Troubleshooting BBS02 that won't stop spinning

[krusty]

So I was moving one of my motors between frames today and all looked good until I tested it on the stand and found that the motor won't stop! It boots up normally but then when I hit the throttle or turn the crank it activates the motor and then just keeps spinning--in fact it actually seems to pick up speed gradually.

A bit of internet sleuthing suggested it might be the PAS system at fault so I took the axle and big gear out to take a look, but there is no obvious problem: everything there looks fine. I know it must be the PAS parts though (or their connection to the controller maybe) because when I ran it minus the PAS magnet the motor behaved normally and obeyed the throttle both on and off.

So my question is: what do I need to replace? All the parts look good as new and the PAS sensor must be doing *something* because it initially responds to the crank and turns on the motor: it just doesn't know when to quit. Still, I'm thinking its probably something wrong with the sensor, not the magnet disc since its hard to see how that could fail (tell me if I'm wrong about this). I haven't opened up the controller itself yet because I was hoping not to have to take the motor off the frame, but I guess I'll have to do that tomorrow; maybe it is all just the PAS connector having worked a bit loose or something like that.

Incidentally when I was positioning the motor I'd tapped it into place with a rubber hammer and now I'm thinking that probably wasn't a good idea I hadn't bench tested it before, though, and this motor has been out of my rotation for a while so I don't know if that was actually the cause or not. It seems to me the motor ought to be tough enough to deal with a few blunt raps, but I'll still be more gentle with it in the future.

 

[gromike]

I'd say the PAS sensor circut has failed, somewhere. I assume the sensor is inside the motor.
The hammer won't hurt, depending on the hammer, and where and how hard you hit it.

 

[docw009]

Is the chain attached? If you spin the pedals with the crankgear free, it will do what you describe. The clutch on the crankgear will drag the pedals around and spin the PAS disk if there is no force on the pedals.

I've never opened my BBS02B, but here's a nice picture I downloaded years ago. Looks to me like the PAS disk is locked to the pedal axle, so it cannot turn unless the pedals move,

 

[krusty]

Thanks @docw009 that's a handy chart! Any chance you have one for the BBSHD as well?

But no, there was no chain, or even cranks, attached and removing part 29 (PAS disc ring) made the problem go away so I feel it is most likely the PAS sensor itself (part 9 in the chart). I just don't get how that could break: are Hall sensors particularly sensitive to shocks or something? And the failure mode seems very weird. I would expect it would send no signals if it breaks, so PAS just wouldn't do anything rather than working in overdrive.

 

[docw009]

But no, there was no chain, or even cranks, attached and removing part 29 (PAS disc ring) made the problem go away so I feel it is most likely the PAS sensor itself (part 9 in the chart).

No chain? Motor is powered on in the second half of video. Never even thought about what was going on til your post today,

There's a clutch bearing in that big gear that lets it turn faster than the pedals., When the chain is off, there's enough drag in the bearing to spin the pedals and the PAS disk.

 

[krusty]

Interesting, that might explain it and in that case there's actually no problem at all--I just need to put on a chain or at least some resistance to the pedal axle. I mostly work with my two BBSHD's and I've never seen this behavior on them even though the design is pretty similar. I often run those without the chain while working on the bike stand because chain->sprocket + accidental motor activation = ouch!

I just finished working on the bike for today but I'll try the back resistance idea tomorrow. Today I opened up the controller housing and checked the PAS sensor connector, but it appears fine. I left the PAS disconnected so I can at least run the bike by throttle but I'll reconnect it and try your idea before I order a replacement sensor. Thanks!

 

[krusty]

A happy update: problem solved! Thanks to the tip from @docw009 I gave it another try with the PAS connector reattached and the problem has gone away: both throttle and PAS now activate and de-activate the motor as they should, even without cranks or anything attached.

I'm 99.9% sure I accidentally fixed it when I reassembled the main gear and shaft assembly and tightened down the two lock nuts on the left (unpowered) crank side much more than they had been originally. In fact I was surprised when I first disassembled it how loose they were, but it didn't occur to me this might be related. When the problem occurred the crank axle must have been spinning along with the drive gear but I didn't notice it at the time.

So in sum, for future reference: if the axle is too loose and there is not enough back-resistance (from cranks or drivetrain), the backside of the ratchet pawls can spin the axle instead of ratcheting past. This makes the PAS disc spin as if pedalling, which causes a feedback loop so the motor doesn't stop after it starts up. This problem is only likely to occur with a bare motor and no drivetrain so it shouldn't be a safety issue.