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You step on the button to activate your trolling motor (TM), then your LCD
sonar begins to draw vertical black lines on the screen, obscuring all else or in really bad cases, goes completely blank!
Welcome friend, to the world of TM/sonar interference! This condition has plagued and continues to plague many thousands of
fisherman. In this article, I will attempt to explain the causes of this condition and some steps that can be taken to reduce or eliminate it.
This is a complex problem and there is
not an easy cure-all that will correct the problem. Often it takes a combination
of things, and even then, in many cases success is measured in terms of reducing it to a tolerable level rather than total
elimination.
What is Affected, What is Not
If
you have this problem, I probably can deduce something about your setup, without even seeing it.
1. The trolling
motor: Your trolling motor probably is one that uses pulse modulation technology to control motor speed. MotorGuide’s
name for this is DuraAmp; MinnKota’s is Maximizer. It features a speed control selector that enables the
motor speed to be selected at any number of points between off and full on,
e.g., there are no pre-set speed selections. Trolling motors that do not use pulse modulation technology to control motor
speed differ in that the speed selector has several (usually 4 or 5) preset speed positions to select from and normally
are not sonar interference sources.
2. The sonar unit: Your LCD sonar unit is probably one with good sensitivity,
easily capable of detecting the weaker sonar echoes represented by things such as weeds, brush, bait fish, etc., that are
positioned between the surface and the bottom. It very likely is a medium to high end unit as many low-end units do not have the sensitivity that causes the interference to become a evident.
I
have or know of a rig that has the types of TM and Sonar discussed above, but it does not exhibit the interference problem.
Why is that?
Many sonar manufacturers incorporate Automatic operating modes in their
products. These units, especially the higher end models, can detect the presence of the interference and compensate for it
by automatically adjusting the unit’s sensitivity settings (lower them) and noise filter settings (increase them) as
necessary to maintain a normal display on the LCD screen. Unfortunately, these adjustments may cause the unit to be unable
to detect weaker sonar signals such as weeds, brush or bait fish that exist between the surface and the bottom. Once the interference
is eliminated (by stepping off of the TM button), the sonar senses that change too, and again adjusts the sensitivity and
noise filter settings to match the non-interference conditions. The result is that a fisherman using his sonar unit solely
in the Automatic mode may never notice the problem, but likely is
missing detail information his sonar is otherwise capable of displaying during the time the TM is activated and until the
unit detects that the interference has lessened and readjusts the settings. (Note: The settings employed by the Automatic
mode itself tend to obscure some detail from weaker sonar echoes, but that issue should probably be the topic of another discussion.)
It is also possible that the rig in question uses one of the newer
digital TM systems that are said to eliminate/reduce the interference problem. My current rig has a MotorGuide Tour
82 TM that utilizes the digital control board. Since acquiring it I have not had a single bit of interference show on
my sonar screen. Another possibility is that you simply have been lucky enough to have a combination of TM, sonar and boat
wiring installation that doesn't exhibit the problem (a condition periodically reported and difficult to analyze without seeing
first-hand). If either of these be the case, congratulations to you!
The
Source of the Interference
The TM’s pulse modulation speed control system is the source of the problem!
The pulse modulation speed control system uses electronics to switch the DC battery voltage On and Off many times per second,
creating a waveform that would look like a square wave (e.g., a pulse) if viewed on an oscilloscope. Motor speed variation
is accomplished by electronically controlling the width of the pulses and/or the relative space between them. For example, wider pulses/narrower relative spacing = higher speed; narrower pulses/wider
relative spacing = slower speed.
As a basic concept of electronics, if the makeup of a square-topped pulse is analyzed,
it will be found to contain a whole range frequencies, many of which are in the range that will interfere with the sonar
unit’s operation. These interfering frequencies are present in/on
the TM’s wiring, electronics and motor and can be electronically induced into any other wiring that is nearby; be it
boat wiring or sonar wiring.
Modes of Introduction
There
are basically two ways in which the interfering frequencies present in the TM can find their way into the sonar unit.
These are:
1. It may be electronically induced into nearby boat or sonar wiring and enter the sonar through its
power wiring.
2. The sonar’s transducer and cable
may act much like an antenna and receive the interfering frequencies via the transducer cable.
For any given case of TM/sonar interference, either one or both of these modes may be a source.
What Can be Done About the Problem?
