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Erratic Speed Sensor Readings

I have a new style, magnetless speed sensor, and lately I've been getting some really erratic readings. Say I'm cruising along on a flat road at 18 mph. The speed on the Garmin will jump up to 21, then down to 17, then 19, settle on 18 for a little bit, and then jump around again. Sometimes it goes totally nuts, and jumps up to like 36. Once or twice, when I was at a dead stop, it showed I was going like 12 mph, until I started moving again. I don't think this happened when I first got the 520, though I can't be sure. Some days are worse than others. I've tried to unpair and repair the sensor, which might help sometimes, for a little while, though again I can't be sure. If I turn off the sensor and rely on GPS for speed, I get smooth, sensible readings. However, I ride in New York City, so GPS readings are often erratic, which is why I got the speed sensor in the first place. I use the Garmin on another bike with an old style speed sensor with magnet, and it doesn't seem to have the same problem.

At first i thought the sensor was wonky, and I called Garmin (after trying to replace the battery, etc.). The sensor was out of warrantee (I got it before I got the 520), but out of the goodness of their heart, Garmin sold me a new sensor for $20. However, when I installed the new sensor, I got the same erratic readings, so the problem apparently isn't the sensor.

Has anyone else seen this happen? I browsed the forum, but didn't see any similar complaints. Does anyone know the cause, or have a solution? I'd appreciate any insight or advice.

0 AdrianRendon over 6 years ago

I had the same problem but change the sensor to the front wheel and that fix the problem I dont know why...

0 9530000 over 4 years ago

I have the same issue with my speed sensor on a trainer. It’s disconcerting as I cannot use this any other way. I am starting to regret buying Garmin if this is how their devices operate. Wahoo speed sensor shows no issues.

0 tigermountrider over 4 years ago in reply to 9530000

It may be magnetic interference with the Garmin. It's been a couple of years since I thought about this but if memory serves the Garmin senses the earth's magnetic field and the Wahoo senses gravity.

0 6624174 over 3 years ago

greetings, could you solve that problem? it's happening to me exactly the same

0 6991279 over 3 years ago

I have 4 of the Garmin speed sensors. All have worked well until recently. I sold one of my bikes which meant i had a spare sensor now. One day, one of my sensors began doing just as described...erratic readings, it would cause my 520 plus to shut down due to a lack of reading. I updated the 520, changed sensor batteries with no change. So i swapped it for my spare sensor. Never had the issue again until I decided to use the malfunctioning sensor on a trainer. Once again, I'm receiving erratic readings while riding my fluid trainer. My personal thought is that these just wear out. My malfunctioning sensor worked fine for hundreds, if not thousands, of off-road XC mountain biking. Maybe the battery contacts are not quite snug enough? I never tried that, but I might now. However, I would still not be surprised if mine is just worn out.

0 aweatherall over 3 years ago in reply to 6991279

The newer speed sensors work off of a magnetometer so if they are placed near an electrical field or other magnetized device they will give strange readings.

0 Wooooooooo!!!!!! over 2 years ago

Same issue here with latest Speed Sensor 2. Had 2 of them. On the front wheel. No magnets near by. Stead speed down a flat road.. Same erratic behavior. Reads correctly then drops to 1/2 or 1/3 correct speed for a few seconds then back again to the right speed. As if it is missing a revolution of the wheel.

Actually have these on all 7 of my bikes and this has always been an issue but I've dealt with it. But recently I was ride leader for a large charity ride where maintaining speed was critical to safety and the sensor went bonkers mid ride. That was the last straw. When and bought a new one and same issue. then another new one and still same issue.

Called CS and they said basically, yea, this happens a lot.... SMH. Really Garmin? 5 years.

This one is going back.