Post by mwardncsu on Sept 14, 2014 20:56:21 GMT -5
I had been suspecting something was up with my sonar and this past Friday when we were on a big school and Tyler started playing with settings we confirmed it.
I had tucked away this article I found somewhere a while back - looks like I need to break it out and so some reading again and try some of the techniques suggested. I figured I'd post it up here for others and future reference.
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Trolling motor interference
mysite.verizon.net/restmvit/tmintf.html
Trolling Motor Induced Sonar Interference - The Fisherman’s Enigma
Note: This article was written several years ago when most trolling motors used analog technology for motor speed control. The content still applies to today's TMs which use digital speed control technology, however with the advent of the digital systems, the predominate interference mode has changed. Rarely now does the interference enter the sonar unit via the power wiring -- the predominate entry point now is via the transducer, so keep this in mind as you read the content and decide which steps are likely to be applicable to your case.
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 to the world of TM/sonar interference! This condition has plagued and continues to plague many thousands of fishermen. 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. TMs with digital speed control circuitry tend to produce less interference, but can also be affected. 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 a 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 thebaseline 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 steps 1 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 14 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. Purchase a suitably sized Ferrite Data Line Filter These are available at many electronics store including Radio Shack and Fry's). There are several models that vary by the size of the cable that they will accommodate. 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).
5. Locate the point where your sonar power wiring is connected to the boat wiring. At the negative (-) connection, splice in a piece of #14 AWG stranded wire of sufficient length to reach the TM motor housing. Attach the wire to the TM housing in the manner described in #4.
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.
TRANSDUCER CABLE FILTERS Some have reported success with eliminating the interference by installing one or filters on the transducer cable. Some methods of installing filters on the transducer cable:
A. Arrange any slack in the transducer cable into a coil ~8" in diameter. Twist the coil to form a figure 8. Fold the top loop over the bottom loop and secure in place with plastic ties.
B. Obtain a toroid coil form from an electronics supply firm (Radio Shack, Fry's, etc.). The inside diameter of the toroid form should be large enough to allow the transducer cable connector to pass through in order to wrap the cable around the form. Make 8 - 10 wraps of the cable through the center of the toroid form, making sure that each wrap is tight against the previous one. Secure the cable to the form with plastic ties.
C. Obtain a short piece (1' or so) of 1" PVC pipe. Attach the transducer cable to the pipe with plastic ties. Wrap cable slack around the pipe, making sure each successive wrap is placed tightly against the previous one. Make at least 8 - 10 wraps around the pipe and then secure the other end of the cable to the pipe so that the coils remain tightly wrapped.
D. Install one or more of the previously mentioned ferrite data line filters on the transducer cable.
If you do one of these steps be sure to compare your sonar's detailed images with what was possible before installation of the filter. This is necessary because there is the possibility that the filter will also attenuate the sonar signal resulting in poor sonar performance. One way to do a before/after comparison:
1. Attach a suitable anchor to enough rope to reach bottom in 25 - 30 feet of water.
2. Attach two empty/sealed plastic bottles to the rope; one ~2 ft. above the anchor, another ~4 ft. above the first.
3. In a clear area in 25 - 30 feet of water, deploy the anchor, attaching a buoy or other flotation device to the rope so that the rope section going down to the anchor is vertical.
4. Put your sonar into manual sensitivity mode, and make several passes beside the buoy. Lower the sonar sensitivity until the two bottles barely show on the screen. Now make another pass or two, increasing the sonar sensitivity until the two bottles show up very clearly. NOTE THIS SENSITIVITY SETTING!
After installing filters on the transducer cable, repeat steps 1 through 4 above. If the sensitivity level required to obtain a solid image of the two bottles is HIGHER than previously, then the filters are also affecting the transducer signals.
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.
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.
I had tucked away this article I found somewhere a while back - looks like I need to break it out and so some reading again and try some of the techniques suggested. I figured I'd post it up here for others and future reference.
-----------------------
Trolling motor interference
mysite.verizon.net/restmvit/tmintf.html
Trolling Motor Induced Sonar Interference - The Fisherman’s Enigma
Note: This article was written several years ago when most trolling motors used analog technology for motor speed control. The content still applies to today's TMs which use digital speed control technology, however with the advent of the digital systems, the predominate interference mode has changed. Rarely now does the interference enter the sonar unit via the power wiring -- the predominate entry point now is via the transducer, so keep this in mind as you read the content and decide which steps are likely to be applicable to your case.
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 to the world of TM/sonar interference! This condition has plagued and continues to plague many thousands of fishermen. 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. TMs with digital speed control circuitry tend to produce less interference, but can also be affected. 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 a 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 thebaseline 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 steps 1 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 14 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. Purchase a suitably sized Ferrite Data Line Filter These are available at many electronics store including Radio Shack and Fry's). There are several models that vary by the size of the cable that they will accommodate. 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).
5. Locate the point where your sonar power wiring is connected to the boat wiring. At the negative (-) connection, splice in a piece of #14 AWG stranded wire of sufficient length to reach the TM motor housing. Attach the wire to the TM housing in the manner described in #4.
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.
TRANSDUCER CABLE FILTERS Some have reported success with eliminating the interference by installing one or filters on the transducer cable. Some methods of installing filters on the transducer cable:
A. Arrange any slack in the transducer cable into a coil ~8" in diameter. Twist the coil to form a figure 8. Fold the top loop over the bottom loop and secure in place with plastic ties.
B. Obtain a toroid coil form from an electronics supply firm (Radio Shack, Fry's, etc.). The inside diameter of the toroid form should be large enough to allow the transducer cable connector to pass through in order to wrap the cable around the form. Make 8 - 10 wraps of the cable through the center of the toroid form, making sure that each wrap is tight against the previous one. Secure the cable to the form with plastic ties.
C. Obtain a short piece (1' or so) of 1" PVC pipe. Attach the transducer cable to the pipe with plastic ties. Wrap cable slack around the pipe, making sure each successive wrap is placed tightly against the previous one. Make at least 8 - 10 wraps around the pipe and then secure the other end of the cable to the pipe so that the coils remain tightly wrapped.
D. Install one or more of the previously mentioned ferrite data line filters on the transducer cable.
If you do one of these steps be sure to compare your sonar's detailed images with what was possible before installation of the filter. This is necessary because there is the possibility that the filter will also attenuate the sonar signal resulting in poor sonar performance. One way to do a before/after comparison:
1. Attach a suitable anchor to enough rope to reach bottom in 25 - 30 feet of water.
2. Attach two empty/sealed plastic bottles to the rope; one ~2 ft. above the anchor, another ~4 ft. above the first.
3. In a clear area in 25 - 30 feet of water, deploy the anchor, attaching a buoy or other flotation device to the rope so that the rope section going down to the anchor is vertical.
4. Put your sonar into manual sensitivity mode, and make several passes beside the buoy. Lower the sonar sensitivity until the two bottles barely show on the screen. Now make another pass or two, increasing the sonar sensitivity until the two bottles show up very clearly. NOTE THIS SENSITIVITY SETTING!
After installing filters on the transducer cable, repeat steps 1 through 4 above. If the sensitivity level required to obtain a solid image of the two bottles is HIGHER than previously, then the filters are also affecting the transducer signals.
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.
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.