Now comes the discussion that is probably your reason for reading this! Note that I am
writing this piece from the perspective of an Eagle/Lowrance user. Your sonar unit may use different terminology for the controls/settings
I will mention. Instructions are also geared toward the most commonly encountered boat wiring/TM wiring configurations. Feel
free to e-mail me at zzclwzz2613olzz@verizon.net if you encounter difficulties and I will attempt to assist and/or help relate my terms and instructions to your situation.
Note: The e-mail address has been encoded to help confound the spammers. Simply remove all the z's; the remainder is the
real address.
Set a baseline! Before attempting fixes to address the problem, I would highly recommend
that a baseline be established so that the impact of each fix can be evaluated. Here are my suggested steps for establishing
the baseline
1. Take your rig to the lake and motor out to
a depth of 20 feet or more, then tie up or anchor and shut off the outboard and all other electronics except your sonar. Lower
your TM into its running position.
2. Place your sonar unit into Manual
mode.
3. Locate the noise filter setting control (usually via
on-screen menu) and disable noise filtering and Surface Clarity Control (if a separate control). If your sonar will not allow
it to be completely disabled, select its lowest possible setting.
4. Note
the current sensitivity/gain setting (again usually via on-screen menu).
5. Set
the TM speed control on a slow speed and momentarily activate it.
6. Is
there interference on the sonar screen? If yes, do step 7; if no, do step 8.
7. While remaining in Manual
mode, decrease the sonar’s sensitivity/gain setting. Repeat this step until the interference seen when activating
the TM barely shows on the screen. Record this setting, identifying it as the interference ON-SET value!
8.
While remaining in Manual mode, increase the sonar’s sensitivity/gain setting. Repeat this step until
the interference seen when activating the TM barely begins to show on the screen. Record this setting, identifying it
as the interference ON-SET value!
Determine which modes are causing your problem: I also recommend
doing some preliminary investigation to determine which of the modes discussed above are causing the problem. If you are lucky,
one mode may be predominant and as a result, it may not be necessary to incorporate fixes for the other mode. If you’re
not lucky, both modes will be contributing significantly and most all of the fixes to be discussed will be needed. These steps
require some work and preparation, and ideally require temporary access to a second transducer and power cable as well as
a small 12V battery (such as used in a lawn tractor or lawn mower). These steps also require on-the-water evaluation, ideally
the same location that the baseline previously discussed was determined.
1. Connect the power cable of your sonar unit directly to the terminals of a small 12 volt battery (DO NOT
connect through the boat’s wiring as this may cause an invalid conclusion to be reached). This is where a spare
power cable is most handy! Once connected, turn on your sonar unit and set it up as per the Set a Baseline discussion.
Repeat steps 2 through 8 of the Set a Baseline discussion. Is the
sensitivity/gain point at which the interference first begins now higher than the baseline setting noted? If yes, then the
difference is due to interference entering your sonar by way of its power wiring.
2. Restore your sonar’s power wiring to its normal configuration by disconnecting it from the small
12 volt battery and reattaching it to its normal point in your boat setup.
3. Connect a temporary transducer
to your sonar unit and position it so that it can be placed in the water somewhere toward the stern of the boat (Note: It
is not necessary to hold this transducer in a normal orientation, all that is required is that it be in the water, preferably
with a foot or so of cable under water). Rout the transducer cable to the stern freely on top of the deck, staying as far
removed as possible from the location of other boat wiring, especially the TM wires. Repeat steps 2 through 8 of the Set
a Baseline discussion. Is the sensitivity/gain point at which the interference
first begins higher than the baseline setting noted? If yes, then the difference is due to interference entering your sonar
by way of its transducer cable.
4. Restore the sonar’s
transducer installation to its normal configuration
If you noticed significant differences in the interference ON-SET
sensitivity/gain setting versus the baseline from both steps1
and 3 then your sonar is affected by interference in both of the modes!
The Fixes
The following
fixes will help to address the interference problem. I will attempt to list them in order of application priority. Some have
claimed success in eliminating the problem at the first step. Others have had to use multiple , or all steps, to see significant
results. For that reason, my recommendation is not to take a shotgun approach doing all of the steps at once. Instead,
I would suggest doing them in order, repeating the baseline tests frequently in order to determine the effect of the
individual and cumulative fixes installed.
