Hi,

 I'm looking at buying a used boat.  It has a Lowrance Flasher mounted in the console (original quipment) and has a Garmin 168 Sounder mounted off to the side of the console.  The flasher gives an accurate depth reading.  The Garmin was on a page that showed course over ground with a track history, speed, etc. but there was a display on the screen that read "Sonar Off".  The Garmin unit comes with a transom mounted transducer, but it looks like there is a temperature sensor there in its place.  I'm going to take a look at the boat again this weekend to see if I can find the two transducers.  My assumption is that since the units are from different manufacturers, they cannot share the same transducer.  If there are two transducers, wouldn't they interfere or can that be cancelled out with different sonar frequencies?

Any help is appreciated.  Thanks.

You're correct, only same model finders can share a transducer.  The old in-dash flasher style units (either Lowrance or Hummingbird) ran at a completely different frequency than todays LCDs which is the reason you can't use the same transducer.  Have you checked the bilge to look for a thru-hull transducer?  

I have a pair of Garmin 160's on my boat, nice units.  There is a separate wire on the harness for the temp probe if you're using a thru-hull transducer (obviously you can't read water surface temp from inside the hull.)  I wouldn't be surprised if you find the transducers for both the in-dash and the Garmin down in the bottom of the bilge.

If the sonars run the same frequency they can share transducers, brand doesn't matter, but not at the same time, there has to be a A-B switch to switch to which ever sonar is wanted to be used.   Some brand sonars run 200 khz, some 192 khz and some 400 khz and some offer the 50 khz deep water option.   The transducers resonate at the frequency they are built for so at any other frequency they either operate at a reduced effecieny or not at all.

As Cart7 mentioned look inside where the bilge pumps are mounted, they may be shooting through the hull.

Even with seperate transducers, with both units on, you can still get interference between them.  My bow mounted and console mounted Lowrance LCDs do a pretty good job canceling out other signals if another unit is on, but the flasher is totally useless if either of the other sonars on.

For the screen you were refering to for showing speed, couse over ground etc requires an optional sensor, it may have just been refering to there was no sensor connected for that option to work.  

There are things you can do to run two sonars at once without noticeable interference. Assuming one on the trolling motor and one on the transom, I tilt the tranducers no more than maybe 5 degrees away from each other, not affecting depth or fish arches significantly. Just tilting the trolling motor transducer ought to fix the problem most of the time unless pointed back at the transom ducer. Since it rotates on the trolling motor, it will mostly be aimed away. Mount it facing forward with trolling motor aimed forward. Then the only time you would get interference is when trolling the boat backwards.

Another tweak is to use a different ping rate between units. I leave the console unit on ping 100 since I mostly use it when outboarding and usually in deeper water needing higher ping rate at higher than troll speeds. A low ping (50) is all you need in shallow water. The chance of two pings arriving at opposing transducers is lowered. There ought to be a "sweet spot" ping rate for the bow unit.

Still another tweak is reduce bow unit sensitivity lower than console unit. The reduced transmit power won't bounce back as strongly from bow to stern, already poorly reflected back if the bow transducer is aimed slightly away from the stern. The beam is shot against bottom at an angle, cutting power of any bounced pulse.

Two different brand transducers might operate on the same frequency, but wiring/plug might no be compatible. A 200 kHz gray Lowrance plugged transducer won't work on a 200 kHz blue connector Lowrance unit.

Jim

Many times, you can order replacement connectors for the transducer cable, giving you the abilily to cut one manufactors off and installing someone elses, I've done that many times when a boat had a through hull transducer I didn't want to knock out and install another. You can also cut off a connector and splice a section of cable with a connector from another that will fit your unit, done that many times when there were no replacement connectors available. So, it's very simple to make a transducer connect to any depthfinder you want. You just have to do the research to make sure the transducer you want to use is the same frequency as the unit you want to plug it into.

A Lowrance 1240A console mount flasher with a through hull transducer doesn't give many options for adjusting out interferance. It will even pick up the TM unit when its stored on the bow if I get the gain to high