Sierra Wiring Harness 18-99422 for Johnson/Evinrude 20, 25, 35 HP Round Plug Outboards, 1977-1981

The Sierra 18-99422 is a replacement engine wiring harness built for Johnson and Evinrude outboards that use a round plug connector. It carries the main engine-to-control wiring for 20 HP, 25 HP, and 35 HP round-plug outboards from the 1977-1981 model years, giving owners a direct way to restore or replace deteriorated, cracked, or corroded factory wiring without splicing in a universal harness. Marine wiring harnesses like this one are a common failure point on older outboards, since constant vibration, saltwater exposure, and UV exposure break down the insulation and connector contacts over time, leading to intermittent gauges, stalling, or a no-start condition. Installing a correctly matched harness restores factory-style connections at the engine and control box, keeping the wiring routed and terminated the way the outboard was originally designed.

This harness is a direct fit for the listed Johnson/Evinrude round plug engines and is cross-referenced to the OEM wiring harness numbers below, so it installs in place of the original part without modification. It ships as a single, complete harness assembly ready to connect at both ends.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-99422
• Category: Evinrude Outboard Electrical System
• Weight: 0.7 pounds
• Connector Type: Round Plug
• Item Dimensions (L x W x H): 7 x 9 x 2.5 inches
• Number of Items: 1
• UPC: 808282394530

Fitment

• Johnson/Evinrude 20 HP, 25 HP, and 35 HP outboards with a round plug connector, 1977-1981

Replaces (OEM)

• 389269
• 387872
• 389764

Frequently Asked Questions

What does the Sierra 18-99422 wiring harness fit? It fits Johnson/Evinrude outboards with a round plug connector in 20 HP, 25 HP, and 35 HP, model years 1977-1981.

Is the 18-99422 harness a direct OEM replacement? Yes, Sierra 18-99422 is a direct replacement that installs in place of the original harness without modification, using the same round plug connection.

What OEM part numbers does 18-99422 replace? It replaces OEM numbers 389269, 387872, and 389764 on the covered Johnson/Evinrude round plug outboards.

How much does the wiring harness weigh? The Sierra 18-99422 wiring harness weighs 0.7 pounds.

What are the dimensions of the harness as packaged? The 18-99422 harness measures 7 x 9 x 2.5 inches.

What type of connector does this harness use? Sierra 18-99422 uses a round plug connector, matching the factory setup on the covered 20, 25, and 35 HP Johnson/Evinrude engines.

Will this harness work on a square plug outboard? No, the 18-99422 is built for round plug engines only; a square plug outboard requires a different harness.

How many harnesses come in the package? One complete wiring harness assembly ships per package (18-99422).

Does replacing the wiring harness require cutting or splicing? No, because 18-99422 is a direct-fit replacement for the OEM harness, it connects using the original round plug terminations without cutting or splicing.

What symptoms indicate a Johnson/Evinrude outboard needs a new wiring harness? Cracked or corroded insulation, intermittent gauge readings, stalling, or a no-start condition on a 1977-1981 round plug engine often point to a failing harness, which the 18-99422 is designed to replace.

Is this harness only for Evinrude, or does it also fit Johnson engines? It fits both Johnson and Evinrude outboards, since the two brands shared the same round plug engine electrical systems covered by part 18-99422.