Fixes directed at
power wiring introduced interference:
1. Make sure your sonar power is NOT being obtained from one of your TM batteries! The sonar unit should normally be powered from the boat's
cranking battery.
2. Look at your sonar’s wiring installation.
Attempt to arrange/rearrange and secure it so as to keep it as far removed as physically possible from TM wiring.
3.
Install new power wiring for your sonar unit. Wiring should be 16 AWG, twisted pair, if possible. A shielded, twisted
pair can also be used and may be beneficial. Connect this wiring directly to your cranking battery; being sure that the red
(+) lead is connected through an in-line fuse of a value required by your sonar’s manufacturer. Connect the black wire
to the battery (-) terminal. If shielded wire is used, also terminate the shield and connect it to the battery black (-) terminal.
Run the wire to the bow using a route that keeps it as far removed as possible from TM wiring and other boat wiring (in many boats, a route through the center bottom section of the boat, under the flooring,
is available and is a good choice). Connect your sonar’s power cable to the bow end of the newly installed wiring. If
shielded wire was used, leave the bow end of the shield unconnected. Position the wiring under the bow panel, attempting to
minimize excess wire length while maintaining as much separation as possible from other boat and TM wiring.
4.
Go to Radio Shack and purchase a suitably sized Ferrite Data Line Filter.
There are several models in their inventory that vary by the size of the cable that they will accommodate. I used their part
number 273-105C, but because of its small size had to remove the jacket from the sonar power wire in order to install. Others
have used a larger size and reported suitable results. These filters have a hinged-clasp construction so that they can be
snapped around wiring without disconnecting one end. Install the filter on your sonar’s power wire by opening it and
making 5 turns of the power wire around one side of the filter, then snapping it closed. Choose a filter location that is
as close as practical to the sonar unit.
5. If #4 reduced but
did not eliminate your problem, obtain a second filter and install it on the sonar’s power wire in the same manner.
Position the second filter immediately adjacent to the first one.
6. As
a last resort, if power wiring induced interference remains after step 5, there is one other course of action that I personally
have had good results with. That is to completely isolate your sonar's power wiring from all other boat wiring by installing
a separate battery just to power the sonar. A small lawn tractor (LT) battery works well for this application, however, I
would recommend this only as a last resort because it brings with it issues pertaining to additional weight and charging of
the battery. If a separate battery is used, the ideal location would be somewhere forward, where the connecting power wire
can be as short as possible. Practically, an aft installation is probably best because of the pounding the battery would take
in a forward position. For an aft installation, simply take the wire installed at #3 above and relocate it from the cranking
battery to the LT battery.
Fixes directed at transducer cable induced interference:
1. Make sure your sonar's transducer is
operating correctly. You should not have to turn your sonar's gain/sensitivity control to really high levels in order to obtain
an acceptable display unless you are in really deep water (e.g. > 30 feet). A "weak" transducer can cause you to have to
elevate the gain/sensitivity control in order to get a usable display. The interference problem only becomes visible at high
gain/sensitivity settings so if you can operate acceptably with a lower gain/sensitivity setting you may avoid the problem.
2.
Make sure that your transducer cable IS NOT tied to
the TM's power cable. Instead, route it separately, along the outside of the TM mount. The reason for this is that the interference
present on the TM wiring can be inductively coupled to nearby wiring and the inductive coupling is facilitated by wiring that
is in close proximity and running the same direction.
3. Install a common ground between the boat/cranking system
and the TM system. Assuming a 2-battery, 24 volt TM system, determine which TM battery terminal is the TM system negative.
This is most easily accomplished using a voltmeter. Set the voltmeter to read 24 vdc with the red probe positive and the black
probe negative/common. Using the red voltmeter probe, touch the (+) terminal of one battery. At the same time touch the black
probe to the (-) terminal of the other battery. Observe the voltmeter reading. If 24 to 25 volts, then the battery
terminal touched by the black voltmeter lead is the TM system negative (Note: The TM plug must be installed into its
receptacle during this test). If the voltmeter reading is significantly different than 24 to 25 vdc, move the red voltmeter
probe to the (+) terminal of the other battery, while
also moving the black voltmeter probe to the remaining (-) terminal. The voltmeter should now read 24 to 25 vdc and the battery
terminal being touched by the black voltmeter lead is the TM system negative. Install the common ground by connecting a 14
AWG wire between the TM system negative and the (-) terminal of the cranking battery. The 14 AWG wire should have an in-line
fuse installed (~10 amp) as a protective device in the event of a failure/short in one of the connected systems.
