Mercury – Adapter Harness | Racing 350 SCI to 400R Fuel/Paddle/Vessel Interface | 84-8M0137199

Overview

Repowering from a 350 SCI to a 400R can leave you with harnesses that don’t quite match, especially around the fuel-level, paddle-wheel, and vessel-sensor connections. The Mercury 84-8M0137199 Adapter Harness makes that transition clean and straightforward. It bridges the older 350 SCI harness layout to the newer 400R interface so the sensors you already have can continue talking to the engine without rewiring the whole boat.

Key Features

• SCI-to-400R Adapter – Aligns the 350 SCI harness layout with the 400R Fuel/Paddle/Vessel interface.
• Fuel-Level Compatibility – Ensures tank sender information reaches the 400R correctly.
• Paddle-Wheel Support – Carries paddle-wheel speed input when equipped.
• Vessel Sensor Link – Allows existing vessel-sensor data to integrate with the newer 400R rigging.
• Plug-In OEM Fit – Factory connectors provide a clean, direct, no-splice installation.
• Rigging Simplification – Eliminates guesswork during repowers or harness upgrades.

Compatibility

• Mercury Racing 350 SCI
• Mercury Racing 400R
• Fuel, paddle-wheel, and vessel-sensor harness connections

Performance & Practical Benefits

This harness smooths out one of the trickiest parts of jumping from a 350 SCI to a 400R—getting all the sensors to communicate the way they should. It keeps your existing fuel-level, paddle-wheel, and vessel-sensor wiring in play, saving time and avoiding unnecessary modifications. Once installed, the system behaves naturally, and the helm displays get the data they expect.

Why Buy OEM

Racing engines depend on precise sensor communication, and Mercury’s OEM connectors are built for the exact pin alignment and shielding these systems require. Aftermarket adapters often cause noise in the signal or fail under vibration. The factory harness fits right, holds tight, and keeps your data steady at every throttle setting.

FAQ

Q1: When do I need this adapter?
When upgrading from a 350 SCI to a 400R and you want to retain the existing fuel, paddle-wheel, or vessel-sensor wiring.

Q2: Does this harness add any new features?
No. It simply adapts your current sensors to the 400R’s interface.

Q3: Will it affect engine performance?
No. It only manages sensor communication.

Q4: Is installation complicated?
No. It’s a plug-in adapter using OEM Mercury connectors.

Q5: Can it withstand racing use?
Yes. It’s made for the vibration, heat, and movement typical of high-performance rigs.