Sierra 18-5709 Marine Rectifier, Replaces OMC 581778, for Johnson/Evinrude Outboard Motors 9.9-40 HP

The Sierra 18-5709 is a marine rectifier built as a direct replacement for the charging system on Johnson/Evinrude outboard motors. It converts the AC output from the outboard's alternator into DC current to keep the battery charged while running, a critical function for reliable starting and onboard electronics. The housing is constructed of metal and plastic with a white finish, and it uses an engine-mount design so it bolts into the same location as the original unit on gasoline outboard motors.

This rectifier is engineered as an equivalent to OMC part number 581778, giving boat owners a straightforward way to source a charging-system replacement without hunting for a discontinued OEM part. It is designed for outboards rated up to 9.9 horsepower and is also listed across a range of Johnson/Evinrude engines from 9.9 HP through 40 HP, spanning multiple production years, including several 2-cylinder configurations. Every Sierra part is backed by Sierra's standard limited warranty, giving buyers confidence in fit and function.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-5709
• Category: Outboard Rectifier/Regulator
• Material: Metal, Plastic
• Color: White
• Weight: 0.16 pounds
• Engine Type: Outboard Motor
• Fuel Type: Gasoline
• Mounting Type: Engine Mount
• Compatible Engine Brand: Johnson/Evinrude
• OEM Cross Reference: OMC 581778

Fitment

• 9.9, 15 HP, 1974-92
• 20 HP, 1989-98
• 25 HP (2 Cylinder), 1977-03
• 28 HP, 1989-96
• 30 HP (2 Cylinder), 1989-03
• 35 HP (2 Cylinder), 1976-93
• 40 HP, 1974-76, 1988-92

Replaces (OEM)

• OMC 581778

Frequently Asked Questions

What does the Sierra 18-5709 rectifier do? It converts the AC current produced by a Johnson/Evinrude outboard's alternator into DC current to charge the battery, which is essential for reliable starting and onboard electrical systems on the motor.

Is the Sierra 18-5709 a direct replacement for OMC 581778? Yes, the 18-5709 is built as a replacement for OMC part number 581778 and mounts in the same engine location as the original rectifier.

Which outboard motors fit the Sierra 18-5709 rectifier? It fits Johnson/Evinrude outboards including 9.9 and 15 HP (1974-92), 20 HP (1989-98), 25 HP 2-cylinder (1977-03), 28 HP (1989-96), 30 HP 2-cylinder (1989-03), 35 HP 2-cylinder (1976-93), and 40 HP (1974-76, 1988-92).

What is the 18-5709 rectifier made of? The housing is made of metal and plastic with a white finish, designed for engine-mount installation on gasoline outboard motors.

How much does the 18-5709 rectifier weigh? The Sierra 18-5709 rectifier weighs 0.16 pounds.

Will the 18-5709 fit a 9.9 HP Johnson outboard? Yes, the 18-5709 is listed for 9.9 and 15 HP Johnson/Evinrude outboards from 1974-92.

Does the 18-5709 fit 25 HP two-cylinder Evinrude motors? Yes, it fits the 25 HP 2-cylinder Johnson/Evinrude outboard covering model years 1977-03.

Is the 18-5709 a genuine OEM part or an aftermarket replacement? The 18-5709 is a Sierra-manufactured replacement rectifier engineered as an equivalent to OEM part OMC 581778, not an original OMC-branded component.

How is the 18-5709 rectifier mounted? It uses an engine-mount design, installing directly onto the outboard motor in place of the original charging-system rectifier.

Does the Sierra 18-5709 come with a warranty? Yes, the 18-5709 rectifier is covered under Sierra's standard limited warranty.

What is included when I order the 18-5709? The order includes one Sierra 18-5709 rectifier unit for Johnson/Evinrude outboard motors.

Will the 18-5709 fit a 40 HP Evinrude? Yes, the 18-5709 fits 40 HP Johnson/Evinrude outboards from 1974-76 and 1988-92.