Alternator Rebuild

With the clarity that comes of retrospection, there were warning signs: The watermaker was running slower than usual, the cabin lights faded earlier each evening, and the recovery time after each fridge cycle was taking longer and longer.

But deny we did, setting off from Mangalonga Island on a 1,200-mile passage from the Solomon Islands to Brisbane, Australia. Less than an hour out, Sparky packed it in. He was crook, as the Aussies say.

With about 99.9 percent of the passage left to run, we spun the helm and headed back to Mangalonga to sort things out.

Voyagers value self-reliance. In developing countries or miles offshore, there is no technical support beyond what is stowed between two bookends or two ears. When something breaks, the first recourse is to fix it yourself. The alternative often is to do without.

Few of us start with a working knowledge of everything on our boats, so some mechanical components become virtual black boxes – single units that can only be treated or replaced as a whole. For me, raw-water pumps, roller furling gear and alternators once were in that category. Necessity is a great teacher.

The alternator is a fairly simple device, especially taken separately from the voltage regulator that controls it. Voltage regulators in many cases really are black boxes, and with limited onboard test equipment, must be replaced as a unit. An alternator, however, can be broken down into easily replaced components using commonly available tools. Spare-parts kits are available for most cruising alternators.

What could go wrong?

There are some common failure scenarios for alternators:

Overheating is probably the number one alternator killer. The alternator has to live in one of the hottest locations on the boat and typically with very limited airflow for cooling. The diodes create heat when they work, adding to the extreme temperature environment.

Normal wear and tear is usually first exhibited by squealing bearings. Some manufacturers recommend replacing bearings every 1,000 hours of operation. The danger of ignoring failing bearings is that eventually the rotor shaft may freeze, destroying the alternator case and possibly damaging the engine. Brushes are subject to wear and should be replaced at the recommended interval or when showing considerable wear.

At some critical temperature and current, a diode will fail. The failure of one diode puts added stress on those remaining, and a cascading failure is likely. This also puts higher-than-design current in the remaining stator windings and will lead to burning and cracking of the stator insulation. (If you suspect a diode has failed and you have an adjustable voltage regulator, lower your charge current – or voltage – to reduce the stresses on your remaining diode/stator components until you can effect a proper repair.)

Finally, removing the load from an operating alternator may cause one or more of the diodes to fail. The sudden open-circuit output that results from turning off the main battery switch, for example, generates a very high voltage spike that destroys diodes.

Fixing alternator failures

Most alternator failures can be repaired in the field with minimal tools and a basic understanding of your boat’s electrical system. Although some specifics of your alternator may vary, the fundamentals
of this repair procedure should be similar. 

If you suspect that your alternator output is lower than it should be, check it by discharging your batteries to  about 12.4volts; turn off any other charging equipment, and run the engine to its normal charging speed. If your alternator is not putting out 80 percent or better of its rated capacity while still cold, there may be something wrong with your charger or your voltage regulator.

If you’ve checked the obvious things – Is it turned on? Are any wires broken? Are connectors loose or corroded? – then you can check to see if the voltage regulator is working. A simple test for the voltage regulator is to see if the alternator rotor has a magnetic field while spinning. Using a normal screwdriver or similar ferrous tool, hold the tip of the blade near the center hub of the alternator. Magnetism in the rotor should pull the blade to the hub – typically a nut holding the pulley. Switch your regulator on and off, or disconnect/reconnect the field wire while testing for magnetism. You should feel a difference when the voltage regulator is operating.

If you have field current and rotor magnetism, the most likely failure is one of the diodes inside the alternator. To test the diodes, you must open the alternator case. (Some very-high-output alternators may have external diodes that can be tested without opening the alternator.)

Disconnect and remove the alternator from the engine. Warning! The positive battery connection to the alternator can supply a dangerous amount of current if it shorts to anything. Be careful when removing this cable. If a fuse is in line, remove it first. If a battery switch completely disconnects it, turn it off. If not, try to disconnect this cable at the other end first. Secure the terminal end away from the engine and wrap with insulating tape just in case.

Splitting the case

Using an indelible marker, draw a straight line across the alternator, marking both case halves and the stator windings in the center. This will be your alignment mark for reassembly. Remove the bolts holding the case halves together and pull the two apart. The pulley/front half will still have the rotor assembly attached. The back will have the stator windings, the field brushes and the diode assemblies in it. The brushes will have popped out of the brush holders; set aside the two brush springs.

