Could be worth a look at the connector on the back of the unit itself. As I discovered a few weeks back, they aren't as saltwater resistant as the IP67 rating would lead you to expect. Compare the pins in the plug with the diagram on page 99 of your manual (also downloadable from http://www8.garmin.com/manuals/GPSMAP198CSounder_OwnersManual.pdf) and make sure all the pins that are supposed to be there actually are. In particular, make sure pin 15 is in a decent state. This one carries power to the unit, but will act as a sacrificial anode in the presence of saltwater, and can rapidly corrode to oblivion. If this is what has happened, you have a bit of a problem, because Garmin, don't appear to be very good at making spares available. I was lucky in that the GPS I was fixing was a lower end model that only used five of the seven pins in its connector, so I was able to rewire to provide a supply through the unused pins. The diagram suggests that the 198C also sports a few unused pins, so you may well be in luck. A.
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