Marine alternators are similar to car alternators in that they produce electricity to power the electrical systems and recharge the battery. There is one main difference: A marine alternator produces more electricity at lower engines revolutions to compensate for the slow speed a marine engine operates at. Marine alternators can have up to four terminals, depending on the sophistication of the electrical systems, but generally they have three: positive, negative and alternator warning light. If they have a fourth it usually is to connect to an external voltage regulator, but many marine alternators have these built in.
Disconnect the positive and negative cables attached to the battery with a wrench before attempting to wire a marine alternator. Move them safely away from the battery.
Locate the marine alternator's terminals and check the type of connection. Some have bolts, others have screws. Later types often have spade or push-in connectors, making the task of wiring the alternator easier.
Loosen and remove the bolts and screws with a screwdriver or wrench, if the alternator has this type of terminal connection. Put them somewhere safe.
Find the cables that connect to your alternator. The positive cable is red, the negative black and the third may be yellow, brown or blue. The third cable is much smaller. The positive cable leads to the battery and starter motor; the negative attaches to an engine bracket or similar metal area as it acts as a ground cable. The third cable is fairly short and extends from a wire loom.
Connect the negative cable to the negative terminal of the alternator. Either push the connector on the terminal, or place the wire eyelet over the terminal and replace the screw or bolt.
Connect the positive cable to the positive terminal of the alternator. Push the connector on, or place the eyelet over the terminal and replace the bolt or screw. Repeat the process for the third wire, then tighten the three bolts or screws with a screwdriver or wrench.
Reconnect the positive and negative battery cables on the corresponding battery terminal to complete the task.