Temporary power outages and motor noise

[Vic Dupont]

Hi there!

My partner’s electric bicycle is no longer working properly.

The symptoms evolved, but to summarize today’s situation, when riding it after stopping to pedal, motor often won’t turn back on, unless we bring the bike to a standstill. And when motor is on, it makes a lot of noise.

Here’s a video:

Complete history of the symptoms​

For the complete history, my partner purchased it second hand, from someone who seemed very trustworthy and very careful with the bike. The bike didn’t have a scratch and was working perfectly for a week.

The only event we can think of between the moment when it was working and when it stopped was a time when we put the electric bike in the trunk of our car, together with my bike, and at some point my bike tipped and fell on her electric bike. But it wasn’t a big shock really.

After that, the motor didn’t work at all. The display lit fine, but the motor never.

I tried to mess with it, unplugged and replugged every cabled, tested a lot of things. While doing so I got the bike to fall from shoulder height when it was hanging from a low tree.

After all this fiddling, now the symptoms are what I described: when we start riding it the motor starts, when we stop pedaling it stops, and when we start pedaling again it often won’t restart, not always though, .

If we then stop the bike, meaning bring its actual speed to 0, without doing anything else, not changing any controller setting or whatever, and start pedalling again, most of the time it kicks back in instantly.

Conversely, if I keep riding the bike without the motor kicking in, even if I'm going downhill for a few seconds and not pedalling, the motor will not kick back in when I start pedalling again.

It will only kick back in after bringing the bike to a standstill.

Also, when motor runs it makes a lot of grinding noise and big vibrations. The whole back of the bike shakes.

When I hang the bike on the tree again and try to reproduce the issue, the motor noise is significantly lower, I wouldn’t notice that it is wrong. Also it always starts back, I couldn’t reproduce that behaviour when bike is hanging from the tree, basically with no real load. I move the pedal by hand, motor starts, I stop pedalling, it stops, I move the pedal again, it restarts, etc. Basically normal behaviour. Here's a video:

Tests I made​

I did all the tests listed here, except for those that require a device to turn the wheels at constant speed: https://electricbike.com/forum/foru...-motor-s-phase-wiring-hall-sensors-and-wiring. All the other tests I did, and they all passed successfully.

I opened up the motor, suspecting loose magnets. Although I didn’t manage to separate the rotor from the stator, looking at them together I didn’t notice anything weird. This is my first time trying to repair an electric bike, but everything seemed normal. The bike is a foldable Megamo Park and I think it’s a Shengyi DGWX2 motor.

I also checked for broken teeth from the planetary gears but didn’t find anything.

The casing where the gears meet with the wheel had a very slight bit of slightly rusty colored stuff, but very little, and nowhere else.

Hypotheses​

The noise made me suspect a problem with the motor, but intermittent power outages don’t seem to me to be related. Power outages would have made me think of a controller issue.

I would also tend to rule out a battery problem, since the display is always lit no matter what.

Does anybody have any idea what the cause could be?

Thanks a lot in advance for your help!

 

[stompandgo]

Rusty colored stuff usually means metal shavings that have been exposed to water. Not good. My guess is a bad bearing causing metal to metal contact somewhere.

 

[Vic Dupont]

Thanks a lot for the swift reply!

I can imagine how that could create the grinding noise, but I don't see how that could cause the temporary power outages. Do you think it could be related?

Thanks again very much for the help!

 

[Vic Dupont]

I have new data, freshly arrived from my latest test: actually when the symptom happens, meaning I pedal, motor starts, I stop pedalling, motor stops, I start pedalling again and motor doesn't kick in again, I just noticed that if I stop the bike completely, meaning I reach speed 0 mph, and start pedalling again, then it kicks back in again.

So it seems it's not a matter of waiting long enough. It seems to be a matter of bringing the bike back to speed 0. After that it kicks back in almost always. Conversely, I can stop pedalling for a while, and keep riding the bike without the motor for a while, it won't kick back in unless I stop the bike entirely. To avoid further misunderstanding: by "stopping the bike" I am only referring to its actual speed, I am not turning anything off, neither controller, nor battery etc. I just reach a standstill and start again, as if at a red light.

 

[stompandgo]

Despite the issues with the metal filings inside the motor, I'm wondering if this bike has brake cutoff switches and one or both of them is malfunctioning.

 

[spokewrench]

I can't speak from experience, but I believe a controller can start a hub motor in a no-sensor mode. When it gets going, the absence of feedback from one of the hall sensors can make the motor noisy. If the controller shuts off the motor because you stop pedaling, and the wheel continues to turn, I think a controller won't start the motor without feedback from all the hall sensors.

I seem to recall checking hall sensors with a multimeter years ago. It was just practice because I had no reason to think anything was wrong.

 

[harryS]

This sounds like a Hall sensor problem. They're inside the motor. Falling over should not affect them as they are tiny. PCEbiker put up this nice pic of his motor the other day. You can see three of them on the outer edges of he circuit board.



What could have happened is you knocked the motor connector loose. You can recheck that. Is it a molded 9 pin connector? They need to be well seated.

The unambiguous test here is electrical test of the Halls. It's tricky if you don't have the right equipment, and those 9 pin plugs are impossible to probe unless you made a custom tester.

You got two ebikes? Do they have the same motor connector? What I have done when I had a noisy motor is plug motor A into controller B etc.
Of course there's a risk that you wind up with two dead bikes.

 

[Vic Dupont]

Thanks A LOT for your help guys, I really appreciate it!

About brake cutoffs: good idea. I disconnected them entirely, to rule this possibility out, and unfortunately they're not the culprits.

About hall sensor feedback, it is really interesting to know that this possibility comes to your minds.

It's also good to know about this no-sensor mode startup thing. I don't think it's exactly what's happening though, because sometimes I start the bike, motor runs, I stop pedalling, motor stops, I start pedalling again, motor starts again. It seems that it's only after putting it through hard work such as climbing a little, that the symptom of it refusing to start again happens.

Still it's interesting, it might be part of the explanation I guess. At least it makes me understand a little bit better how it works.

@harryS, About loose motor connector, do you mean the black cable outside? Yes I unplugged it and replugged it several times.

About testing hall sensors, I did all the tests listed here: https://electricbike.com/forum/foru...-motor-s-phase-wiring-hall-sensors-and-wiring

Would these cover the possibilities you have in mind?

Unfortunately I don't have a second ebike

Thanks again very much for all your help!