George Van Parys

Mar 1, 2000
RO Number


Start your testing by checking the belts for wear and belt tension. Many alternators have been replaced due to a loose or glazed belt. With today's high amperage alternators, belt condition and tension are critical in proper alternator performance. Connect the voltmeter leads to the battery post, not to the cable ends. Crank the engine and increase the rpm to a fast idle. Observe the voltage reading. On most systems the correct voltage will read between 13.8 volts and 14.2 volts. Some systems with gel cell batteries may have a lower voltage setting of 13.5 and 13.8 volts.


You can use an ammeter to check for alternator output, it is an easy and safe test, but this cannot tell you the specific problem, only give you an indication of what the may be the issue. If the voltage reading is below 13.5 volts, connect the positive voltmeter lead to the output post of the alternator and the negative to the ground post of the alternator. You must be very careful to connect only to the output post. If the lead touches the alternator case, it can short out your alternator. Crank the engine and increase the rpm to a fast idle. Check the voltage reading. If the voltage reading is within range, resistance in the charging circuit is the problem. This is a common problem with many marine alternator systems, especially if an in-line amp-meter is being used. The manufacturer of the boat runs many charge circuits through plug-in connections for easy installation. These connections work fine when they are new. After time they tend to corrode and cause severe voltage drops. When a standard alternator is replaced with a higher output alternator, you must replace the charging circuit wiring with the proper gauge wire to handle the higher output, otherwise you can cause a fire. In order to test the output circuits for resistance you must discharge the battery somewhat so the alternator will charge at its full output. The best way to accomplish this is with a battery load tester. Another way is to turn on a few accessories for a short period of time. Turn off the accessories and connect the positive voltmeter lead to the positive output terminal of the alternator and connect the negative voltmeter lead to the positive battery post, not the battery cable end. Crank the engine and increase the rpm to a fast idle while observing the voltmeter. If the reading goes above 0.2 volt the circuit has excessive resistance. Connect the positive voltmeter to the negative output terminal on the alternator and the negative voltmeter lead to the negative post of the battery and perform the same test as done with the positive output side of the charge circuit. Most alternators today require some type of ignition or switched voltage in order to start charging.


If you are not sure how to test all the circuits on an alternator, call someone who is familiar with them and ask for help. It can save you a lot of time, trouble, and expence if you do something wrong.