Garmin Fishfinder 120 Installation Instructions

Installation Instructions

Please read through these instructions thoroughly before attempting installation. Make sure 

you completely understand these instructions before you begin. When in doubt, seek profes-

sional assistance. Be sure the wiring harness reaches the unit and transducer location before 

beginning installation. The Garmin Fishfi nder 120 hardware allows for installation on either 

the transom or trolling motor. Please check the packing list below. If you are missing any items, 

please contact your Garmin dealer.

Fishfi nder 120

Selecting a Transom Mount Location

For your sonar to operate properly, the transducer has to be located in clean (non-

turbulent) water. The transducer should be mounted as near the center of the boat as 

possible.

DO NOT

 cut the transducer lead, this will void your warranty and may cause the 

unit to not operate at optimal levels. 

DO NOT

 mount the transducer behind strakes, rivet lines, struts, fi ttings, water 

intake, discharge ports, eroding paint, or anything that creates air bubbles or causes the 

water to become turbulent. It is important that the transducer be as close to the center 

line as possible in calm, non-turbulent water for optimal performance. Avoid mounting 

the transducer in locations where the boat may be supported during launching, haul-

ing, trailering, or storage.

Mount the transducer away from the path of the prop on single drive boats. On 

twin drive boats, mount the transducer between the drives if possible. Do not mount 

the transducer directly in the path of the prop. The transducer can cause cavitation that 

may degrade the boat’s performance and damage the prop.

Testing the Transom Mount Installation

This test is to be performed after the unit installation. See unit operating instruc-

tions for additional use information. Since water is necessary to carry the sonar signal, 

the Fishfi nder 120 does not function properly with the transducer out of the water. 

When you place your boat in the water CHECK FOR LEAKS around the screw holes 

that are below the water line. DO NOT leave your boat in the water for an extended 

period of time without checking for leaks. 

1.   Begin testing the installation at a slow speed. If the sonar appears to be working 

properly gradually increase the boat’s speed observing the sonar’s operation. If the 

sonar signal suddenly is lost or the bottom return is severely degraded, note the 

speed at which this occurs.

2.  Return the boat to the speed the signal was lost. Make moderate turns in both 

directions and see if the signal improves.

3.  If the signal strength improves while turning, adjust the transducer so that it 

extends another 1/8" below the transom of the boat. It may take several adjust-

ments to eliminate the degradation.

4.   If the signal does not improve it may be necessary to move the transducer to a 

different location.

Mounting the Transducer On a Trolling Motor

1.   Slide the large Cable Tie through slot on the Transducer Mount with the ridges of 

the band facing up until equal lengths extend on both sides of the mount. (For 

cold water or heavy timber/debris areas, a metal 4-5” worm gear clamp is recom-

mended.)

2.  Position the Mount Gasket on the curved top of the Mount.

3.   Place the transducer assembly against the motor body of the trolling motor with the 

front of the transducer pointed away from the trolling motor propeller. 

4.   Place the pointed end of Cable Tie through the fastener hole on the 

opposite end and pull through (you will hear clicks) until snug.

5.  Position the trans duc er so that it is parallel with the bottom 

when in use. Make sure the Gasket is aligned properly. Pull 

the Cable Tie end until tight. Excess may be trimmed off if 

needed. Tighten the 10-32 locking nut until it touches the 

mounting bracket and then tighten 1/4 turn more (do 

not overtighten).

6.   Route the 20’ (6.1 m) transducer cable using 

the supplied cable ties to secure the cable 

to the motor shaft. If desired, the forward-

facing portion (except the Cable Tie pocket) 

of the Transducer Mount may be filled with 

sealant to avoid debris accumulation.

Make sure the transducer is below 

water level when the boat is on plane 

at high speed.

Apply marine sealant to all screw 

threads to prevent water from 

seeping into the transom.

Mount the transducer parallel with the 

water line.

Packing List:

A - Fishfi nder 120 Sonar Unit (1)

B - Swivel Mount Bracket (1)

C - Swivel Base (1)

D - Mounting Knobs- Short (1), Long (1)

E - Mounting Knob Spacer (1)

F - Transducer With Power Cable (1)

G - Transducer Mount (1)

H - Trolling Motor Mount Gasket (1)

I - 5 mm Flat Washers (2)

J - 5 x 30 mm Screws (2)

K - 10-32 Lock Nut (1)

L - 4 x 12 mm Screws (4)

M - 10-32 x 1.75 Screw (1)

N - 1/4” Cable Clamps (2)

O - Plastic Spacer (1)

P - 1/4” Rubber Washer (1)

Q - Cable Tie, 5.6” (4)

R - Cable Tie, 20” (1)

S - Snap-Ring (1)

A

C

D

E

I

J

K

L

M

N

O

P

G

F

Q

H

R

B

(Cable not shown)

Assembling the Transducer 

1.   Insert the Rubber Washer and Plastic Spacer  

in the transducer at the same time. DO NOT  

lubricate the Rubber Washer.

2.  Route the cable towards the back and 

slide the transducer into the mounting bracket.

3.   Place a 5 mm Flat Washer on the 10-32 x  

1.75" screw and insert the screw through

the Mounting Bracket, Spacer, and Rubber 

Washer.

4.   Place the remaining 5 mm Flat Washer on  

the exposed end and install the 10-32

Lock Nut finger tight. The transducer is

tightened further after installation on boat.

