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  1. #1
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    Polar S520 Speed Sensor Problem

    I have been using a Polar S520 on my mountain bike. For about a year speed/distance calibration looked quite reasonable. Then about 2 weeks ago it seems like the calibration suddenly changed. Instead of 15 mph speeds I'm showing 25 mph speeds and distance went up accordingly. I checked to make sure the bike setup didn't get changed ( I use it with my road bike too). I checked the calibration number for the wheel/tire size. Both are right where I set them.

    Does someone have a suggestion on what else to look for? Is there a battery in the speed sensor that could cause this? Could the magnet to sensor distance cause such a problem and why would that have changed? Everything I can think of would cause the speed to show slower, not faster. Any help would be appreciated.

    I'm comparing the speed/distance to both a Cateye Enduro and a GPS track. Cateye shows 10.5 mile loop, GPS shows 9.8 mile loop. Polar use to show 11 mile loop but suddenly changed to 19 miles. I tend to trust the Cateye more than the GPS due to tree coverage over the trail.

  2. #2
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    Found the problem. I'm posting it here so that others don't have to learn the hard way.

    When the battery on the speed sensor gets low, the indicated speed goes UP. That's not what I expected but replacing the speed sensor battery solved the problem. In spite of the warnings in the Polar manual, replacing the battery wasn't very hard.

    Now if I could solve my rear der / shifter problem as easily...

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