4.
Verify/install grounds of TM components necessary to improve shielding and reduce interference radiation. Do this by using
an ohm-meter to verify a near-zero ohms continuity reading between the various metallic TM components (e.g., motor shaft segments
and motor housings) and the TM system negative. If tests show no continuity between the TM system negative and individual TM components, an external ground wire should be installed. The wire
should be 14 AWG and should be routed from the TM system negative forward to the TM components. Attachment to the TM shaft
can be accomplished using a clamp attached to the shaft segment, then securing the wire end to the clamp with a screw and
locknut (a crimp lug should be used on the wire end). Often there is no continuity between the shaft and the motor housing,
which necessitates continuing the ground wire also down the shaft to the motor housing. Attachment here is usually made by
drilling a small hole in the motor housing skeg, removing the paint from an area around the drilled hole and securing the
wire/lug with a screw and locknut. In all cases, the installed wire should be neatly dressed and secured to the TM cable and
shaft (leaving a rotation loop where necessary).
Note: Prior to
the advent of the "digital" TMs, steps #3 and #4 rarely had any effect for me, however with the digital TMs my experiences
show that these steps will usually eliminate (or drastically improve) the interference issue.
5. I have not personally verified the effectiveness
of this fix, however some have reported a significant reduction in interference by isolating the transducer from the motor
housing by installing a piece of rubber between the transducer mount and the motor housing. Similarly, some also recommend
that the transducer mount be secured to the motor housing with plastic ties rather than the commonly used metal clamp.
6.
If the previous steps fail to eliminate or materially reduce
the problem, consideration should be given to finding an alternate location for the transducer installation. Transducers installed
on the motor housing are in close proximity to the interference source, thus are most susceptible to it. If the user can tolerate
the down side of a shoot-through-hull transducer installation in a position at or aft of the front seat, many boat types will
accommodate it and it will greatly reduce the susceptibility to the interference.
Now a caution! Some individuals
have reported success with eliminating the interference by installing one or more of the Ferrite Data-line Filters also
on the transducer cable. I would encourage anyone who reads this NOT to take that step. I have no doubt that the
filter will prevent the interference from being visible, but being a passive device it cannot distinguish between the
unwanted noise (interference) and the desirable sonar signals on the transducer cable. It will attenuate all within its effective
range by the same amount. This has the equivalent effect that the sonar in Automatic mode provides (e.g., reduced sensitivity).
However, unlike the Automatic mode, it will cause the sonar to be unable to detect and display weaker echoes all
of the time, not just when the interference is present. So - if you can live with the reduced sensitivity during periods
of TM interference; don’t do any of the things discussed here; just operate in Automatic mode and let the sonar unit attempt to compensate!
Conclusion
The fixes discussed here should help to reduce the impact of your TM interference
problem, hopefully to a tolerable level. If really lucky, they may virtually eliminate the problem. If not, be aware that
you are not alone. There are a lot of folks around that have
been unable to find an effective remedy for the problem. At any rate, I hope that the information presented here has been
somewhat informative in terms of understanding the nature of the problem.
Now For a Real Example!
First a caution! Don't let the results I am listing here deter you from doing
your own investigation regarding your situation, hopfully using a structured approach as I have described above. The factors
that contribute to interference can be complex and can vary from case to case, so the results I obtained may or may not apply
in your situation.
My previous application
of the troubleshooting techniques and the "fixes" described above involved a MotorGuide Brute 62 TM and a Lowrance X-85 with
the transducer mounted on the TM. In that case, I found that the interference started showing up whenever the sonar sensitivity
was at 70% or more. I also found that most all of my interference was getting into the sonar via the boat wiring and sonar
power wiring; very little was being picked up by the transducer or its cable. The "fix" that had the most dramatic affect
was the Radio Shack Data Line filter. Installing one of these did not totally eliminate the problem, but reduced it to the
point that I could easily fish water up to 30 ft. in depth with no problem, involving X-85 sensistivity settings up to 90%.
This represents an improvement that rendered the situation tolerable for my application.
