Maintenance Matters

Alternator and voltage regulator problems.


In the days of steam gauges, an alternator failure was inconvenient. In the age of glass panels, it is much more serious. Now if the alternator stops producing power, it is just a matter of time—and maybe not very much time—before you no longer have any instruments at all. Keeping your charging system happy and tracking down problems needs to be in a high position on your list of priorities—even if you are a day/VFR pilot.

The insides of an alternator are pretty standard. The field windings magnetize the rotor shown in the left half, and the stator windings, diodes, and brushes are in the right half. The field windings get their power through slip rings shown on the exposed end of the rotor. The crankshaft pulley is on the other end.

What Do the Gauges Tell You?

Two instruments can tell you something about your charging system: the ammeter and the voltmeter. Depending on how your plane is wired, the ammeter will tell you how much current is going in and out of your battery or how much current your alternator is producing. The voltmeter will tell you the voltage at the main electrical bus. A voltmeter is required equipment if you have electronic engine gauges: tachometer, manifold pressure, oil pressure, etc. These two gauges tell you quite a bit about the state of your alternator in particular, and your electrical system in general.

Normal voltage readings should be around 14 volts for a so-called 12-volt system. This is because if the voltage output of the alternator is not greater than the battery voltage, current will not flow into the battery, which on its own should produce about 12.8 volts when fully charged. If you normally see system voltage that is above 14.5 or below 13.8 when flying, you need to find out why. Too little voltage will not charge the battery properly, and too much voltage can damage it permanently. When the alternator fails, system voltage will drop to battery voltage, which will be at 12.8 or less, and declining as the battery loses charge. Somewhere around 9 to 11 volts, your electronic instruments and radios will fail. This will vary from one device to another and is a number that you should know for your equipment, especially if you fly IFR. In the case of radios, transmitting will fail before receiving because just keying the mike will cause system voltage to drop a bit. This is why a voltmeter is required with electronic engine monitoring systems.

On alternators with F1 and F2 terminals, you will need to connect one lead of the meter to each terminal. The resistance reading is a little high because this alternator has been sitting on a shelf for several years. Some regular use should remove the oxidization from the brushes and slip rings.

A slight increase in voltage may indicate resistance building up in the field circuit for some reason. A corroding connection or a partially-broken wire could be the cause. The failure of one of the stator coils could also cause voltage to rise. The important point is that just as decreasing voltage is bad, so too is increasing voltage. If it changes, there must be a reason.

Normal ammeter readings will vary depending on what you have installed and turned on, and depending on how you have wired your ammeter. It is very wise to have a good idea of how much current each device draws, just in case you need to shed load in a hurry to preserve your battery. When flying sometime, try turning off each device in your plane and see how the current changes. Then make a note of it in case of an emergency. Pitot heat, landing gear, and landing lights are typically the big power hogs in most planes.

Why the Lights Go Out

Failed alternators cause relatively few electrical failures. The various alternators likely to be used by amateur airplane builders will last for 2000 hours, the life of a typical aircraft engine, and will require no service during that time unless abused or improperly installed. Of course, relatively few is not the same as none, but look elsewhere first unless the alternator displays obvious signs of distress. Wires and connectors are the most likely culprits when things stop working. Just as with starter problems, track down each wire, switch, and connector to see where resistance is high—above 0.2 ohms across any switch or connector—and/or where voltage drop is excessive—more than 0.5 volts. Get a good multimeter that will reliably measure tenths of ohms and learn how to use it.

There are some mechanical problems that could also cause your alternator to underperform or fail. If you hear a high-pitched squealing sound when you place a heavy load on the alternator, you may just have a loose belt. Lycoming has a service instruction that tells you how to properly tighten your alternator belt. Keeping your alternator belt properly tightened helps ensure a long life for the belt. Since replacing a belt means taking off the propeller, getting a long life out of it is really important.

Alternator Belt Tension: Use a torque wrench on the pulley belt and adjust the tension so that you get the specified torque just as it starts to slip. Turn the pulley nut in the tightening direction so you don’t risk loosening it. Reference: Lycoming Service Instruction 1129A

Be sure to check the tightness of your alternator belt yearly and whenever you are having alternator problems.


