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Author Topic:   SONAR: Faulty Depth Readings
montaukman posted 06-22-2006 10:49 PM ET (US)   Profile for montaukman   Send Email to montaukman  
Hi. Depthfinder and tranducer guru's help me out. I have a Garmin 178C GPS chartplotter on my 170 MONTAUK. I had it installed last year when the boat was new. I kept the boat on a trailer all last year and never had any problems with the unit. This year I am keeping the 170 MONTAUK in a slip. I notice, that when I go about 10-MPH, about half the time I get false readings on the depth gauge. It either shows two or three feet of water when I know there is over 20-feet of water, or it just shows a blank area for where the depth should be. I also lose the contour of the bottom. If I go 5-MPH or less the depthfinder shows [the correct depth], and, if I get above 10-MPH, it also clears up. What causes this?

I did notice that there was some growth on the transducer, and I rubbed as much off as I could. But it still didn't seem to make it work right. The manual said to possibly adjust the gain to a lower setting, and that it could be cavitation causing this. Why would it start all of a sudden? The only difference between this and last year was the bottom paint. Could the angle of the transducer have changed when they painted the boat? Would that matter?

Any ideas? Is my next step a call to Garmin? How do I clean the transducer without scratching it or damaging it?

Thanks for any help you can offer

All the best,


jimh posted 06-22-2006 11:22 PM ET (US)     Profile for jimh  Send Email to jimh     
Check the orientation of the transducer. Perhaps it has been bumped.
Livingwater posted 06-23-2006 01:47 PM ET (US)     Profile for Livingwater  Send Email to Livingwater     
Make sure that it didn't get hit and flipped up alittle out of the bracket. Some transducer brands are made to flip up if it comes in contact with a solid force and/or it gets loose and the high speed knocks it up alittle.
bsmotril posted 06-23-2006 04:00 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
First check the angle of the transducer as others here have said. If that looks good, then try this. Some transducers have a hygroscopic coating that must be well wet for it work. Put some dish detergent on your fingers and then rub it over the transducer face before launching. That will wet out the surface completely and eliminate air bubbles. The old Heathkit neon flasher units were pratically un-usable unless you tried this trick. Learned it from the good old Heathkit manual. BillS
Hoosier posted 06-23-2006 08:14 PM ET (US)     Profile for Hoosier  Send Email to Hoosier     
I have a similar problem with a Garmin 240. One thing I noticed
this weekend was that the Auto Range didn't work. If I set the depth range
manually I could get the unit to track the bottom but the digital depth
readout would not lock at any depth more that 60'. When I put it in Auto the scale went to 1000' and the bottom trace started coming in at 120'. In a previous life I was a sonar engineer, this looks like the echo is too weak to trigger the auto detect circuit, ergo, either a bad power amp or a bad transducer. The unit is out of warranty but Garmin sent me a new transducer to "try out" (and keep). Hmmmm.....
The unit worked fine last year, but after wintering over in the Upper Peninsula, it doesn't work correctly. I wonder if freezing the transducer can break it. That would be a big design defect.
crabby posted 06-25-2006 11:49 PM ET (US)     Profile for crabby  Send Email to crabby     
Did they paint the transducer with regular bottom paint? If so that could be your problem.
montaukman posted 06-27-2006 11:26 PM ET (US)     Profile for montaukman  Send Email to montaukman     
Hi Everyone,

Thanks for your help.

I called Garmin about this and was told that it could possibly be angled wrong, or there may be algae interfering with the reading, and that I should switch the frequency to the less deep frequency...not sure if it is 50 or 200.

They also suggested I lower the gain and slow down the reading speed. I think it is set to auto now.

Other than that, they weren't sure. I will try a few things and see what happens. it was not painted nor was it bumped out of position. I saw no crack either. My gut tells me a cleaning may remedy it.

Hopefully i'll check it out by the 4th of July. Rains are threatening here again.....never saw so much rain. Too bad BW doesn't have an ark kit.

Thanks again for the help,


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