[C320-list] Instruments gone awry

[William Russell]

On my 1998 C320 (#545) I have ST40 Plus instruments (speed, wind, depth,
autopilot).  They've always worked fine in previous years.  This year, I'm
getting very unreliable (or non-existent) performance from them.  Speed
seems to work, but I've not checked it against a GPS to see how accurate it
is.  Wind direction doesn't work at all, until partly through a race it
starts to read, though not consistently or accurately compared to the
Windex.  Wind speed reads, but again, I can't tell how accurately, and it
often seems way off. Depth is completely inaccurate - sitting at the dock
in 6-7 feet of water, it showed 14-16 feet, and changed minute by minute
even though we weren't moving.  The compass on the autopilot reads 20-30
degrees differently than the binnacle compass, and the "wind" setting on
the autopilot won't work without an accurate apparent wind direction.
I can't figure why I would get such variable and unreliable performance
from all of the instruments at once.  They were installed by the (very
knowledgeable) previous owner, so I don't really know where to start to
troubleshoot.
Any suggestions?
Bill Russell
"Little Wing," 1998 C320, #545
Cayuga Lake
Ithaca, NY