With any kind of troubleshooting, there are a couple of rules of thumb that are worth considering. First, start with the simple stuff, then move to the complex items. And second, which is similar, start with the cheap stuff first, then move to the more expensive things. It is just easier to fix simple things, even if it is not always easier to find simple problems. And if, in desperation, you devolve from mechanic to parts changer, you sure as heck want to start changing the less expensive things first. These are not hard and fast rules, but really more like guidelines. Sometimes the problem is obvious, and it is obviously expensive, but when you are having a hard time figuring things out, these rules can help you keep things in perspective.

This airplane uses a B&C solid state voltage regulator. It will work better if it is installed inside the cabin instead of in the engine compartment. One advantage of the B&C regulator is that the voltage is easily adjustable.

Common Alternator Problems

If you have a mechanical whining that increases in pitch with increased rpm, you may have a bad bearing in your alternator. If so, sending it back to the manufacturer or a repair shop for overhaul is probably your best bet. Mechanical noise is different than radio noise, so turn your radio off if you aren’t sure of the origin of the noise.

If the alternator just stops charging, the first thing to suspect, assuming it isn’t making some terrible noise or hasn’t blown the main breaker, is the field. If you are not getting 12 volts or more to the field terminal of the alternator, you are not going to be making electrical power. Check the connector at the alternator, the wire, the voltage regulator if you have an external voltage regulator, and then the ALT side of the master switch. If you don’t have 12 volts at any of these points when the master switch is turned on, find out why and fix it. Loose connectors can be the toughest problems to find, so be sure to tug on each one to make sure it is secure. If the field breaker has popped, suspect a chafed wire making contact with the airframe somewhere. Trace the entire path of the field wire until you find the short.

A short from the field to the alternator case inside the alternator can also cause the field breaker to trip. Disconnect the wire from the field post of the alternator and use your ohmmeter to check for a short by placing one lead on the field terminal and the other on the alternator case. If there is no, or almost no resistance, you have a short inside the alternator. You should expect to see at least four to six ohms resistance, assuming a 12-volt system. In some cases the field is not internally grounded. You can tell this by the existence of two field terminals on the alternator (usually marked F1 and F2). In this case measure resistance between F1 and F2.

It can be quite difficult to access some field connections on alternators installed in airplanes. Here a pigtail has been made to make checking resistance easier.

When testing an alternator with only an F terminal, one lead goes to the case (ground) and the other to the F terminal. This B&C alternator shows 5 ohms field resistance, which is exactly right.

In other cases you may have a poor connection between the field brushes and the slip rings inside the alternator. To track this down disconnect the field wire and the alternator belt. Then turn the alternator over by hand while checking resistance between the field post and the alternator case (or F1 and F2). If resistance is consistently high, the contact between the slip ring and the brushes inside the alternator is not good, which again means the alternator needs to be repaired. You may be able to cure this problem by using a fine Scotch-Brite pad on the slip rings. If you have an open circuit across the field (infinite resistance), you may have brushes that have fallen apart or a broken wire in the field winding—time for some major alternator repairs.

A quick test to see if the field is working is to power up the field and see if the rotor shaft is strongly magnetized. A tester made up of two paper clips shows that it is.

Because of their extremely low operating current, digital multimeters do not do a good job of measuring resistance in the field circuit, so it is best to use an old-style analog meter for this. Even if you get a reading of 50 ohms or so, there is no reason to panic. Spin the alternator a few times and see if the resistance goes down. You may also notice that the reading varies as you spin the alternator. This is pretty common and not necessarily a reason for concern if the alternator is otherwise working well. A thin layer of oxidation on the slip rings can easily produce some extra resistance. Higher resistance indicates a possible problem worth further investigation.

Make sure that all battery wires and terminals are clean and secure before worrying too much about your alternator. Note the ground bus bar in front of and below the battery. Good grounding eliminated many potential problems.

If the field wire and the alternator wire short against each other, you will get a potentially very damaging overvoltage situation. A partial stator failure can also cause the field to trip if it has caused an overvoltage spike. Make sure wires are properly routed and secured, and that they can’t rub against each other. The price of this mistake can easily run into the thousands of dollars. To guard against this you should install an overvoltage protection module available from B&C Specialty products or other sources for about $35. It is quite easy to install, so there is no reason not to provide yourself with this protection. Rapid loading or unloading of the alternator can also cause overvoltage surges. Some of the current models of Plane-Power alternators have internal overvoltage protection, but others do not. Be sure to find out if this is included in your alternator, or if not, add it to your electrical system.

