I wrote the below over a year ago. I follow this and my FF works like new and the tiny connector pins are as bright and shiny as the day I bought it. I've even rolled it in the surf. Only thing I do otherwise it freshwater rinse it.
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As kayak anglers, we regularly do something to our fish finders that the boat guys rarely ever do, we disconnect our head unit from the cable. So what’s the big deal? First, I don’t believe many of those connectors were designed for repeated connection/disconnection. Second and more importantly, when you power down your head unit, there is still power going to the connector.
Since every manufacturer uses a different connector, the severity of these issues varies but none are completely immune to corrosion issues, especially in saltwater use. There is much debate on which manufacturers have the best connectors, but there is little debate about whose is the worst. That unfortunate title goes to Lowrance at least according to the countless internet forums I have read on the subject. The Lowrance connector is a single connector for both power and transducer, so if the power terminals corrode out, it’s nearly impossible to replace the connector and you’re stuck with buying a whole new transducer set. The pins on connector are also fairly thin and a bit delicate. Finally, unlubricated, the connector can be tough to plug in which only makes bending the pins more likely.
But don’t let all that scare you off of Lowrance if you want one. I have a Lowrance Elite 4 an am very happy with it. Just follow the below steps and you shouldn’t have any issues. Some of the below is specific to Lowrance, but can be adapted to other fish finders.
Ample use of dielectric grease on the connector almost every time it is connected/disconnected. Dielectric grease protects the pins from corrosion and makes the connector slide in and out effortlessly. Trust me, it's not a little difference, it's all the difference and it is HUGE.
Absolutely disconnect the battery before you remove the connector from the fish finder head unit. Corrosion is an electrochemical reaction. Having unprotected thin metal pins in a highly ionic fluid (salt water), connected to a power source and allowed to short out is going to be an issue for any connector.
Cover the end of the connector when not in use. A 3/8" screw protector from Lowe's works well for this on Lowrance units.
Even when it's all connected up, its best to disconnect the power to the unit (either with a separate switch or disconnecting the battery) if you aren't using the system.