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Alternator failures are a fact of cruising life. Here is what to do when the unsung heroes of your electrical system stop functioning.


Imagine that you just arrived in the Bahamas for a month of cruising. Two days after crossing the Gulf Stream you notice the batteries are not getting fully charged, leaving you to wonder if the alternator has stopped working. How can you quickly test it and what can you do if it has failed? Has your cruise come to an early and abrupt end? Before we answer these questions it will be helpful to talk about alternators.

Marine alternators function unnoticed in a harsh environment, spinning at thousands of rpm while producing high operating temperatures. Given the combination of conditions and complexity, alternator failures are not rare.

An alternator functions on the basic principle that a magnet will induce a flow of current in nearby wires. Inside an alternator the core becomes magnetized and spins inside hundreds of windings of fine wire, producing electrical current. A field current excites the core, varying the magnetic intensity, which then affects the current generated. A regulator manipulates this field current, controlling the output of the alternator. This process produces AC current, and a rectifier converts the AC to the DC current needed to charge your batteries.

Before we go into troubleshooting the alternator, we need to step back and talk about your batteries. Prior to leaving the dock, it pays to know the answers to the following questions:

How many batteries/battery banks do you have? You might, for example, have a large bank of batteries for the house, a single battery for starting the engine and another single battery for starting the generator. Some older boats have two equal battery banks, either of which can be used to start an engine, and either of which can be used for house supply. In addition, there might be another bank dedicated to supplying the inverter.

How are the batteries identified at the main electrical panel? Does “1” refer to the starting battery or house bank?

How are the separate batteries/banks charged? The alternator might be set up with a charge divider, so that the house batteries are given priority, with all charge going to the house bank and a trickle charge to the starting battery.

The multimeter reads 14.0 volts between the positive output and the negative ground terminal, indicating a healthy charge.

By what means is the generator’s starting battery charged?

Which batteries are charged by the 120 VAC charger?

If you know the answers to these questions in advance, when trouble occurs you can focus on finding the problem instead of wasting hours trying to figure out how the system should work.

Let’s imagine that you are cruising and you suspect the alternator is not charging. In order to determine whether or not it is working you need a baseline, a reference point, and that will be the resting voltage of the batteries. To measure the resting voltage, you must turn off all charging sources and all loads. If the generator is running, shut it down. If you are plugged into shorepower, turn off the AC main breaker. If you have an inverter, turn it off. Turn off all DC circuit breakers. You can now read the resting DC voltage of the battery on the voltmeter at the main distribution panel. As an alternative, you can use a multimeter to read voltage at the battery. In either case, you will likely see a reading of between 12.0 and 12.8 volts. This provides your reference point.

Next, start the engine and increase the rpm to a fast idle, say 1200 rpm. Read voltage again. If the voltage increases above the resting voltage and continues to rise, then the alternator is charging. If the voltage does not increase, or it decreases, then we know the alternator is not working or its charging current is not making it to the batteries.

You also can put a voltmeter across the alternator’s positive terminal and case (or negative, if isolated ground), read the voltage, crank the engine, rev it up and read again. If the voltage goes up, the alternator is doing something. Whenever working around a spinning alternator use caution: clothing, hair, and wire leads can create hazards. If the alternator is not charging, there are four possible reasons: alternator failure, regulator failure, faulty electrical connection or failed battery. Let’s assume the batteries are good and focus on the other three.

Starting with the simpler options, first check the alternator belt. Assuming the belt is not broken, there should be approximately 1/8 to 1/4 inch of deflection under moderate finger pressure for every 12 inches of span. A loose belt might prevent the alternator from spinning fast enough to generate the necessary charge. If the belt is okay, the next stop will be checking the wiring connections on the back of the alternator.

Before removing wires, power to the alternator must be disconnected. If there is corrosion on some of the terminals, it will be best to remove each conductor, clean the terminal and reattach. Take a photo with your smartphone before disconnecting any wires. If no corrosion is evident, try to wiggle each wire connector on the terminal—if any of them move they should be tightened.

One more simple check worth making: The largest positive wire (probably red) leaving the alternator carries output current to the batteries. This may be wired back to the nearby starter motor solenoid, but it may also be wired back to the batteries by some other path, in which case the best practice calls for this wire to be protected from over-current by a fuse or circuit breaker near the battery (within 7 to 72 inches depending upon the situation). Locate the over-current protection (OCP), pull the fuse and test it with an ohmmeter (it should read 0 ohms), or make sure the breaker has not tripped. If both are okay, proceed to the next step. At this point, two possibilities remain: The alternator has failed or the voltage regulator has failed. For most cruisers, unless you have a spare alternator, an alternator service kit or a spare regulator, knowing which has failed will not do much good. So before we dig more deeply, let’s look at two workarounds that are available.