If you have an overvoltage situation, and you have overvoltage protection, it should trip the field circuit breaker. In such a case you will need to turn the power to the field off (ALT side of master switch) to reset the overvoltage module. Of course, the next thing to do is find out why you had a voltage spike, so it won’t happen again.

If your electrical system and/or alternator do not already have overvoltage protection, this module available from B&C is inexpensive and easy to install. It is pretty risky not to have overvoltage protection in your electrical system.

Rotax Alternators

The Rotax charging system is a special case and calls for its own diagnostic and repair procedures. There are some simple resistance checks that you can perform to check the alternator. First disconnect the alternator at the plug. Check resistance between the yellow wires and between each yellow wire and ground. Yellow to yellow should measure 0.1 to 0.8 ohms. Yellow to ground should be open (infinite resistance). Next check the resistance between the red wire and ground. This should be in the range of 3.2 to 4.5 ohms. The two colored wires are for the ignition system. If the unit does not pass these basic checks, it most likely needs to be removed and further checked and/or repaired. For more detail on these procedures, please refer to the Maintenance Manual (Heavy Maintenance) for Rotax Engines Types 912 and 914 Series, which is available online. Troubleshooting is also covered in the Rotax maintenance training courses, which are highly recommended to anyone who works on Rotax engines and are required for SLSA Rotax mechanics. The nice thing about the Rotax troubleshooting procedures is that they are very straightforward and can be performed with a good multimeter.

Radio Noise

If you are getting radio noise that seems to be alternator-related, you can use your multimeter to do a quick check of the alternator output voltage. Set your meter to AC voltage and measure from the main bus to ground while the engine is running. If you see more than one volt in the AC scale, suspect a blown diode inside the alternator, or possibly bad brushes. Remember, the alternator is only supposed to output DC voltage. This isn’t a perfect test, but it is quick and easy. A ripple meter is an even better way to check stator and diode condition, but most amateur builders will not have one in their tool box.

Suddenly occurring and persistent radio noise is probably caused by a bad diode or a broken stator wire in the alternator. This means a trip to the alternator shop. Intermittent noise is more likely caused by a ground loop, which in turn is likely caused by a bad connection to ground somewhere. Electrical systems that employ a central ground bus are seldom plagued by ground loop problems, but using the airframe for a ground leaves you more vulnerable. These problems can be tough to track down, but simply making sure every single ground connection in your plane is secure and corrosion-free is where you need to start.

Where to Find Help

As with most items made for aviation, the manufacturers of these products are ready to help you sort out your particular problems with their products. Use these resources to speed up the troubleshooting process. Another great resource for all things electrical in aircraft is Bob Nuckolls’ AeroElectric Connection. There is no other one single source that has so much good information about aircraft electrical systems in one place. For $20 plus shipping it is hard to think of a better investment for an amateur airplane builder.

Leading manufacturers of alternators for Experimental builders include B&C Specialty Products and Plane-Power. Hartzell Engine Technologies now handles the legacy brands such as Prestolite and Kelly Aerospace. In addition they now have a lightweight product line suitable for amateur builders.

Dave Prizio is a Southern California native who has been plying the skies of the L.A. basin and beyond since 1973. Born into a family of builders, it was only natural that he would make his living as a contractor and spend his leisure time building airplanes. He has so far completed three—a GlaStar, a Glasair Sportsman, and a Texas Sport Cub—and he is helping a friend build a fourth, an RV-8. When he isn’t building something, he likes to share his love of aviation with others by flying Young Eagles or volunteering as an EAA Technical Counselor. He is also a member of the EAA Homebuilt Aircraft Council.

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Dave Prizio
Dave Prizio has been plying the skies of the L.A. basin and beyond since 1973. Born into a family of builders, it was only natural that he would make his living as a contractor and spend his leisure time building airplanes. He has so far completed four—two GlaStars, a Glasair Sportsman, and a Texas Sport Cub—and is helping a friend build an RV-8. When he isn’t building something, he shares his love of aviation with others by flying Young Eagles or volunteering as an EAA Technical Counselor. He is also an A&P mechanic, Designated Airworthiness Representative (DAR), and was a member of the EAA Homebuilt Aircraft Council for six years.


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