Marine Alternator Troubleshooting

If you're having problems with your marine alternator, or any high performance alternators, there are some general tests you can do to determine what your problem is. Marine alternators are like any other; they charge the battery and ensure a proper flow of electricity to all parts of your boat. Therefore they can be tested the same as most any other alternator.

The most important thing to remember is to start all testing at the battery. Boats can be stored for months without charging the battery, and all batteries self discharge during storage. This self discharge is even higher when moisture is present. To make the smart aleck comment, I hear there is water around boats. When a battery becomes completely discharged sulfation occurs. This is a chemical reaction that occurs when batteries are stored for a long time and remain in a state of discharge for a long time. They are hard, bulky, crystal-like substances that form on the lead plates in the battery, causing them to become clogged and useless.

In order to test the battery you will have to apply a load that is 50% of its cold cranking amp rating for 15 seconds and maintain a terminal voltage of 9.6 volts. The C.C.A. rating for a battery used to crank a V8 inboard is 650 amps. To test this battery you would need to apply a 325 amp load for 15 seconds. Be sure you are using a tester that is capable of applying this load, at least a 300 amp load will be needed for most V8 gas or diesel inboards. Smaller engines such as outboards and 4 cylinder gas engines can likely be tested with a handheld 100 amp load tester. You can buy these at most any automotive store as they are typically used on most cars.

The first consideration when testing the charging system is to check the belts for proper tension and wear. If they are loose or glazed replace them, too often an alternator has been replaced when all that was needed was a new belt. If they are fine then connect the voltmeter leads to the battery post and not the cable ends. Crank over the engine and increase the RPM to a fast idle while observing the voltage reading. The correct one is usually between 13.8 and 14.2 volts. Gel cell batteries can be lower at 13.5 to 13.8 volts. If you are reading below 13.5 volts you should connect the positive voltmeter lead to the output post and the negative to the ground post of your alternator. Crank over the engine again and increase the engine RPM to a fast idle. If you see that the voltage is within the desired range for your battery then resistance in the charging circuit is the problem. The cause of this can be due to the boat manufacturer running many charge circuits through plug-in connection. They work fine when new, but corrosion can cause severe voltage drops. This can be a common issue with a marine alternator, especially when in-line amp meters are being used.