How to Test a Mercury Outboard Solenoidby Don Bowman
The solenoid on a Mercury outboard motor is used to actuate the starter. It is nothing more than a remotely activated switch. It takes considerable amperage to operate the starter, which in turn requires a large diameter wire to carry the load. There are four terminals on the solenoid--two large-diameter posts and two small-diameter posts. A large wire (4-gauge or larger) goes from the B+ post on the solenoid and the other large post has the large diameter wire going directly to the starter. When the solenoid is not energized, there is an open circuit between these two large wires. When power from the key is sent to the small terminal of the solenoid, it energizes and causes the solenoid to create a closed circuit to the starter.
Set the voltmeter to 20 volts. Remove the cowl or engine cover from the engine to gain access to the solenoid. The solenoid on an outboard is usually on the right side of the engine, looking at it from the back looking toward the front of the boat.
Ground the black lead on the voltmeter to the block and touch the terminal end of the large red wire at the solenoid with the voltmeter's red lead. This wire must be the wire from the battery. The wire can be seen coming up through the lower cover. The other large terminal can be seen going to the starter. There should be 12 volts at the terminal from the battery. If there isn't, there is a problem with the battery or terminal.
Check the small wire on the solenoid by removing it with a small wrench. With the black lead grounded as before touch the red lead to the small wire end as someone turns the key to start. There should be 12 volts every time the key is moved to the start position. This is the voltage to actuate the solenoid. If there is no voltage when the key is turned to the start position, there is a problem with the wire to the switch exits or a bad switch.
Reconnect the small wire to the solenoid. If there is 12 volts to the large wire from the battery, and 12 volts to the small wire when the key is in the start position, the solenoid should click and there should be voltage on the other large terminal. Check this terminal if the solenoid does not click for 12 volts. If there is no voltage, the solenoid is bad and needs to be replaced. If there is 12 volts then the starter is bad.