Cherokee Pilots' Association

Troubleshooting the Cherokee Charging System

By Robert M. Adkins

Light aircraft charging systems are similar in many respects to their automotive and marine counterparts and are quite simple in nature. There are a couple of notable differences however.

Unlike a car or boat, an aircraft electrical system is controlled by a separate switch (the master), not the engine ignition starter switch. In addition to the battery Master switch a separate switch is provided to allow the charging system of an aircraft to be shut down while still leaving the battery on.

This separate switching is the cause for the majority of the problems and the short life expectancy (high failure rate) of light aircraft charging systems. There really is little difference in all of the other parts of the charging system and apart from mechanical problems, electrically an alternator that is used in an aircraft should last just as long (if not longer) than the same alternator that is used in an automobile or boat.

If you have doubts about the statements just made, read on. I think by the end of this article you will find substantial justification for that statement, and proven suggestions that you can use in virtually any light-piston single engine or twin to extend the life expectancy (and reduce the failure rate) substantially of your charging system. This might just bring a little more peace of mind next time you find yourself in heavy IFR conditions at night near freezing level, not to mention the savings in maintenance costs.

I will explain the function of each of the major components found in an aircraft (and for that matter most all other) charging systems. I will also describe the most common failure of each of the individual components and the symptoms that may be observed. I will show some basic troubleshooting tips for several of the most common charging system failures and the likely causes for each failure.

Please note that I have taken great care to arrange the troubleshooting and parts list in a logical and cost effective order. You may chuckle a little when I suggest that you verify the operation of a switch, circuit breaker or connection, but these checks take very little time and effort to perform and will not cost you anything.

These components do play mayor roles in the charging system. We may not want to admit it, but a lowly switch, circuit breaker or connection can completely disable a charging system. If you don't believe me you might want to reread the tale about David and Goliath and ask Murphy his opinion while you're at it.

Anyway if a problem is found with one of these components they will be the least expensive components to replace. I assure you switches, circuit breakers and connections do fail, especially in older aircraft or aircraft that are exposed to damp or corrosive environments.

If after checking all of these basic parts you conclude that the problem is elsewhere at least you will feel a bit more confident about dropping $100 to $300 on voltage regulators and alternators.

The Major Components

Aircraft charging systems consist of the following major components:

1) Alternator.

2) Battery.

3) Voltage Regulator.

4) Over voltage relay.

5) Battery Master Switch and Master Relay.

6) Alternator Switch.

7) Field and Output circuit breakers.

The charging systems in late model (post 1964) Piper Cherokees and most other light aircraft use relatively standard off-the-shelf automotive parts. Most of the alternators used may be Chrysler, Delco or Prestolite. The alternator is the business end of the charging system. Alternators typically produce their rated power at 5,000 to 6,000 rpm.

In automotive applications the alternator drive is usually reduced 2 to 1 to achieve optimum power output at typical cruise speed engine rpms. In an aircraft installation the drive ratio is typically 3 or 4 to 1. For this discussion I will assume that the alternator is always turning at the optimum rpm which would at any time allow the alternator to produce its maximum rated power.

The Battery and the Alternator

An alternator consists of three basic components, the rotor, stator and rectifier.

The rotor and stator are windings made up of varnished copper wire, the varnish acts as an insulator. The rectifier is made up of six diodes, arranged in pairs. Each pair of diodes rectify the current from each of the fixed phase windings in the stator.

Most alternators do not have fixed magnets and therefore do not produce any power on their own. In effect an alternator is a form of power amplifier; it can turn a small amount of electrical power into a larger amount of electrical power by using mechanical force (the engine drive).

An alternator requires a small amount of external power to produce a magnetic field in the windings of the rotor. The strength of this magnetic field determines the amount of power (current) that may be produced by the stator windings.

The strength of the magnetic field produced by the rotor is controlled by controlling the amount of current that it draws. Most rotor field windings can draw up to four Amps.

