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Speed / Distance cuts out during cycling - Think its related to the altimeter

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  • Speed / Distance cuts out during cycling - Think its related to the altimeter

    Over the last 2 weeks I've had my Garmin 935 stop recording speed (drops to 0.0km/h) and stop recording distance while cycling. This has happened three times and has previously never happened before (I've had the watch for almost two years now and have done heaps of cycling). The watch automatically pauses the cycling activity when this happens, however I've tried turning auto-pause off and noticed that the heart rate and cadence sensors keep recording. So presumably the watch is still working.

    Also, one of the times this happened was during a triathlon (really annoying!) and when I got to T2 after the cycle and to the run the watch functioned correctly - recorded speed and distance and everything else during the run. Once again, makes me think that almost everything is functioning correctly.

    I can upload the data if that helps, but what I've noticed is that in all three scenarios by elevation has dropped significantly at the time of the malfunction. During the triathlon the elevation went from +40 to -90 in a matter of seconds (see attached picture).

    I've also noticed that all three times I was going relatively quickly (30+km/h) with a headwind coming at me. My watch is mounted on my handlebars with the triathlon quick release thing. So this leads me to believe this has something to do the altimeter/barometer (maybe the pressure from the headwind is messing up the readings). I've tried playing with all the settings but nothing seems to helping.

    Has anyone had similar experiences or is this most likely a warranty issue?
    Attached Files
    Last edited by samharris212; 11-10-2018, 10:25 PM.

  • #2
    Do you have a speed sensor? If so, it is more likely that the speed sensor has dropped out.

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    • #3
      No. No separate speed sensor. All tracked through the watch. all external sensors are going fine (cadence, heart rate).

      Also, not sure if that would explain the massive elevation drop as well.

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      • #4
        The watch does not reboot mid activity? Do your tracks look OK? The 130 meter elevation drop would imply 15-16 hectopascal rise in ambient air pressure which is AFAIK unlikely to be due to a weather event, so that might indeed point to a sensor issue. It's always hard to isolate where the problem is, but if it still happens after a master reset with no ANT+ sensors in use, it may be time to call Garmin Support.

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        • #5
          Thanks for the replies. I've done a bit more googling and I didn't realise that the Giant RideSense sensor I have is both a cadence & speed sensor. So it looks like the most likely culprit is this external ANT+ sensor.

          I think the drop in elevation can be explained by the fact that I'm unpausing the timer while I'm on the bike and I'm guessing maybe the altimeter does a quick elevation check. Since I'm getting big headwinds on the bike the barometer makes it seem like the air pressure is much higher, hence the 130m elevation drop.

          My tracks look OK on the GPS, so I think you're right that its not a GPS issue. And the watch doesn't reboot mid-activity (just pauses but I think that's because auto-pause while not moving is on).

          I'll play around the the ANT+ sensor and do some tests with/without using and report back.

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          • #6
            A low battery in the speed/cadence sensor is probably the 1st place to look. Possibly also whether the speed magnet is still properly aligned.

            Perhaps worth turning autopause off for cycling, so that it doesn't pause until GPS speed kicks back in.

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