One more little piece of info that might shed some light on what is going on inside the motor. MK says that even using the TM battery to power the depthfinder, such as in a boat with no outboard can cause this problem. This isn't just a little leakage either, as some owners have reported burned wiring and evidence of the nose cone of the motor being hot.
That seems to say that the reference for the transducer negative is tied to something that is not TM battery negative, but that it is held at a particular voltage with respect to TM negative. That could be TM battery positive, but more likely it is some in-between voltage.
What causes the problem isn't connecting the cranking battery negative and the TM battery negative, but connecting TM battery negative to the fishfinder at all.
It even happens if you use one of the TM batteries as the cranking battery for a kicker motor that doesn't share a wired path to the big outboard. It appears that having both lower units in the water provides enough current in some situations to cause problems.
Hmmm...
