Your boat's alternator is a critical piece of your engine's electrical system.  It supplies power to the ignition system, provides on-demand power needs and conducts battery charging. Marine alternators are designed differently than an automative alternator and include important safety concerns as well.  A better understanding of your boat's alternator will help you to troubleshoot issues and identify the correct alternator for your application.

SAFETY FIRST: If you are not experienced in working on boat engine electrical systems, we strongly recommend that you consult a professional marine technician. Failure to follow proper safety precautions can result in catastrophic damage and cause injury or death.

and Meeting the Marine Standards

Marine alternators being used on inboard and stern drive gasoline engines must meet marine industry standard of ignition protection which is not part of an automotive alternator's design.  Marine alternators must meet the standards set by the Society of Automotive Engineers and be certified by the US Coast Guard.  Alternators that meet these specific safety standards are identified by the certification code "SAE J1171" printed or stamped on the body of the alternator.

Internal brushes in marine alternators create spark while rotating and the alternator is charging.  The sparks inside an enclosed engine room can be really dangerous.  If there isn't proper ignition protection, this can be a hazardous situation.  There can be flammable gases present in a gasoline engine compartment from a fuel leak, which can cause potential for explosion.  Alternators must be tested for proper ignition-protection to suppress spark in order to be certified for marine use. Automotive alternators are not, and they can create spark exposed directly to the enclosed engine compartment.

Marine Alternator Styles

The two most popular styles of marine alternators are Motorola styles that use internal brush covers and the Delco styles that use flame arrestors on the case. You can these styles below:

Motorola-Style Marine Alternator with BrushesMotorola-Style Marine Alternator with Brushes
Motorola-Style Marine Alternator with Brushes
Delco-Style Marine Alternator with Flame ArrestorDelco-Style Marine Alternator with Flame Arrestor
Delco-Style Marine Alternator with Flame Arrestor

Alternator Circuit Diagrams

In order to begin charging, most alternators need to receive a 12 volt signal from the ignition circuit of the engine.  Many times, the failure of the alternator can be traced back to improper, corroded or loose connections.  A good place to start with troubleshooting your alternator or charging system is by verifying that all the required connections are on the proper terminals. You should also test that all connections have the correct signal in order for the alternator to charge correctly. There have been a variety of of alternators and circuitry used in marine applications over the decades.  Below are some of the most common circuit configurations:

Late Model Motorola/Prestolite Marine Alternator CircuitLate Model Motorola/Prestolite Marine Alternator Circuit
Delco Self-Exciting Marine Alternator CircuitsDelco Self-Exciting Marine Alternator Circuits
Early Motorola Style Marine Alternator CircuitEarly Motorola Style Marine Alternator Circuit
Late Motorola/Prestolite Style Marine Alternator CircuitsLate Motorola/Prestolite Style Marine Alternator Circuits
Valeo/Paris Style Marine Alternator CircuitValeo/Paris Style Marine Alternator Circuit
Delco Style Marine Alternator CircuitDelco Style Marine Alternator Circuit
Early Prestolite Style Marine Alternator CircuitEarly Prestolite Style Marine Alternator Circuit
Late Model Mando Style Marine Alternator CircuitLate Model Mando Style Marine Alternator Circuit

Ask for Help

As stated above, fixing electrical issues on your boat engine shouldn't be approached lightly.  Consult with an experienced and ceritfied marine technician if you have questions.  You can also give our ABYC-certified experts a call at (866) 388-0390.  To shop our offering of high-quality and thoroughly-tested marine alternators, click here.