Help- Engine interference with depthfinder?

[mackeral5]

I know this is mainly a high performance forum but I do know there are some fisherman/speed freaks on here as well. I am having problems with my depthfinder, it is a Lowrance LMS320DF. Engine is an '02 Yammie 115 V4. The depthfinder doesn't read worth a crap, loses bottom at 40' or so, no matter what settings I use on surface clarity and noise rejection. It tends to change with RPM. for example, at a standstill in neutral, 8' of water, as I raise RPM's it will progressively start giving false depth readings all the way up to 120+feet. Does the same thing at speed once I get in 15+feet of water. I'm about to replace the 'ducer with a single frequency 200mhz model as I really don't need a dual 50/200 for the fishing I normally do. Also, even with the engine off it won't read bottom in more than 40' and shows very little fish, structure, etc. I've heard that the charging/ignition systems can cause this as well and was interested in the board's opinion prior to putting $$$$ in any direction.

I know many of the fishing boards will have ideas but i did want to see if anyone here thought the charging/ignition systems could be causing my probs.

Thanks in advance.

[2springers26]

RFI interferance is a issue with mercs , fishfinders & ign components... the old plugs for some modles were bp8hs-10 now they are bpz8hs-10.. the z plug is for rfi interferance...

[150aintenuff]

do this test... take a volt meter and ground it to the hull (NOT THE BATTERIES) and drop the positive lead in the water.. any reading over .200 volts indicates a bad grounding system and can cause magnetic interference with a transducer as well as scare off and repell fish. the fact that you cant hold bottom in greater than 40' even with engine off tells me you have a grounding issue not just a RFI suppression problem. set the finder in fully automatic mode and note the sensitivity level that it reads the bottom at.. should maintain bottom echo to around 1/3 of possible sensitivity max and at about 90-95% sensitivity max it should begin to fill screan with surface clutter and by max the clutter should extend to near the bottom but still have a defined bottom line below it with all filters turned off.. if you cannot do that with your current unit there is powwer noise and grounding issues as well as RFI comming from the engine.. solve the grounding problems and RFI will be little to zero noticible on screen. BTW most fiberglass hulls will leak out up to .150V on average with a good grounding system.. aluminum hulls are slightly higher with .175-.250V being IDEAL due to more metal surface in the water for the water to react to.

[Todd]

Is the transducer shooting through the hull or outside on the back. If you are shooting through the hull, make sure it is on the bottom floor and not on the top. Needs a good epoxy if through the hull with no air bubbles trapped (push it down hard when mounting).

If it does this with the engine off, I don't think the charging system is the problem. If you have a 2nd battery for troll motor or radio.., hook it up to that and try or just un hook one of the battery feeds to the engine when not running.

Has it ever worked correctly or is this a new install?

[150aintenuff]

never stated charging system is the problem... just grounding... poor grounding can cause magnetic feilds to build in wire which give off their own interference and can blank out a transducers feild of view... cancel the sound wave with any other wave of like or oposite frequency ... no bottom line... everything from batteries to transducer mounting has to be double checked and not just on the engine or unit..

[mackeral5]

Thanks for the input. I'll give the suggestions a try this weekend. When I originally installed the unit I connected it straight to the batteries as I didn't see the nice fuse and ground distribution blocks mounted high in the console, you have to lay on your back and look up to see them. I rewired the power/ground as it should have been done originally and no change. I'll give the other suggestions a try this weekend.

This sounder has never really read right and it could be related to the items mentioned here. I'll find out this weekend and post results.

[mackeral5]

I forgot to answer the questions about the transducer. It's not a thru hull. It's the 50/200 skimmer hung off the transom. I have it mounted where the bottom of the boat is approximately in line with the middle of the transducer. The unit does the same crap at rest as it does when moving, with regards to not holding bottom and showing fish very well.