The Mercury Marine / Mariner / Force ignition key switch from Buckshot Racing #77 is a direct replacement for OEM part number 87-17009A5 and its common aftermarket equivalents. It’s built for modern 3-position, 6-wire magneto systems and lets you prime (choke), start, and stop your engine cleanly—without the dead shorts and flaky kill circuits that often show up in old switches on the black/yellow wire.
 
This switch utilizes durable, potted internals to prevent moisture and corrosion, ensuring it withstands vibration, spray, and salt. It comes with two keys, a mounting nut, and a color-coded wire lead set that matches standard Mercury wiring, making replacement straightforward.
 
Wire Functions
 
The Buckshot Racing #77 ignition switch follows the standard 6-wire Mercury layout:
Red is the 12 V battery positive.
Black is ground.
Purple is key-on “hot” 12 V for ignition and gauges.
Yellow/Black is primer or choke (push-to-choke when you push the key in).
Yellow/Red drives the start/solenoid circuit when you turn to START.
Black/Yellow is the ignition kill/ground used to shut the engine off.
 
If your current key switch uses the same six colors, you can wire this one in color-to-color.
 
Part Numbers & Cross References
 
This ignition key switch is intended to replace typical Mercury 6-wire push-to-choke switches, including:
 
OEM Mercury / Mariner / Force: 87-17009A5, 17009A5, 17009A2
Common aftermarket numbers: MP51090, MP51092, 1803
 
If you see any of these on your old switch, parts diagram, or in your manual, this Buckshot Racing #77 unit is the correct style replacement.
 
Help: Will This Fit My Engine?
 
You have a Mercury, Mariner, or Force outboard (or control box) that uses a 3-position key switch with push-to-choke.
Your existing switch has six wires in the same or very similar colors as listed above.
Your parts lookup or old switch ties back to 87-17009A5, 17009A5, 17009A2, MP51090, or MP51092.
 
If one of those doesn’t line up, it’s worth checking your engine’s serial number against a Mercury parts diagram before ordering.
 
Help: Basic Install & Wiring Tips
 
Always disconnect the battery before working on ignition wiring. Remove power (starting with the negative cable), then pull the old switch from the dash or control box. As you disconnect each wire, label it by color or take a clear photo so you can replicate the layout.
 
Mount the Buckshot Racing #77 switch in the same hole with the supplied nut, then connect each lead to the matching color in your harness using proper marine-grade connectors. Once the battery is reconnected, test each function:
 
In OFF, the engine and gauges should be off.
In RUN, gauges and ignition should power up.
In START, the engine should crank.
With the key in RUN and pushed in, the choke/primer should activate.
Turning back to OFF should cleanly shut the engine down via the black/yellow kill wire.
 
If the engine won’t stop with the key, or a fuse blows immediately, recheck especially the black/yellow and red circuits for a mis-wire.
 
When to Replace Your Ignition Switch?
 
This style of replacement switch often fixes issues like no-crank with a good battery and starter, intermittent starting that changes when you wiggle the key, loss of push-to-choke function, or an engine that won’t shut off with the key. Visible corrosion, melted plastic, or signs of overheating around your old switch are also strong clues it’s time to swap in a new Buckshot Racing #77 unit.
 
Part Numbers:
MP51090, 1803, MP51092, 17009A2, 17009A5, 87-17009A5, 17009A5
 
Contact Mike Hill with any questions at +1-714-697-1716 or mike@buckshotracing77.com