In Speedometer 6y5-83570-s6 there are two black wires which is ground and which goes to fuel tank?

My Raymarine E80 depth display kept freezing up (display then says "DSM connection lost"). Of course, this seems to happen more often when entering a tight shallow anchorage in Alaska's inside passage. Raymarine tech support recommended that I update the display software and then update the DSM300 sounder software. I did this to no avail. Then Raymarine told me to install a dashboard mounted toggle switch inline with the positive power input of the DSM300 module so I could do hard re-sets (turning off the system power or powering down the displays does not reset the sounder module). This I did. Now when entering a tight anchorage, the display still freezes, but at least I can do a hard re-set with the new toggle switch and have the unit back up in about 40 seconds (a life time in a tight channel with the current running, etc.). Now, Raymarine says that if I send in the DSM module to them, they will replace it with their new circuit boards that solve this chronic problem that they have known about for a long time (hello, this is a significant safety issue - why no recall?).
Don't mess with this - send your DSM300 in to Raymarine!
Bob S.