Electrical Interference with Depth Finder

[lancelot9898]

I have an old 25 year old Impulse digital depth finder at the helm with the transducer installed in a thru hull fitting. It's worked fine for the past 24 years, but recently it gives bad readings whenever the engine is running. Under sail it still does fine so there must be some degradation in the transducer cable as it passes through the engine space. Is there a simple and easy fix such as wrapping the cable with some shielding as it goes through the space? Replaceing that cable would not be easy since it's difficult to access on both ends plus not sure if even possibe since the transducer end in potted in epoxy.

[heyniceguy]

wrapping the cable in metal sheilding/braiding is only the first step to protecting the transducer signal from EMI. step 2 is to connect one end (and only one end) to a grounding source, preferably at the helm rather than at the transducer.

but if you're running in your engine compartment, then it's likely the EMI is coming from your alternator. in order to generate electricity, the alternator creates a very strong electromagnetic field to induce current in the secondary windings. and that field dissipates the further you get away from the alternator.

another source of EMI can be high-power lines that are running parallel to your transducer line over a continuous length in close proximity. thats why in home install wiring, LAN cables only cross 120VAC lines at right angles and are never run in the same raceway. if you ran them together, the 120VAC would induce noise in the LAN line.

that said, i would try first to relocate the transducer line to another part of the engine compartment and, if possible, not in a bundle with other wires.