Brian,
To be able to select which transducer your 1198c Si unit is going to use for the Si sonar you will have to use a TS3 Transducer Switch.
The AS-Si-DB-Y cable is only for use when you want to use a second transducer as the source for the 2D sonar that your Si unit will use.
Sounds like you either have some damage to the transducer cable of your transducer or GPS Receiver cable or that the power switch is starting to fail.