The output of the stator windings is three-phase alternating current. A three-phase full-wave diode rectifier (two diodes per phase) rectifies the AC voltage produced by each winding of the stator to usable DC voltage.

The battery plays two main roles: it supplies current to the rotor field windings to produce a magnetic field and it acts as a capacitor to both draw and smooth the rectified power (current) from the stator of the alternator.

Alternators are generally very reliable. They do however have one main enemy - heat. Overheating or overloading an alternator will melt the varnish insulating the copper wire in the field windings of the stator very quickly and will also weaken the diodes that make up the rectifier.

With the exception of the bearings that support the rotor the field brushes are the only other part that is subject to wear. The brushes are not actually a brush but rather a precisely machined piece of carbon with an imbedded wire and which is quite fragile.

The brushes make contact with the rotor rings to supply current to the field windings. Each brush is held in contact with the rotor ring with a spring. Carbon dust that accumulates as a brush wears can cause it to stick in place and eventually the brush will not make contact with the rotor ring. This is perhaps the most common alternator failure, the second most common is worn out brushes.

Low output power is also a fairly common type of failure. This is usually caused by failure of one or more diodes in the rectifier. An alternator will continue to produce some power as long as at least one pair of the three phase diode rectifiers is still functioning. In such cases replacement of the diode rectifiers usually restores the full output power of the alternator.

The Voltage Regulator and Overvoltage Relay

The purpose of the voltage regulator is to maintain the electrical system voltage to a preset level. The voltage regulator preforms this function by controlling the amount of current that is supplied to the alternator field windings in the rotor.

This device may a be a mechanical relay type unit or a solid-state transistorized unit. Either type performs basically the same function. Most 12-volt system regulators are set to maintain the electrical system voltage at 13.8 volts.

So-called 28-volt systems are a bit of a misnomer since the battery voltage is usually around 24 volts (12 cells instead of 6), however the voltage regulator is set to maintain the electrical system voltage at 28 volts.

Voltage regulators accomplish this task by controlling the current that is supplied to the alternator field windings in the rotor windings and the amount of power that is generated by the alternator stator windings. This is the only function of a voltage regulator.

Solid-state voltage regulators respond faster and more accurately to loads on the electrical system than do the mechanical types and have the benefit of no moving parts to wear out. There are however two distinct differences in the way these units may fail.

Mechanical (relay type) voltage regulators almost always fail open-circuit either because a relay coil or a resistor burns out. In very rare cases the relay contacts may weld closed.

Solid state voltage regulators tend to be of a 5050% type failure - sometimes they short circuit and sometimes they open circuit. I have found more short circuit failures than open circuit failures so I tend to lean towards the possibility of a short circuit failure on the solid-state voltage regulators.

Q. What happens when a voltage regulator (mechanical or solid-state) fails with an open circuit?

A. No current can be supplied to the rotor field windings which turns off the magnetic field and the stator windings produce no power. The alternator is effectively turned off.

Q. What happens when a voltage regulator fails with a short circuit?

A. Assuming that this is not a catastrophic internal failure which would cause the alternator field circuit breaker to trip) the field windings would be able to draw the maximum possible current and would effectively turn the alternator ftill on. In this condition the alternator is producing its maximum rated power. You might think that this is not a problem, but due to rules governing inductors and capacitors an even bigger problem will develop: runaway output voltage. If there is not sufficient electrical load to use all of the available power from the alternator the voltage in the alternator stator windings will rise uncontrollably.

Both of these failures are a problem. The second failure, however, could do potential damage to the radios and other appliances in the electrical system. This is the purpose of that sometimes mystical and poorly understood overvoltage relay.

The overvoltage relay will open the alternator field drive circuit when the voltage in the electrical system rises above a preset point. In 12-volt systems this is usually 15 to 18 volts; in 28-volt systems it is usually 30 to 32 volts.

The over-voltage relay does not cut off the alternator output, instead it cuts off the alternator field drive which effectively turns the alternator off. This device is usually a non-resetting relay.

