Interesting, thanks Larry.

Oddly enough, sailing from the UK to Holland last night our Airmar P79 transducer hooked up with our HDS-7 stopped displaying depth. The plug into the HDS7 was fine, and the internally mounted transducer was full of fluid as required. The depth reading has worked perfectly from day 1 about 4 years ago, and last night it stopped. I suspect it might be software within the HDS-7 because in the middle of the night my wife said the HDS-7 stopped working, and we ended up rebooting it. It returned to normal operation except for the depth. I've checked the settings and believe they are set correctly for the type of transducer, etc. Not sure exactly what else I can do.

Oh well, the joys of boating. I'll call Lowrance tomorrow. Maybe a new head unit is in order, as a backup if nothing else. Perhaps I'll do the same swap you did.

If anyone has had a similar issue and has any suggestions, I'd be interested. Not really sure what to troubleshoot, there isn't much there to troubleshoot.