Sierra Wiring Harness 18-99420 - Replaces 84-73369 - Mercury/Mariner 20 HP (1977-1981), 50/80/85 HP (1976-1979)

The Sierra 18-99420 is a wiring harness engineered for Mercury/Mariner outboard engines, built with copper conductors for reliable electrical continuity between the engine and control system. It is designed to fit 20 HP (2 cylinder) outboards from 1977-1981 as well as 50, 80, and 85 HP (4 cylinder) outboards from 1976-1979. The harness uses spade-style connectors for a secure, corrosion-resistant connection point and is designed to exit through the side of the lower cowling, matching the factory routing on these engines.

This harness is a direct replacement for a number of original Mercury/Mariner part numbers, making it a straightforward swap for boat owners dealing with a cracked, corroded, or failed factory harness. Copper wiring construction supports dependable current flow to ignition and charging components, helping restore consistent engine electrical performance. The 18-99420 measures 7 x 9 x 2.5 inches and weighs 1.3 pounds, and it ships as a single complete harness ready for installation.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-99420
• Material: Copper
• Connector Type: Spade
• Item Dimensions: 7 x 9 x 2.5 inches
• Weight: 1.3 pounds
• Cable Type: Auxiliary
• Harness Exit: Side of lower cowling
• Unit Count: 1 harness

Fitment

• Mercury/Mariner 20 HP (2 cylinder), 1977-1981
• Mercury/Mariner 50, 80, and 85 HP (4 cylinder), 1976-1979

Replaces (OEM)

• 84-76295
• 84-76295A1
• 84-73447
• 84-73447A1
• 84-64997
• 84-73369
• 84-73369A1

Frequently Asked Questions

What engines does the Sierra 18-99420 wiring harness fit? The Sierra 18-99420 fits Mercury/Mariner 20 HP (2 cylinder) outboards from 1977-1981, and Mercury/Mariner 50, 80, and 85 HP (4 cylinder) outboards from 1976-1979.

Is the Sierra 18-99420 a direct replacement for the original Mercury harness? Yes, the Sierra 18-99420 is designed to replace the OEM Mercury/Mariner harness on the applicable engines, with a matching electrical layout for a direct fit.

Which OEM part numbers does the 18-99420 cross-reference? The Sierra 18-99420 replaces Mercury/Mariner part numbers 84-76295, 84-76295A1, 84-73447, 84-73447A1, 84-64997, 84-73369, and 84-73369A1.

What material is the wiring made from? The 18-99420 harness uses copper wiring, which provides reliable conductivity for engine electrical circuits.

How big is the 18-99420 wiring harness? This harness measures 7 x 9 x 2.5 inches and weighs 1.3 pounds.

What type of connectors does this harness use? The Sierra 18-99420 is fitted with spade-style connectors for making secure electrical connections at the engine.

How does the harness route through the engine cowling? On the applicable Mercury/Mariner engines, the 18-99420 harness is designed to exit through the side of the lower cowling, matching the original factory routing.

Will this harness fit a Mercury outboard outside the listed year ranges? The 18-99420 is specified for Mercury/Mariner 20 HP (2 cylinder) engines from 1977-1981 and 50/80/85 HP (4 cylinder) engines from 1976-1979; fitment outside those years is not confirmed, so buyers should verify against their engine's original part number before ordering.

Does the Sierra 18-99420 come as a complete harness? Yes, the 18-99420 ships as a single complete wiring harness ready to install, with a unit count of one.

Is Sierra 18-99420 a genuine Mercury part or an aftermarket replacement? The 18-99420 is a Sierra-brand replacement harness built to match and replace the original Mercury/Mariner factory harness, rather than being sold under the Mercury brand itself.

Why would I need to replace my outboard's wiring harness? Wiring harnesses like the Sierra 18-99420 are typically replaced when the original harness has cracked insulation, corroded connectors, or intermittent electrical faults affecting ignition or charging circuits on the applicable Mercury/Mariner engines.