: the one problem with the bike was that when upshifting to the smaller gears on the rear cassette, the bike wouldn't always shift immediately, almost as if the shift cable was sticking when I gave it more slack. Downshifting was no problem. So I cleaned out and gear shift sensor housing, lubed the sensor wheel, and roughed up the cable and sensor wheel and now shifting is much better. But I can't tell if the gear sensor is actually cutting power, since maybe I've never had a working one and I'm not sure what it's supposed to feel like, and there doesn't seem to be any gear sensor indicator lights on the display.

Any insights much appreciated. Also, would getting rid of the stock BBSHD front chainring help with the delayed shifts?
 

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So helpful, thank you ebuilder! I'll check out the cable tension and other potential issues, too. Do any of your BBSHD builds have an gear sensor indicator light that goes on when you shift? Is it the same indicator as the brake cut-out?
 
It's definitely broken. Got the bike up on a stand and it wasn't cutting power. There may be an indicator light on the actual sensor housing that doesn't light up either. Anyway to troubleshoot it? I'm guessing it's a problem with the sensor cable, perhaps a too-tight bend in the routing to the motor? Just get a new one and see if that works?
 
AFAIK these sensors work the same way the HWBS (hidden wire brake sensors) do: there is a magnet inside it that "sticks" to the cable that's passed thru it, and a hall sensor in a position to detect the movement of that magnet. The specifics of the two are likely different inside, but generally the problems seen with this design are mechanical. For instance, the magnet might have too much friction on the inside of the case and not move with the cable; this could be caused by contamination that got inside the housing, distortion of the housing (heat, mechanical pressure, etc), etc.

You can test the sensor with a piece of cable thru it, moving it back and forth, to see if the light on it (if it has one) shows a detection, while it's hooked up to the system. Or if the system indicates it detected a gearshift (if it has a way to show that). If it doesn't detect anything, regardless of how much moving of the cable is done, either it's mechanically stuck inside, or electrically faulted (no power to it, no signal from it, broken ground, failed sensor, etc.)
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I moved the cable while it was connected and got no response. I also took a pretty strong magnet and waved it all over the sensor body while it was connected and also got nothing. I bought a new one and will hook that up in place. Hopefully the cable slides more easily and it works to sense shifts.

Otherwise I'll just remove it and replace the cable housing. If I go that route, I was thinking of adding a 5cm piece of cable housing with ferrules on the ends to make up for the section of housing that was removed to add the gear sensor--that way I won't have to run a whole new cable, which currently is pretty intricately routed under the motor and taped together with the other cables.
 
In case anyone comes across this thread in their own searches, the problem was:

1. Dead gear shift sensor. New one lit right up.
2. Messed-up rear derailleur cable ferrule dragging on the cable, messing up shifts.
3. The BBSHD motor itself pinching the rear derailleur cable housing where it ran under the bottom bracket. I'd recently zip tied the motor to keep it from working loose and the pressure was dragging on the cable. Some tight zip ties on the cable also seemed to be affecting shifts.

Working perfect now.
 
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