Trim & Tilt Relay Harness | 3-Wire to 2-Wire Conversion Instructions
- Mike Hill
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- Jul 22
- 4 min read
Updated: 7 hours ago
The Buckshot Racing #77 Trim & Tilt Relay Harness by API Marine, Part Number WH476, is designed to install a compatible two-wire trim motor in place of an older three-wire motor. It includes the relays and color-coded wiring needed to reverse motor polarity for trim-up and trim-down operation.
A traditional three-wire trim motor commonly uses separate blue and green leads for the up and down windings, with the motor grounded through a third wire. A two-wire motor changes direction by reversing polarity across its blue and green leads. The WH476 harness uses two relays to perform this polarity reversal when the trim switch is activated.
API Marine specifies the WH476 for converting compatible Mercury and Yamaha three-wire systems to two-wire operation. It is intended for use with API Marine trim motors including PT-475TN-4, PT-476NMK-3, PT-485NMK-3, PT-495NK-3, PT608NK, PT609NK, and PT612NK. API Marine also states that these two-wire replacement motors have a lower amperage draw than the corresponding original three-wire motors.
Complete Two-Relay Harness
The WH476 is supplied as a prewired assembly, reducing the need to fabricate a relay circuit from separate components. Its color-coded connections simplify installation:
Solid blue: two-wire trim motor blue lead
Solid green: two-wire trim motor green lead
Blue/white: trim-up control circuit
Green/white: trim-down control circuit
Red: fused positive battery supply
When the trim-up switch is pressed, the relays apply voltage and ground in one direction. Pressing trim-down reverses polarity, causing the motor to rotate in the opposite direction.
The harness includes two replaceable relays and inline circuit protection. Confirm the fuse rating supplied with the exact harness before replacing it; never install a fuse larger than the manufacturer-specified rating.
Mercury and Mariner Applications
The WH476 is commonly used when replacing an older round Prestolite or square Eaton three-wire trim motor with a compatible API Marine two-wire motor on select Mercury and Mariner integral trim-and-tilt systems.
Applications may include certain 35 through 225 HP Mercury and Mariner outboards, including select inline engines and 2.0L, 2.4L, and 2.5L V6 two-strokes. Fitment is determined by the trim-system design and replacement motor—not horsepower or displacement alone.
Verify all of the following before ordering:
Original trim motor uses a three-wire connection
Replacement trim motor uses a two-wire connection
Trim pump and mounting pattern match the selected motor
Harness connectors match the engine control wiring
Relay mounting location matches the engine configuration
Do not assume that every Mercury 2.0L, 2.4L, 2.5L, or 3.0L powerhead uses the same trim system. Engines with single-ram, three-ram, side-fill, aft-fill, remote-pump, or later factory two-wire systems may require different components.
Important Part-Number Clarification
Mercury numbers 882751A1 through 882751A9 identify various trim-relay assemblies or kits; they should not automatically be treated as direct cross-references for the complete WH476 conversion harness. Mercury 882751A1, for example, is cataloged as a trim relay assembly.
The correct conversion harness should be selected according to the existing trim system and the API Marine two-wire motor being installed. For many older Mercury three-ram applications, another commonly listed conversion harness is Mercury 84-819514A15, which is associated with specific 1985–1992 side-fill systems and a different 30-amp harness configuration.
Cross-reference numbers must therefore be verified individually rather than grouped together solely because they relate to trim relays.
Installation Overview
Disconnect the negative battery cable before beginning. Support the outboard securely and keep all wiring clear of the clamp bracket and other moving components.
Connect the replacement motor’s solid blue and green wires to the matching blue and green harness leads. Connect the harness blue/white and green/white wires to the engine’s matching trim-control wires.
Route the red power lead to a constant positive battery connection, normally the battery-side terminal of the starter solenoid or another manufacturer-approved positive terminal. The connection must remain protected by the harness fuse.
Depending on the engine configuration, the relays may mount on the starboard side near the air silencer or toward the rear of the powerhead using an appropriate coil-plate mounting location.
After installation:
Secure every terminal and insulate exposed connections.
Keep the harness away from the flywheel, throttle linkage, exhaust components, and sharp edges.
Reconnect the battery.
Test trim-up and trim-down operation.
Cycle the engine through its complete trim range while watching for stretching, rubbing, or pinching.
Should the switch operate backward, do not randomly change control wiring. Recheck the blue and green motor connections against the supplied diagram.
Critical Wire-Stack Positioning
CAUTION: The wire stack must be positioned correctly so it cannot be crushed by the clamp bracket as the engine is lowered.
Install the wire stack with the product label facing outward toward the installer. Slowly cycle the engine through its full trim-and-tilt range and confirm that the harness remains clear of the clamp bracket at every position.
Incorrect routing can damage the insulation, short the circuit, blow the fuse, or cause the trim system to stop operating.
Marine Electrical Installation Tips
Use only marine-grade terminals, heat-shrink connectors, and corrosion protection suitable for the installation. Do not use household wire connectors or leave butt connectors unsupported.
Check the battery, grounds, trim motor, hydraulic pump, fluid level, and mechanical condition before blaming the relays for slow operation. Low voltage, corroded battery cables, poor grounds, a worn motor, or a binding trim assembly can produce similar symptoms.
Inspect the fuse holder, relays, connectors, and power terminals regularly—especially on boats used in saltwater.
Final Thoughts
The API Marine WH476 Trim & Tilt Relay Harness provides a clean method of installing a compatible two-wire API Marine trim motor on select older Mercury, Mariner, and Yamaha three-wire systems. Its two-relay polarity-reversing circuit, color-coded connections, and fused power lead simplify the conversion while retaining conventional blue-for-up and green-for-down operation.
Fitment must be based on the original trim-system design and replacement motor number. Confirm the application before ordering, follow the supplied wiring diagram, and carefully check harness clearance through the complete trim range.
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