Heurofosh Starter Solenoid Relay 817109A2 for Mercury 40–250 HP — 12V Direct-Fit

The Heurofosh 817109A2 starter solenoid relay is a direct-fit 12V replacement for Mercury outboards from 40 to 250 HP, built for reliable starting and simple installation.

If your Mercury or compatible outboard is struggling to engage the starter or you need a like-for-like replacement for worn or corroded hardware, this Heurofosh relay matches common OEM specs and mounting dimensions. Engineered to accept standard marine battery and motor terminals and the familiar switch and ground posts, it aims to restore dependable starter engagement without modification. Below we answer practical buyer questions about fit, connections, durability, and installation.

Will this relay fit my Mercury 40–250 HP outboard?

Yes — the Heurofosh 817109A2 is designed to replace a wide range of Mercury solenoid part numbers and compatible aftermarket equivalents. It suits many 3-, 4-, and 6-cylinder models across 40 through 250 horsepower ranges, matching common mounting and terminal layouts.

• Replaces Mercury 89-817109A1/A2/A3 and Sierra 18-5834.
• Also compatible with Arco SW109, Arrowhead SMR6010, J&N 240-22131, WAI 67-731.
• Fits 3, 4, and 6 cylinder engines rated 40–250 HP.
• Mounting hole spacing of 68 mm center-to-center for direct replacement.

How do the electrical connections and terminals work?

The relay provides the standard heavy-current battery and motor posts plus separate switch and ground studs for the ignition circuit. Terminal sizes and layout mirror common marine relays so you can reuse existing cables in most cases.

• 5/16-18 battery and starter motor terminals for high-current connections.
• 10-32 switch and ground terminals for control wiring and chassis ground.
• Isolated base reduces stray conductivity and simplifies wiring routing.
• Durable terminal hardware minimizes loosening or corrosion under vibration.

How durable is the Heurofosh starter solenoid for marine use?

This 12V solenoid relay is built with marine environments in mind: robust posts, an isolated mounting base, and a design intended to resist the electrical and mechanical stresses of starting systems. While no electrical component is indestructible, this part offers a reliable service life when installed and maintained correctly.

• Sturdy metal terminals to handle repeated high-current cycles.
• Isolated base reduces corrosion pathways and potential shorts.
• Mounting centers match OEM spacing to avoid stress from misalignment.
• Engineered for the repeated cycles typical of outboard starting systems.

What should I expect during installation?

Installation is straightforward for anyone familiar with outboard starter wiring. Disconnect the battery, note terminal positions, remove the old relay, and transfer cables to the corresponding Heurofosh posts. Tighten to manufacturer torque recommendations and confirm operation before finishing.

• Disconnect battery negative before starting work for safety.
• Label wires or photograph the old relay to preserve terminal order.
• Use the 68 mm mounting centers to align the new relay without bracket modification.
• After installation, verify starter engagement and re-check terminal tightness.

Who is this for: Boaters, marine mechanics, and DIYers who need a dependable, OEM-compatible starter solenoid replacement for Mercury and similar outboards. If your engine spans 40 to 250 HP and requires the common 5/16-18 and 10-32 terminal arrangement with a 68 mm mounting pattern, this Heurofosh relay is a practical swap-in solution to restore reliable starting performance.