T&T: Airmar B744

[Garrett Lambert]

I believe I have the same unit. Here's a copy of my post from a few months
ago. Hope it's relevant.

About a month ago, the depth sounder reading on the pilothouse chartplotter
disappeared for no apparent reason, until a message on the screen said the
sonar was switched off. Rebooting the chartplotter returned it, but I had a
bad feeling that proved to be well-founded. The depth display was lost with
increasing frequency until a few days ago, it seemed gone for good.

With a lot of patient assistance from tech support at both Garmin and
Airmar, I was able to determine and resolve the cause at no expense other
than a lot of time, most of which was traveling back and forth between house
and marina.

The boat has two Garmin 2010C chartplotters, one at each helm. Two Garmin
GPS antennae feed each one independently. So long as they were working, I
had paid no attention to them. I learned through tracing wires and a
combination of tech support that data indicating depth, speed through water,
and water temperature are provided to both chartplotters from a single
Airmar transducer via a Garmin GSD20 sounder module. The GSD translates the
Airmar's data stream into a Garmin format and amplifies it to support up to
2 displays in either split- or full-screen presentations. "Features include
auto-ranging, 2X and 4X auto and manual zoom, optional fish symbols and
See-Thru(R) technology, which separates fish from thermoclines and
structure."

The first step was to determine the source of the failure, i.e.
chartplotter(s), GDS20, transducer, or wiring/connections. With the same
chartplotters at each helm, it was simple to switch them back and forth and
conclude that they were fine. A Garmin tech thought it might be the GSD20,
told me about an LED on the unit, and gave me the codes to interpret its
several messages. It was signaling a software failure, so he next suggested
I update the software. Since I have the proprietary data card reader and an
extra card, I downloaded the update from Garmin's web site, and after
applying it at the lower helm, ended up not only with no depth display, but
a new message indicating no signal from the GPS antenna as well. Yikes! Yet,
this chartplotter worked with the GPS antenna on the upper helm. Another
call generated the information that Garmins have a "Boot block" feature, and
that I should use it to apply the update again. Back to the boat, insert the
data card, and this time, turn the chartplotter on while pressing the up
arrow. This sent the unit directly to the data card, and the update loaded.
The GPS was now working again, but, sadly, still no depth info, and the
GSD20 was still signaling a software failure.

I called Airmar and connected with a very knowledgable lady tech who passed
on some info that none of the Garmin techs had known, i.e. that the Garmin
GSD20/21/22, for whatever reason, first sense the temperature data from the
transducer. If that temperature data is unavailable, the GSD's won't process
the depth and speed data, and simply shut down. The temperature data is
provided by a thermister embedded in the transducer. She gave me two tests
to determine if the thermmister had failed. I had mentioned the independent
depth sounder, also a Garmin with an Airmar transducer. She said it had the
same connector, and suggested I plug it into the GS20 briefly - because the
GSD20's power would cause it to overheat if left on too long - to see if
that corrected the problem. Alternatively, I could measure the resistance
across the thermister terminals. A meter reading significantly higher than
about 20 ohms would indicate the thermister had failed. I went for option
one, plugged in the second transducer for a couple of minutes, and saw a
bottom contour form on the chartplotter, but no depth data. She later said
that was because the much smaller single frequency transducer didn't have
enough power to drive the GSD20, but that the thermister was clearly the
problem.

She went on to say I now had 2 choices. If the water temperature reading was
important to me, I'd have to replace the through-hull transducer. (Not so
daunting as it might seem, since I'd already pulled it once to try to get a
part number off it.) If not, I could short out the thermister leads, and
thereby 'fool' the GSD20 into processing the depth and water speed streams.
Since water temps in the Pacific north-west vary only a few degrees between
summer and winter, and a new transducer would cost about $200, I went with
option two. BTW, incomprehensibly, there is no part number on the transducer
bodies, only on a tag attached to the transducer cable, and, of course, that
tag is often knocked off. However, she was sure she knew which transducer it
is, and pointed me to the wiring diagram on Airmar's website. From there it
was easy to slit the cable open, locate and isolate the colour-coded
thermister leads (white and brown), short them out, and tape everything up.
With chartplotter on, the depth display was back to normal, and the LED on
the GSD was signaling all OK. Happiness. And well deserved kudos to Airmar
and Garmin support.

Cheers, Garrett