Yes, that is actually happening. I have a Volvo Penta MD22P which only connects gound to the engine when starting or stopping, otherwise it's isolated. However, every time I hit the kill button it blows the 5a fuse labelled Seatalk on the AP Ccourse computer. All Seatalk instruments promptly go to sleep.

I can cut instrument power before stopping the engine as temporary work around but this is puzzling. The only electrical changes since last year were to add an NMEA 0183 to NMEA 2000 converter, on the NMEA 0183 output of the AP Ccourse computer. But I don't understand how this might affect Seatalk or tie into the engine cutoff.

Am going to call my mechanic and raymarine tomorrow. Meanwhile, ideas?