Avoid being left out on the water by testing the rectifier diode on your boat motor. Testing the rectifier will give you time to order a new part if it needs replacing. Generally, a bad rectifier leads to starting problems and loss of battery power. Rectifier diodes allow electrical current to flow in only one direction and convert alternating current (AC) into the direct current (DC) that the boat's devices require. Testing a rectifier's resistance in both directions indicates whether it needs to be replaced. Resistance measures the ability of a device to limit current flow in ohms.
Turn off power to the outboard motor. Locate the back box with a red and black connected to the motor. Disconnect the red and black wires by hand. Theses are the leads of the rectifier diode.
Plug the black test wire of the multimeter into the negative port of the device. Plug the red test wire of the multimeter into the positive port of the device.
Turn on the multimeter. Rotate its measurement dial to the resistance setting marked by the capital Greek letter omega. Capital omega is the symbol for ohms.
Touch the red wire of the multimeter to the end of the red lead of the outboard rectifier. Touch the black wire of the multimeter to the end of the black lead of the outboard rectifier. The resistance measured should be between 0 and 1 ohm, or the rectifier needs replaced.
Connect the red wire of the multimeter black lead of the outboard rectifier. Connect the red wire of the multimeter red lead of the outboard rectifier. The resistance measured should be infinity or the maximum reading possible. Otherwise, replace the rectifier.