Challenge: Depth Sounder Fails, but Only when Engine Is Running

[Cat-a-tonic]

Got a Stowe depth sounder on my 33 ft catamaran (1986) which works fine when sailing but frequently (but not always) goes to default 199 feet when the engine is running.

Tried a 12v regulator on the instrument feed in case it was not tolerating the high charging current (14.8v) - but no change.

Tried an isolated and dedicated 12v battery supply - no change.

The transducer is through hull mounted about 15 feet forward of the instrument whilst the engine, batteries and charging system are all about 6 feet aft of the instrument.

Any constructive suggestions ?

[Boracay]

Is your transducer plastic or bronze?

If bronze, is the body of the transducer grounded?

Is the negative wire from the transducer grounded?

Are any wires from the transducer in poor condition?

[Cat-a-tonic]

I think it's bronze -

transducer not grounded as far as I can tell

Neg wire and screen go to instrument terminal block without grounding unless internally within the instrument.

Transducer wire is fine as far as I have traced it. The ducer ends of the wire are sealed into the ducer.

Any clues?

[AnchorageGuy]

Cat, We had this same issue and it turned out to be the alternator causing the problem. When you did an isolated battery was it still connected to the alternator? If so try to connect to a battery completely isolated from the system. We eventually had to replace the sounder. New one worked fine with no issues.

[starbolin]

Since an isolated supply didn't cure it your problem is noise pickup in the transducer or transducer cable. Either the transducer has developed a problem, the cable has an open or the cable is connected to the unit incorrectly. Replacing the transducer is probably the next step.

A engine that is generating excessive noise could be complicating the problem. Bypass capacitors for the engine coil and the voltage regulator do go bad over time and should be replaced regularly. Most mechanics will change the coil capacitor at tuneup but the bypass for the regulator often never gets changed.

[Wotname]

Many alternators generate a high level of AC on the DC output due to aging (leaky) diodes. Put an AC millivoltmeter on its output, you should see less than 30 mV.
Yes, the battery is meant to filter the AC level but does this less and less as the internal resistance of the battery increase over age.

I would like a dollar for every "overhauled" alternator I have seen still with leaky diodes. A lot of shops just don't check for AC ripple from the diodes, they just check they are not shorted DC wise.

This MIGHT be causing a transducer/sounder problem as others have mentioned.

[Cat-a-tonic]

Thanks for the quick feedback folks.
When I tried the isolated battery test (8 x 1.5v torch batteries) I connected them at the main fuseboard as a convenient and accessible point to re-connect the live and ground wires to the instruments.

Am I right in having the screen and neutral wires from the 'ducer connected together at the instrument head? There are only two terminals available for live and neutral.

I'll check out the alternator output for stray AC ripple when I can beg/borrow/steal an AC millivoltmeter.

I'm obviously loathe to rip out the old transducer until totally convinced that all other possibilities have been eliminated. This is a fixed 'ducer type and not easily pulled.

[cacagolf]

Originally Posted by Wotname

Many alternators generate a high level of AC on the DC output due to aging (leaky) diodes. Put an AC millivoltmeter on its output, you should see less than 30 mV.
Yes, the battery is meant to filter the AC level but does this less and less as the internal resistance of the battery increase over age.

I would like a dollar for every "overhauled" alternator I have seen still with leaky diodes. A lot of shops just don't check for AC ripple from the diodes, they just check they are not shorted DC wise.

This MIGHT be causing a transducer/sounder problem as others have mentioned.

Makes sense, I agree with your point of view, thank you for sharing, good luck to you!

[LifeIsAReach]

Originally Posted by Cat-a-tonic

Thanks for the quick feedback folks.
When I tried the isolated battery test (8 x 1.5v torch batteries) I connected them at the main fuseboard as a convenient and accessible point to re-connect the live and ground wires to the instruments.

Am I right in having the screen and neutral wires from the 'ducer connected together at the instrument head? There are only two terminals available for live and neutral.

I'll check out the alternator output for stray AC ripple when I can beg/borrow/steal an AC millivoltmeter.

I'm obviously loathe to rip out the old transducer until totally convinced that all other possibilities have been eliminated. This is a fixed 'ducer type and not easily pulled.

Most likely it is AC noise from the alternator as others have noted. This is not necessarily on the DC output, it could be RFI. Try adding ferrites to the transducer cable; this might solve your problem. Re-routing the transducer cable, if possible, could also help. Lastly, a Faraday Cage around the engine would fix it for sure!