Diagnosing an brake sensor issue

[chalmero]

Hi all

First time posting here. Excited to learn from the wealth of knowledge in this community!

I have a Yuba cargo bike that I converted 3 years ago. I added a bafang mid drive (BBSHD) system and it has given me endless joy and utility. A couple months ago, i encountered a strange symptom where in the middle of my ride, the motor started to "stutter" (cutting in and out rapidly) and the display indicated that I was out of battery, even though I had 75% charge left on it. The system then shut down completely (motor + display) and I could only get it to start up again after letting it sit for a while. But I quickly encountered the same issue again and again.

After much tinkering (taking the motor apart, buying a new controller, testing the battery), I ended up finding the problem in a place where I obviously should have started: the brake sensor. Once I unplugged *one* of the brake sensors on my more-recently installed Tektro hydraulic brake, the bike works now works beautifully.

My question: How can I be sure *where* the problem is occurring in the brake system. I've purchased a new 3-pin sensor cable that feeds into the brake lever, but if the problem is occurring elsewhere, I'd rather not go through the trouble of installing it. Any advice??

 

[amberwolf]

AFAIK the sensors just short the brake signal to ground when they are activated. They use only two pins (even if there are three in a connector). You can verify this by measuring the pins on the working sensor with it unplugged with a meter set to ohms or continuity. If the meter shows continuity or low ohms only when pulling the lever, then this is that type.

That makes it easy to see the problem in the other sensor--to see where it is failing, just use the meter to measure across the signal and ground wire of the failed sensor connector. It'll show shorted when it should not.

But regardless of where it is shorted, it's within the disconnected wire / connector / sensor unit, so replacing that whole part would fix the problem. (you can verify this by plugging your working unit into the plug where the failed unit was).


Another type of sensor is actively powered, using three wires, and has a hall sensor like those in a motor, which grounds it's output (just like the other kind) when a magnet is in the right position near it. The only operational difference is it requires 5v to power the sensor (the other kind doesn't); the effect and solution are usually the same.



Note that if you have that kind of sensor (or any magnetic type), and the magnet is not in the right place, it can cause the brake to engage when it shouldn't from too little lever movement, vibration, etc. If it's a magnet position issue, removing the magnet will cause the problem to go away even when the sensor is still plugged in.

A couple months ago, i encountered a strange symptom where in the middle of my ride, the motor started to "stutter" (cutting in and out rapidly) and the display indicated that I was out of battery, even though I had 75% charge left on it. The system then shut down completely (motor + display) and I could only get it to start up again after letting it sit for a while. But I quickly encountered the same issue again and again.

FWIW, the symptoms you describe should not be possible from a brake sensor failure, but only from either a battery connection or battery problem (such as a cell group imbalance or defect causing voltage drop under load, below the point at which first the controller reaches LVC (motor stutter), and then the BMS (power shutdown).

But if it's working after only disconnecting the sensor....it's good.

 

[beemac]

Hi all

First time posting here. Excited to learn from the wealth of knowledge in this community!

I have a Yuba cargo bike that I converted 3 years ago. I added a bafang mid drive (BBSHD) system and it has given me endless joy and utility. A couple months ago, i encountered a strange symptom where in the middle of my ride, the motor started to "stutter" (cutting in and out rapidly) and the display indicated that I was out of battery, even though I had 75% charge left on it. The system then shut down completely (motor + display) and I could only get it to start up again after letting it sit for a while. But I quickly encountered the same issue again and again.

After much tinkering (taking the motor apart, buying a new controller, testing the battery), I ended up finding the problem in a place where I obviously should have started: the brake sensor. Once I unplugged *one* of the brake sensors on my more-recently installed Tektro hydraulic brake, the bike works now works beautifully.

My question: How can I be sure *where* the problem is occurring in the brake system. I've purchased a new 3-pin sensor cable that feeds into the brake lever, but if the problem is occurring elsewhere, I'd rather not go through the trouble of installing it. Any advice??

On the tsdz2 osf when you engage the brake the motor cuts out for a certain minimum time (not sure if it's a second or more or less) even if you only tap the brakes very briefly - you can't make the motor stutter too quickly using the brake as the software doesn't allow it. Not sure how the bbshd works - but if it's similar then like amberwolf I'd say it's not the brake causing the problem and the battery has to be the most likely culprit based on the symptoms. You say you tested the battery - how did you test it - under a load?