Garmin GPS 128 Service Manual

The Garmin GPS 128 is a marine GPS receiver featuring a waterproof case, high-sensitivity GPS receiver, NMEA 0183 interface for connection to autopilots and chartplotters, built-in worldwide basemap, waypoint and route storage, track log recording, and external antenna capability. This service manual provides technical information for installation, configuration, operation, maintenance, and troubleshooting. Below are key sections covering features, installation, operation, specifications, and service procedures.

Features at a Glance

Key components: Waterproof housing (IPX7), LCD display, front-panel buttons, external antenna port, power/data cable interface, internal GPS receiver.

Installation

Unpack and inspect contents. Select mounting location with clear view of sky.

1. Mounting: Use included bracket or optional flush mount kit. Ensure surface is flat and secure.
2. Power Connection: Connect red wire to positive (+), black wire to negative (-) of 10-40 VDC source.
3. Antenna: For best reception, mount external antenna (if used) away from obstructions.
4. NMEA Connection: Connect NMEA 0183 output wires to compatible chartplotter or autopilot.
5. Sealing: Ensure O-ring is properly seated before closing case for waterproof integrity.

WARNING! Ensure power is off during installation. Use appropriate gauge wire for power connections. Do not overtighten mounting hardware.

Initial Setup & Configuration

Apply power; unit will begin satellite acquisition. Follow on-screen prompts.

1. First Fix: Allow several minutes for initial GPS satellite acquisition outdoors.
2. Set Units: Navigate to SETUP menu to select nautical miles/statute miles/kilometers, feet/fathoms/meters.
3. Set Time: Configure time format (12/24 hr), time zone, and daylight saving.
4. Configure NMEA: Enable desired NMEA 0183 sentences (GGA, GLL, RMC, etc.) and set baud rate (4800 default).
5. Calibrate Compass (if applicable): Follow magnetic variation setup.

CAUTION! Perform initial setup with clear view of sky. Ensure waterproof seal is intact before use in marine environment.

Technical Specifications

• GPS Receiver: 12 parallel channel, WAAS enabled.
• Update Rate: 1 second.
• Accuracy: Position: <15 meters RMS, Velocity: 0.1 knot RMS.
• Acquisition Times: Hot start: ~15 seconds, Warm start: ~45 seconds, Cold start: ~2 minutes.
• Interface: NMEA 0183 version 3.01, 4800 baud default.
• Power: 10-40 VDC, 1.5W typical.
• Waterproof: IPX7 (1 meter, 30 minutes).
• Operating Temperature: -15°C to 70°C (5°F to 158°F).
• Storage Temperature: -40°C to 85°C (-40°F to 185°F).
• Dimensions: 5.3" W x 3.2" H x 2.2" D (135mm x 81mm x 56mm).
• Weight: 12.3 oz (350 g).

Controls and Interface

Front Panel: POWER, PAGE, ENTER/MARK, GOTO, IN/OUT, UP/DOWN arrows.

Display Pages: Use PAGE button to cycle through: Satellite Status, Map, Navigation, Highway, Position.

Menu System: Press and hold ENTER to access main menu: Waypoints, Routes, Tracks, Setup, System.

Setup Menu Options: Units, Time, NMEA Interface, Alarms, System (Backlight, Contrast, Tones).

Navigation Functions: GOTO (direct to waypoint), MOB (Man Overboard), Route Navigation.

NMEA 0183 Interface Setup

For connection to autopilots, chartplotters, sounders, or VHF radios.

1. Access Setup > Interface > NMEA.
2. Select NMEA Output: ON.
3. Select Sentences: Typically GGA (fix data), GLL (lat/long), RMC (recommended minimum), VTG (course/speed).
4. Set Baud Rate: 4800 (default, confirm compatibility with connected device).
5. Connect Wires: NMEA Out (+) to device In (+), NMEA Out (-) to device In (-).

Tip: Verify polarity. Incorrect connection will result in no data transmission.

Waypoints, Routes & Tracks

Supports creation and management of navigation data.

1. Create Waypoint: Press and hold ENTER/MARK at current location. Edit name, symbol, location.
2. Edit Waypoint: Menu > Waypoints > Select > Edit.
3. Create Route: Menu > Routes > New. Add existing waypoints in sequence.
4. Navigate Route: Menu > Routes > Select > Navigate.
5. Track Log: Menu > Tracks. Set recording interval (Time, Distance, Auto). Save active track.
6. Clear Data: Menu > System > Clear Memory (select Waypoints, Routes, or Tracks).

WARNING! Clearing memory is irreversible. Back up important data if possible.

Alarms & Navigation Features

Anchor Watch: Set alarm if unit moves beyond specified radius. Arrival Alarm: Sounds when within specified distance of destination. Off Course Alarm: Activates if deviating from route beyond set limit. Proximity Waypoint Alarm: Warns when near a stored waypoint. Access via Setup > Alarms.

Maintenance & Waterproofing

Unplug power before servicing. Inspect O-ring regularly for cracks or debris. Clean with mild soap and fresh water; dry thoroughly. Use silicone grease on O-ring sparingly if needed. Check cable connections for corrosion.

CAUTION! Do not use high-pressure water spray. Do not open case unless necessary; improper reassembly compromises waterproof rating.

Regulatory & Compliance

FCC Part 15: Class B digital device; complies with limits for marine use. CE marked. Contains GPS technology subject to U.S. export laws. Copyright © Garmin Ltd. or its subsidiaries. For support, visit www.garmin.com/support or contact Garmin Product Support.

Troubleshooting & Service

Reset: Menu > System > Reset (options: Reset Settings, Clear Memory, Master Reset).

Service: For repairs not covered here, contact Garmin Authorized Service Center. Do not attempt internal repair unless qualified.