Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
What transducer do you have, working with the Garmin, and has it ever worked in your experience (I realize the boat is second-hand, so maybe it never has worked in your experience) ?
(With that information, I ought to be able to give you a good answer to this question. Please Reply letting me know. Thanks.)
The capability of the transducer for sonar, depends upon whether it also has a (big, black, squared-off) CHIRP sonar module in the circuit. (Otherwise known as a Lowrance SonarHub, or similar device/box.)
Actually, on further research, it may be this GPSMAP doesn't need a 'black box' to get CHIRP sonar, with this transducer.
The only question in my mind is, does a Lowrance transducer work well with a Garmin device? In this case there's no reason to believe it *doesn't* - yet - since you do get some readings of some sort even now.
It's more likely that the connection has become slightly dirtied. Corrosion is a constant enemy of electronics and metals in any maritime or water-going context, especially in salt-water or brackish waters.
Trace the connection from the Garmin to the transducer (transom-mounted). See if the wires/cables are at all damaged, severely bent, or even slightly cut, anywhere along the entire length. (And see whether there's a 'black box' involved, en-route.)
Power Off the system.
Then disconnect the connections to the transducer from the Garmin, one at a time only! and scrub both sides of each connection with a clean dry brush for 30 seconds or so, then wipe down both sides with a clean dry rag cloth until the metal shines, and re-connect firmly, then on to the next connector, and proceed similarly.
(Also check the transom mount of the Airmar transducer.)
Then power On the system, and try it again.
Not necessarily needed, no. The Garmin model you have is 'independently CHIRP capable'.
Let me know how it goes. Thanks.
Did you try the scrub-with-brush and wipe-with-cloth on the connectors for the cable to the transducer, also?
If you did and it still obstinately doesn't work, then it could be a defective transducer. Taking it to a shop (where they can test it by putting it into a busket of water, to some extent, hooked up to another device or display) and having it tested would make sure of the diagnosis.
If you need recommendations or listings to a local shop that could do testing on it, let me know your local zip code please.