In the fall of 2002, I replaced
the Motorguide with a Minnkota Maxxum 74, with the universal built-in transducer. While this setup provided a very clean installation (e.g., no transducer and cable to
install on the trolling motor), I found that the interference had returned and appeared to be as bad as, if not worse than
ever before. This was in spite of the Radio Shack filter being left in place.
The following is the result of subsequent
application of the trouble-shooting steps and “fixes” presented above to the MinnKota installation:
1.
Baseline Test: I observed and recorded the X-85 sensitivity setting at which the interference lines first became evident
on the sonar display (Note: I will hereafter refer to this point as the ON-SET value). For this and all remaining tests,
the X-85 was operated in “Manual” mode, chart range was fixed at 0 - 30 feet (I was anchored
in 23’ of water), ASP was “off”, and SCC (Surface Clarity Control) was at “minimum”.
2. Temporary Battery Test: I disconnected
the X-85 power leads from their normal location and reconnected them to a small twelve volt battery placed on the deck adjacent
to the sonar unit and again observed and recorded the ON-SET value. The X-85 power leads were then reconnected normally.
3. Common Ground Test: When I talked
to MinnKota tech. support about the problem, they said that installation of a common ground between the boat’s wiring
system and the trolling motor’s wiring system would probably fix the problem. Per their instructions, I connected a
ground wire between the negative terminal of the cranking battery and the negative terminal of the trolling motor battery
(e.g., the 24V system’s negative terminal). Again the ON-SET value was observed and recorded.
4.
Grounded TM Housing Test: I then extended the common ground forward and made physical connection to the metal trolling
motor housing (physical connection verified with an ohm-meter.) Once again the ON-SET value was observed and recorded.
5. External Transducer Test: Next,
I removed the MinnKota universal transducer connection and plugged a temporary transducer into the X-85, routing
the transducer cable along the top deck and letting the transducer dangle in the water at the stern of the boat. Once again,
the ON-SET value was observed and recorded.
6.
External Transducer Mounted on TM Test: This time, I attached the temporary transducer to the trolling motor (as would
have to be done without the built-in universal transducer) and routed the cable along the trolling motor shaft. Once again,
the ON-SET value was observed and recorded.
Actual observations for each of these tests are in the following table:
Test Condition |
ON-SET
Value |
Comments |
1. Baseline Test
|
70%
|
Radio Shack Data Line Filter from previous installation
still in place. |
2. Temporary Battery Test |
70%
|
No change vs. Baseline Test indicates existing Data
line Filter is adequate, no interference contribution entering on power wiring.
|
3. Common Ground Test
|
70%
|
MinnKota recommended common ground no help! Left in place for subsequent testing. |
4. Grounded TM Housing Test |
70%
|
Grounding the TM housing doesn’t help either! |
5. External Transducer Test |
Max
|
Max. X-85 gain, no interference. Clearly, transducer
proximity to the TM is the primary interference factor, with the "built-in" transducer being the most interferernce prone! |
6. External Transducer Mounted on TM Test |
87%
|
Transducer mounted on outside of TM clearly much better than built-in transducer, but may still be evident in deeper water, where
sonar sensitivity setting must be increased! |
From my perspective, the table tells it all! What I am dealing
with is a trade-off between the uncluttered, but highly interference-susceptible installation of the built-in transducer versus
the more cluttered but less interference-prone external transducer installation. Further, to completely eliminate the interference,
the transducer would have to be moved to a location that is not in close proximity to the interference source (e.g., the trolling
motor).
Since the grounding schemes (test 3 & test 4) didn’t help, there are no other known alternatives.
A final note: The Grounded TM Motor Housing Test was repeated with the External Transducer Mounted on TM Test;
again with no impact, e.g., the ON-SET values were the same whether the TM housing ground was connected or not!
Credits
The information I have presented here is a compliation from a
variety of sources. Some was gained by contact with manufacturers such as MotorGuide, Lowrance and MinnKota. Some was gained
from technical articles in publications such as Bass & Walleye Boats magazine. Some was gained through dialogue with individuals
across the country who also have found themselves trying to deal with this perplexing problem (much of that contact occurred
on the BFHP pages). Finally, some stems from my "hands-on" experiences as I have tried to deal with the problem on my own
rig. In preparing the article, the objective was to compile the "bits and pieces" of information into one place where it could
be seen and possibly be of use to others "fighting the TM interference wars".
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