New answers tagged garmin
Garmin 510 uses ANT+ to communicate to the add-on devices. I would remove and re-pair the devices first to see if they are actually seeing each other first. The speed and cadence sensor should pair as a single device and not speed and cadence separately.
I would try another speed cadence to see it works with that or try yours with another garmin (friend?). Sometimes the sensor shifts a bit, I found that it has to be really close to the peddle to function properly. Folks in bike shops are often very friendly/helpful with such things. Battery could also be an issue. ~510 owner.
Try re-pairing the 510 with the speed/cadence sensor. It seems like it's not connected and the speed data is coming from the GPS rather than the sensor. As suggested by others, the alignment of the speed/cadence sensor is important, but it must be close since you seem to get a little cadence data before it drops out. Perhaps it's too close and your crank ...
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