Speed sensor showing wrong or no speed, but only when not pedaling

[ichauchtt]

I have the following issue on my YT Decoy. It all started with error code W011 (speed cannot be read). I removed, cleaned and reinstalled the speed sensor, magnet and everything looks good and undamaged. The result is that the error disappeared, however, the correct speed is only shown as long as I am pedaling. When I stop, the speed display starts jumping up and down and eventually goes to 0 kph. The distance is also not measured correctly.

The bad thing is that the motor on the YT is mounted directly to the frame, I would have to remove the whole motor to get to the connectors. I talked to YT and they said that I should not try to remove it myself as the warranty would be voided.

This behaviour is very strange, how can the speed only be measured when I am pedaling? If it would be the sensor or cabeling, speeds would always be wrong or not shown at all?

Anybody came across a similar issue or has any hints?

 

[urastus]

I did have a similar issue, of my own doing. I installed a larger rotor on the rear. It didn't come with a magnet, so I epoxied one on in the right location. It is only 5mm x 1mm; about 1/3 or 1/4 of the area of the original. Anyway it worked fine...for two days Like you, I applied logic, cleaned everything, reseated the connector (mine is easy to get to), etc, all to no avail. Then, I just whacked another magnet on top of the existing magnet - voila! I have no idea why it worked trouble free for two rides. I have since superglued that magnet in place on top of the existing magnet, done a couple of rides, all good. If you have the spoke type magnet, consider it's location - try and get it as close as possible to the receiver. Or buy some 10mm x 1mm rare earth magnets from ebay - they are stupidly cheap. And, re the pedalling - there is probably some flex - cluster on the right; under power pushes front of rear wheel left inside the swingarm bringing magnet closer to sensor? It's a theory - so check that your rear axle is tight!

 

[ichauchtt]

Thanks for the answer. The idea about the flex could really be the issue. I noticed when removing and screwing the sensor back on, if I tightened the screws too much, there was no speed measuring at all... Could it be that a fraction of a mm makes the difference? Wow, that would be a thing! I will try your trick of whacking another magnet on top of the existing one.

BTW, my magnet is on the brake disc not on the spokes.

The axle is tight and there is no wobbling.

 

[ichauchtt]

I just tried with a fridge magnet as a temporary solution and indeed, it works! How can it be that this problem is not appearing more often if it is really about the distance (or strength) of the magnet? Now I'm gonna get some of these neodym magnets to make the fix permanent.

@urastus - you superglued thet 2nd magnet, no issues yet of it falling off?

 

[urastus]

I just tried with a fridge magnet as a temporary solution and indeed, it works! How can it be that this problem is not appearing more often if it is really about the distance (or strength) of the magnet? Now I'm gonna get some of these neodym magnets to make the fix permanent.

@urastus - you superglued thet 2nd magnet, no issues yet of it falling off?

Still good. The first one I put on with two part expoxy. The 2nd one I didn't want to wait, and it was still a theory at that point and needed more testing, so I wasn't going to go all anal with epoxy again I did wipe over the old expoxied magnet with some isopropyl alcohol to clean it.

 

[Peaky Rider]

I have two e-bikes (both Shimano) and added second magnets to them both as soon as I got them. Super glue and their own power has kept them in place for two years now. I also carry a spare when I go out.

 

[ichauchtt]

Yesterday, when trying it on the stand with a simple magnet, it seemed to work. However, today, I took my baby out and in real life conditions, the issue still exists with the wrong speeds. Even when pedaling without any pull on the chain, the speed shows correctly, so it can't be the flex either. I tried several magnets, flipped them around, changed positions on the brake disc..., nothing helped.

Sometimes, a wrong speed is shown with the pedals standing still and it appears to be always a fraction - 1/2, 1/3, 1/4 of the correct speed, so the sensor works, but not on every turn.

I guess I need to take the bike to a Shimano service center, either it is the sensor itself or something worse with the motor.

 

[urastus]

I still use basic computers on my acoustic bikes; magnets on the spokes, sensor on the forks. The only problem in 20 years or so is the pickup getting knocked now and then. A few seconds to put it back in range of the magnet and it's good again. I don't know how ebike manufacturers manage to make something that was so reliable, unreliable. What is it with ebikes? Good luck.

 

[ichauchtt]

Eventually, I did not take the bike to a Shimano service center but replaced the speed sensor myself today. It is a bit painful because you need to remove the motor on my YT Decoy to get to the cable. Took me a couple of hours, but now it works again. So, after all, it was just this 25 Euro piece of plastic which caused the trouble. One would really think that such a simple part should not be faulty and work forever.

Well, anyway, I am happy that everything works fine again