Use a volt-ohmmeter (VOM) to check the resistance between the stator terminals and the alternator case. Set the meter to ohms/resistance/impedance. The reading should be very high (infinite) in one direction and very low (less than 1 ohm) with the VOM leads swapped (normal diode behavior). If the reading is zero in both directions, you may have a shorted diode or one or more of the stator windings may be shorted to the case. If the reading is infinite in both directions, check your VOM battery.

The diodes must be isolated from the stator windings in order to be tested individually. Remove the stator winding wires from the three terminals in the back case. Now the stator can be removed. Although you can check the resistance between the stator terminals, this check doesn’t tell you much. The typical VOM is not sensitive enough to detect differences between windings, and delta-wound stators have two paths between each pair of terminals anyway, making it difficult to detect open windings.

Check the stator visually instead. The lacquer coating on the wires should be glossy, not black, charred or chipped. Hold the stator in two hands and give it a gentle twist. If you hear cracking, the insulation probably has been burned. Stators can be rewound at a fraction of the price of a new alternator. It does require special equipment; check with a local automotive electrics shop.

Testing diodes

With the diodes isolated from the stator windings, you can check the individual six diodes. These diodes normally use their case as one of their electrical leads, being press-fit into a metal plate (the +battery connection) or the back case itself (the ground connection).

Touch one lead to one end of a diode and the other lead to the mounting plate or back case. Then switch the two leads. One reading should be less than 1 ohm and the other near infinity. Repeat the test for the five other diodes.

If you find a diode that reads zero in both directions, it is short-circuited and must be replaced. If you find a diode that reads infinity in both directions, it is open-circuited and must be replaced.

You should have two groups of diodes, based on which direction gives a low reading and which reads infinite. The three in the back case should behave the same, and the three on the +battery connection should behave the same. These two groups differ in the direction in which they transmit current.

Presumably by now, you’ve found one or more open-circuit diodes – the most common failure – that need to be removed from the case or mounting plate. Be sure to support the area surrounding the diode case; the mounting material is probably an aluminum alloy and can shatter if handled improperly. I’ve found that variously sized sockets work well for supporting the mount material. A hammer and a smaller socket complete the tools required.

Replace the bad diodes with new ones from your extensive onboard spares collection that you assembled prior to voyaging. Be certain the diode you are installing matches the remaining diodes in that group, as determined by direction of flow measured by your VOM. Use the same support socket method while pressing the new diodes in place. Use a small socket to insure that you are pressing (hammering) on the solid metal rim of the diode and not on the insulating material.

Use your VOM again to check that all six diodes are functioning properly and that the three in each assembly are the same type.

Replacing bearings

If your alternator has more than 1,000 hours on it, and especially if it is starting to make noise, replace the bearings, too. The rear bearing is easily accessible in the center of the back case. To replace the front bearing, first remove the pulley from the rotor shaft and then remove the shaft from the front case. The front bearing is now easily accessible as well. Use a similar socket support and press technique on these parts.

Inspect the brushes for even wear. If they are worn, replace them. The whole brush assembly can be removed from the case for this.

With the new brushes in place or with the old brushes reinserted (and don’t forget those springs from earlier) depress each brush into its holder, and starting from the rear of the alternator, slide a straightened paperclip or wooden matchstick into the retaining hole of the brush assembly. This holds the two brushes in a retracted position until the rotor has been reinstalled. Replace the brush assembly in the rear case. Make sure your brush retainer “tool�VbCrLf protrudes from the back of the alternator case.

Replace the stator in the rear case, paying attention to the alignment mark drawn prior to disassembly, and reattach the stator windings.

If you removed the rotor from the front case, set the rotor into the rear bearing and then slide the front case onto the rotor shaft. If the rotor and front case are still assembled, simply insert the rotor into the rear bearing, and the front case will slide into place. Rotate the front case to align your marks.

Insert and tighten the assembly bolts. Reattach the pulley (if removed). And finally pull out the brush retaining device. You should hear each brush snap into place in turn as you slide out the retainer.

Measure the resistance across the two main terminals. Again, it should be a low value (less than 1 ohm) in one direction and infinite in the other.

The alternator is now ready to be remounted to the engine, the wires reattached and the fridge cooled down for a rewarding cold drink. One more mysterious piece of equipment has been demystified and your ability to handle an unexpected failure increased.

Jeff and Raine Williams completed a five-year circumnavigation onboard their J/40 Gryphon in 2004. See their complete story on CD, available at

By Ocean Navigator