Question about Humminbird 798ci SI Combo GPS Receiver

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Humminbird 798 has erratic gps speed readouts at slow speeds between 1 and 3 miles per hour. Speeds bounce up and down between 0.1 and 3.0 mph even when sitting in the boat in my driveway!

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  • imalyshko Mar 15, 2013

    Thanks for your reply. My problem was solved with adding an external antenna. This solved the problem immediately. The internal antenna seemed to lose signal at slow speeds. I have now replaced my unit with a 900 series Humminbird with external GPS antenna. Works like a charm.

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I had the same problem. I talked to 3 different technicians on the Humminbird support line about it. All 3 of them confirmed that this is how their technology works. They ping the GPS satellites continuously, and at low speeds the GPS accuracy causes the speed measurement to fluctuate wildly. It smooths right out when you go faster than 3 MPH, since your boat moves far enough at that speed that the GPS accuracy is no longer a problem.

Seems like they could easily overcome this issue by averaging GPS readings over a longer time period at low speed (i.e. give the boat enough time to move far enough to eliminate the accuracy issues), but they don't do that.

It was a big issue for me since I do a lot of trolling at low speed and need an accurate speed indicator, so I returned the product and bought a Lowrance instead. Lowrance doesn't have this issue.

Posted on Mar 14, 2013

  • imalyshko Mar 15, 2013

    Thanks for your reply. My problem was solved with adding an external antenna. This solved the problem immediately. The internal antenna seemed to lose signal at slow speeds. I have now replaced my unit with a 900 series Humminbird with external GPS antenna. Works like a charm.

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hydetech
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SOURCE: Humminbird NS25 GPS

This is a common problem on lots of boats with any brand of electronics. Most likely it is being caused by a bad crimp in a wire connector, or by having too many connectors on a terminal in the power supply bus. Start at the power supply source and ground and replace any butt or terminal connectors with high quality heat shrink connectors (such as Ancor). Make sure that the connector is the right size for the wire.

On Humminbirds the problem could also be in the swivel mount that holds the power and data cables. The cables should be held in place by a metal bracket which is located under a panel held onto the swivel bracket by to small philips head screws. If any of those components are missing, the plug may be loose in the socket which could cause it to disconnect temporarily when the boat hits a wave. While you have it apart check the plug and the socket for corrosion.

I hope this helps. Please contact me if you have any more questions.

Posted on Feb 11, 2008

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