Sounder sensor or cable fault?

[jackt]

Q: Depth sounder (in steel yacht hull) works fine...for about 1 hour, then metres/feet display starts flashing, stabilizes for a few minutes, flashes again, and after an hour or so longer, loses all numbers (dashes). It is NOT weed on the transducer as the manual suggests for this symptom. At this stage (after 1.5 hrs), turning off then on gives no numbers even, only dashes.

Model: Raymarine ST40. I have swapped out the display with a borrowed new model - same display...but I get the exact same problem.
I CUT the sensor cable near the thru-hull sensor to replace cable. The old cable passed under engine and was very stiff and thus I thought damaged/resistance high/or??. But a new cable I spliced to 30 cm of remaining old cable at the sensor gives same problem. I tested volts at display. Volts seem fine (>12.3 rest/>13.4 engine running).

My splice is a test splice - twisted wires/taped plus tin foil to complete the shield over the gap. I have not tried a full solder but the connection at rest should be sufficient to test?? The new (and old which worked for years) cable is about 12 ft/4 m long.

Q. What gives such a symptom of working then becoming faulty after running for +1 hour? Does resistance/heat build up? Piezoelectric issues? Do you think it must simply be the thru-hull sensor that has failed? The time-frame before failure is interesting.
I have done the self-tests as described in the service manual. It passed all incl. the transducer stage:
http://cirspb.ru/pdf/ST40%20Instrume...e%20Manual.pdf
I have not hauled out to grind out the old thru-hull transducer to fit a new one - I am trying to avoid/delay that as long as possible as it will be a major.

[boatbod]

Weak transducer that needs to be replaced.

Want proof? Obtain a new transducer and temporarily wire it to the display. Lower the transducer over the side of the boat so it is just submerged. Should give good readings.

[jackt]

Thanks Boatbod. Yeah, I suppose so...but do you think it still could be the connection to the new wire where I cut it? The last check I need to do is to solder or at least crimp-connect the wires. Could a twisted wire splice be enough to somehow create resistance - esp. as it happens after an hour...what could be going on electronsonically after an hour??

[jackt]

Originally Posted by boatbod

Weak transducer that needs to be replaced.

Want proof? Obtain a new transducer and temporarily wire it to the display. Lower the transducer over the side of the boat so it is just submerged. Should give good readings.

I actually did try and hook up a transom sender 9a spare I carry) and sure, it worked. I really do not want to have to haul out and fiddle with holes and glue and steel...

[boatbod]

Originally Posted by jackt

Thanks Boatbod. Yeah, I suppose so...but do you think it still could be the connection to the new wire where I cut it? The last check I need to do is to solder or at least crimp-connect the wires. Could a twisted wire splice be enough to somehow create resistance - esp. as it happens after an hour...what could be going on electronsonically after an hour??

The splice either works or not, but what you are seeing is a weak signal from a dying transducer. It happens... I've replaced more of them than I care to remember.

[jackt]

Any thoughts, anyone, on the time course - all okay for an hour then it loses the signal. Is that an issue of heat...or mechanics...what is it that's dying? Elan vital??

[jackt]

ps. The thru-hull sensor is 20+ years old - "Autohelm" brand I guess as that was the depth display unit brand that I also had to replace with the equivalent Raymarine ST40 - display replaced a decade ago.

[boatbod]

Originally Posted by jackt

Any thoughts, anyone, on the time course - all okay for an hour then it loses the signal. Is that an issue of heat...or mechanics...what is it that's dying? Elan vital??

Think of the transducer as a sensitive ear. As it ages it gets less sensitive, rather like humans loose their hearing as they get older.

Why does it work for 1hr? Who knows - probably because it's right on the edge of it's operational envelope - unfortunately the fix is still to replace the darned thing.