In some cases an alternator will fail in a dramatic way, with the bearings seizing and the pulley no longer spinning. This type of failure announces itself with the smell of burning rubber and smoke. If the alternator and the water pump share the same belt you now have two problems: No alternator and no engine cooling. Some cruisers carry a spare idler pulley that can be installed in place of the failed alternator, allowing the belt to continue driving the water pump. A spare alternator would be the only other option,

although in an emergency you may be able to drive the water pump with a homemade belt made from a pair of nylon tights or small- gauge nylon rope wrapped tightly around the crankshaft pulley and water pump pulley and tied off in a way that will ensure that the loose ends don’t snag anything. On some engines the water pump is gear-driven, or on a dedicated belt. In that case, you can cut the belt from the alternator, and the engine will still run.

Speaking from experience, once when leaving the lock in Port St. Lucie, smoke and odor announced the seizure of the alternator. The John Deere engine had a gear-driven water pump and I was able to cut the belt and keep going. If the alternator has failed but still spins freely and if you have a generator, the cruise can continue. You will need to monitor battery voltage when under way to make this work. When the battery voltage reaches 12.2 volts, it’s time to fire up the genset and use the charger to bring the batteries back up to a fully charged state.

If the alternator has not seized and you want to perform a repair, your options are directly related to the specific cause of failure, the spare parts on board, and your own troubleshooting and hands-on skills. In addition, you should have on board the manufacturer’s instructions for the alternator and the regulator (some are internal to the alternator).

Spares on board might include an alternator, or replacement parts for it, and a regulator. Not many cruisers go so far as to carry a spare alternator. Some manufacturers, Balmar for example, offer an Offshore Kit, which contains replacement brushes, diodes and bearings. Troubleshooting instructions are available from the manufacturers. For an excellent in-depth discussion on this topic, check out the Boatowner’s Mechanical and Electrical Manual by PassageMaker’s Technical Editor, Nigel Calder.

The decision on how many spares to carry should be driven by the type of cruising planned and the availability for workarounds if a component fails. A generator provides a workaround for a failed alternator, as long as the alternator does not seize and prevent the water pump from cooling the engine. By monitoring battery voltage under way you can keep cruising and deal with the alternator when you reach a good service yard.

Ask the Experts


I share Steve Zimmerman’s appreciation of The Pocket Ref (“Troubleshooter: In Defense of the Pocket Reference” PassageMaker, November/December 2014), but I’m curious about the kind of wet exhaust systems he works on. If I understand his example correctly, the entire 8-foot run of 4² ID exhaust hose would be completely full of water, necessitating 1,356 cubic inches or

5.87 gallons of muffler internal capacity. The exhaust lines on boats I’ve owned carried mainly exhaust with only a modest stream of water injected for cooling and quieting. How does his engine expel exhaust gases if the hose is full of water?

—Terry L. Johnson Professor of Fisheries, Marine Advisory Agent Alaska Sea Grant Marine Advisory Program

School of Fisheries and Ocean Sciences

Terry, while you are correct that the hose contains a mixture of gas and sea water, it is safest to size the lift muffler based on the assumption of a full column of water. The ratio of air to water will vary, and at times higher concentrations of water will occur, especially when water surges out of the collector into the hose. Under normal running conditions the mix will be roughly 25 to 30 percent water. So why size the muffler based on the assumption of 100-percent water? The answer can be found, in part, in the question you asked: “How does his engine expel exhaust gases if the hose is full of water?”

The hose will not be full of water, but the muffler might be. In order to reduce pressure inside the muffler when starting the engine, the muffler should not be more than 50 percent full of water. By working off of a calculation of a hose full of water, the necessary reserve is factored in. This method of calculation also provides a reserve against water backfilling into the engine.

Keep in mind that we are not talking about the entire hose run, but rather the length of hose from the collector to the highest point before it runs downhill to the exit point in the hull. I had difficulty recreating your math. If you are referring to 4-inch-diameter hose, Pocket Ref tells us that the volume would be 3.14 x 22 x 96 inches = 1,206 cubic inches, or 5.2 gallons. Looking at one of the muffler manufacturer’s websites, we see that the standard muffler for a 4-inch inlet and outlet has a volume of 1,356 cubic inches, or roughly 6 gallons, which falls right in line with the hose-volume calculation based on a full column of water.

—Steve Zimmerman