It sounds like you may have to send it out for service, but before you you do there are a few things you can try.
Low voltage can cause erratic behavior. Since you already wired directly to the battery, check the plug for corrosion.
Try unplugging the transducer cable to see if the display still behaves erratically. You wont see depth readings but you should still see the screen going black. If no erratic behavior with transducer unplugged then problem is in the transducer cable or the plug. A common source of failure is at the plug because the added stress of plugging and unplugging cables can cause the socket to twist slightly which can cause a short.
Make sure you have the latest software update.
How to see if your LCX-19C is already using version 1.8.0 software:
- Press the MENU button twice on your LCX-19C.
- Select SYSTEM SETUP.
- Select SOFTWARE INFORMATION.
- The software version for your unit is located under the line that says "LCX-19C" (ex. "Version 1.8.0").
- If your version is the same version listed above, there is no need to download the software.
If it is lower than 1.8.0 you can download the update from this link: LCX-19C UPDATE
Copy it onto a blank MMC card and install it in your GPS. The GPS should prompt you on how to install the update.
If you need to send the unit in for service click this link: LOWRANCE SERVICE