How to Test a Marine Outboard Motor Coilby Ryan Hotchkiss
An outboard motor is an engine mounted on a boat that performs independent of other mechanical or electrical devices on board. The coil is a critical part of an outboard engine's ignition system. A coil is composed of a primary and a secondary circuit. A faulty coil means the engine will turn over, but not fire -- or fire, but not correctly. If your outboard motor will not fire or runs hard -- chugs and lurches -- test the resistance of the coil using a voltmeter. The values given by a volt meter are in OHMS.
Disconnect the wires from the positive and negative terminals on the coil. These terminals are connected to the primary circuit. The primary circuit is connected to the battery and creates a charge.
Disconnect the spark plug lead from the coil tower. The coil tower is connected to the secondary circuit. The secondary circuit sends the electrical charge from the coil, to the distributor where the voltage is then sent on to the spark plugs to fire the engine. Do not test the coil with any of these wires connected to the coil.
Turn on the volt meter. Turn the dial on the face of the meter to OHMS, not AC or DC voltage. You are testing resistance, not amps or current. If your meter has one, look at the screen. If an upside down horseshoe appears, the meter is ready to read OHMS -- resistance. For an outboard motor, turn the meter dial to 200. Other options include 2,000, 20,000 and greater. These values are too high.
Test the primary circuit. Place one probe from the amp meter on the positive terminal and the other probe on the other terminal. It is irrelevant which probe touches which terminal because you are testing a circuit. The values of an outboard motor must fall between .02 and .04. If your OHMS do not fall within that range, your coil is bad.
Test the secondary circuit. Place the negative probe -- usually black -- on the negative terminal. Place the positive probe -- usually red -- on the coil tower. The amp meter must read between 8 and 11. If the OHMS of the secondary circuit do not fall within this range, your coil is bad.