Cummins Marine Diesel Repower Specialists › Forums › Instrumentation & Electronics › Alternator Electrical field and electrical interference

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  • June 18, 2020 at 5:41 pm #99687

    rconover
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    Over last weekend I noticed that when I started my engine after a long drift that the sounder at low chirp became very noisy and the Garmin xsv quickly dialed down the gain till it cleaned up on auto. I was in approx 1000 ft of water so the gain was reasonably high. The last 2 days I did the following. Upon testing at the dock on a higher gain I could easily reproduce the issue. I also noticed that the higher the rpm the higher the interference. No setting on the sounder, filter, or interference settings made any difference. And its only there on the lower frequency range. The HF end of the transducer is unaffected. The sounder is grounded to the grounding circuit of the boat. Because the interference was only there with the motor running and was rpm dependent I had a pretty good idea where to look. 1st thing I did was hook up the sounder to an external battery. No change – still a bunch on noise and rpm dependent. I then pulled the transducer cable and ran it away from all other cables. Still no change. Next I unhooked the power cable off of the alternator and that did it. Sounder was powered off of the external battery and transducer cable was laying on the deck. Played with it a minute and that was definitely the cause. So there is some electrical field hitting the transducer or the cable. The transducer is a thru hull and only a few feet from the alternator. I asked a local alternator shop and they said that it was possibly a bad diode. Has anyone had any experience with this? Would something like a diode cause a larger than normal electrical field off of the alternator? Unless someone has another suggestion my next step was to pull the alternator and take it in to be checked out. I was thinking about taking some sheet metal and trying to shield the transducer and see if that made a difference as well. Boat is a single engine cummins 6CTA. Electronics are Garmin XSV and trandsducer is a B275LHW. Thanks

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