Mounting the Transducer On a Transom

Tool List (not included)

 - Drill, 3/8" Wrench or Socket, 5/32" and 1/8" Drill Bits, 

Masking Tape, #2 Phillips Screw Driver, Marine Sealant

1.   Position the transom mount at the selected location, making sure the transducer is 

parallel with the water line. Mark the center locations of each hole on the transom 

mount. (See figures to the right.)

2.   Using the 5/32" bit, drill the pilot holes approximately 1" (25 mm) deep at the 

marked lo ca tions. To avoid drilling the holes too deep, wrap a piece of tape around 

the bit 1" from the point of the bit. 

3.   Apply Marine Sealant to the 5 x 30 mm screws. Attach the transducer to the transom 

using the 5 x 30 mm screws. Adjust the transducer to extend beyond the bottom 

of the transom approximately 1/8" (3 mm) on fiberglass hulls or 3/8" (10 mm) on 

aluminum hulls. Adjust the transducer to be aligned parallel with the water.

5.   Tighten the 10-32 locking nut until it touches the mounting bracket and then 

tighten 1/4 turn more (do not overtighten).

6.   Place the first cable clamp on the transducer cable approximately one third of the 

distance between the transducer and the top of the transom. Mark the location. 

Using a 1/8" bit, drill a pilot hole approximately 3/8" (10 mm) deep.

7.   Attach the cable clamp using a 4 x 12 mm screw. Coat the screw with marine seal-

ant before installation. Repeat steps 6 and 7 using the other cable clamp. 

8.  Route the transducer cable as needed to the sonar unit. DO NOT CUT THE CABLE. 

Try to avoid routing the cable with electrical wires or other sources of electrical 

interference.

OK

Level

Drill Pilot Holes Here

V

ertical

Bottom of T

ransom

Align with transom 

bottom. Transducer should 

extend 1/8” below fiber-

glass hulls or 3/8” below 

aluminum hulls.

Keep Parallel With Water Line

Installing the Unit On the Mounting Bracket

1.   Align the slot on the back of the unit with the long mounting knob and slide in 

place. It may be necessary to adjust the long knob to spread the bracket arms 

apart. (Turn counter-clockwise to widen the bracket arms, clockwise to tighten.)

2.   Adjust the unit angle and tighten the long mounting knob until snug.

3.   Tilt the unit by loosening the long knob on the right side of the bracket assembly.

4.  Rotate the entire bracket by twisting it left or right. (You will hear clicks as you turn 

the bracket.)

5.   Tighten all knobs once the desired viewing angle is obtained.

Unit Installation — Selecting a Proper Location

Choose a location that provides optimal viewing while operating the vessel and 

allows easy access to the unit’s keypad. Select a mounting surface strong enough to 

support the weight of the unit and protect it from excessive vibration or shock. DO 

NOT mount the bracket in a location where the unit is exposed to extreme tempera-

ture conditions. When installing the mounting bracket, be sure to allow room for the 

connection/routing of the power cable.

Mounting the Bracket Assembly

Tool List (not included)

 - Drill, Screwdriver (Phillips or Standard), three #8 pan head 

machine bolts with matching nuts and washers and a 5/32” drill bit, OR three #8 pan 

head self-tapping screws and a 1/16” drill bit

1.   Using the swivel base as a template, mark the location of the three holes which 

are used to secure the bracket to the mounting surface.

2.  If securing the base with machine bolts, drill three 5/32” holes at the locations 

you marked. OR, If securing the base using self-tapping 

screws, drill 1/16” starter holes at the locations you 

marked. Starter holes should generally be no deeper 

than half the screw length.

3.  Secure the swivel base with three bolts or screws. 

DO NOT OVERTIGHTEN.

4.  Place the rest of the mount over the swivel base 

and secure with the short knob. 

Wiring Harness Installation

The Fishfinder 120 comes with a wiring harness that connects the unit to power and 

the transducer with one easy-to-remove connection. If it is necessary to extend the power 

wires, use 22 AWG wire. DO NOT cut the transducer cable. This will void your warranty. 

If your boat has an electrical system, it may be possible to wire the unit directly to an 

unused holder on your current fuse block. If you are using the boat’s fuse block, remove 

the in-line fuse holder supplied with the unit. You may also wire the unit direct to the 

battery. 

CAUTION: The Fishfinder 120 input voltage is 10-18 volts DC.

 DO NOT 

exceed this voltage as this may damage the unit and void the warranty. 

Installing the Wiring Harness:

1.  Determine the polarity of the power source using a Test Light or Volt Meter.

2.   Install the Red (+) wire on the Positive fuse holder or battery terminal.

3.  Install the Black (-) wire on the Negative fuse holder or battery terminal.

4.   Install a 2 amp fuse in the fuse holder (fuse block only).

5.  Align the notches on the cable plug and back of the unit. Insert the cable into the 

connector and turn the lock ring counter-clockwise unit it stops.

Unit Care

Cleaning - 

Clean the unit housing using a cloth lightly dampened with a mild detergent 

solution and then wipe dry. Avoid chemical cleaners and solvents that may damage plas-

tic components. Do not apply cleaner to electrical contacts on the back of the unit.

Storage -

 Do not store the unit where exposure to temperature extremes may occur, as 

permanent damage may result.

Do not mount the transducer behind strakes, 

rivet lines, struts, fittings, water intakes, or 

discharge ports.

S