As soon as the overvoltage relay opens the field drive and the alternator is turned off the electrical voltage will drop back to about 12 volts, however the field drive is not reactivated. Otherwise the electrical system would oscillate in a high-low-high-low voltage condition.

In order to reset an overvoltage relay the alternator (or master) switch must be turned off for a few seconds and then turned back on.

By the way you might find it interesting to know that most automobiles do not have an overvoltage relay. Auto manufacturers don't think that an overvoltage condition is very likely to occur in a car. However, when it does it usually burns out all of the lights. Most other components (fans, heating elements, etc.) will survive the condition.

The Battery Master Switch and Master Relay

The Master switch is actually two switches in one. The left side controls the battery (by turning the electrical system on and off) and the right side controls the alternator. The battery must be turned on in order for the alternator to be turned on. Turning on only the alternator side will do nothing.

There is one very important distinction that should be made. The Alternator switch turns the voltage regulator on and off, not the alternator output. The voltage to the alternator switch comes from the alternator field circuit breaker which is tied directly to the aircraft battery through the master relay.

The battery master switch handles very little current, usually less than 0.5 Amp, the current it takes to drive the master relay coil. The master relay handles all of the electrical load (including the engine starter) which amounts to 200 to 400 Amps maximum (when starting the engine) and 30 to 70 Amps under normal operating conditions.

The Alternator side of the master switch is a different story. This switch handles all of the alternator field drive current (usually around two to five Amps). This is a significantly higher load on the switch. Yet this switch has a fundamentally more significant role than the battery side of the master switch.

The condition of the contacts (the contact resistance) in the alternator switch will affect the voltage that the voltage regulator will see. This is where Ohm's law starts to have an effect and this is where just about everyone seems to get lost and misinterprets the symptoms.

Using Ohm's law we can calculate the voltage drop across a resistor based on a specific current. So what does this have to do with a switch? Plenty!

If I could have a dime for every voltage regulator and alternator that were mistakenly replaced because of this switch I would be a very rich man. You see as this switch degrades due to oxidation of the contacts caused by internal arcing when it is switched on and off - the contact resistance increases.

Say for example the switch has developed 0.5 (which is very small) ohms of resistance and the maximum current required to drive the alternator field is four Amps. What voltage would be detected at the voltage regulator?

V drop = 4* 0.5 = 2 volts

V reg = 12 volts - 2 volts

V reg = 10 volts

Now the Vreg result is not strictly accurate, however the voltage drop across the alternator switch is. That means that the voltage regulator will see two volts less than the rest of the electrical system when four Amps is flowing to the alternator field.

A more accurate way to look at this is to say what voltage the rest of the electrical system is at.

With the voltage regulator preset to maintain a 13.8 volt level, then the rest of the system is at 13.8 volts + 2 volts or 15.8 volts when the regulator is seeing 13.8 volts!

This insidious problem only gets worse since the resistance in the switch causes it to heat up and further degrade the contacts. You may be surprised to know that this same 0.5 resistance at 15.8 volts and 4 Amps must dissipate 8 Watts of power!!! This may not seem like a lot but we are taking about a device that is not designed to dissipate heat.

Most of the time when a switch gets this bad it starts to act like (you guessed it) a Christmas tree light. As the switch contacts heat up the metal warps and the contacts open, the contacts cool down and make contact, the cycle repeats and presto, Santa is coming to town.

The result is a charging system that is being turned on and off just as if you were flipping the switch) and the result is a fluctuating Ammeter (load meter).

A second type of failure will also produce the same result. If the switch contact resistance increases significantly the voltage regulator may not be able to flow sufficient current to the alternator field before the voltage drops below the point at which the voltage regulator will operate.

When the voltage drops too low the voltage regulator turns off and stops flowing current to the alternator field. This allows the voltage level to return to normal, at which time the voltage regulator turns back on and the cycle repeats.

Once again you're flying a blinking charging system, except in this case the rate at which the system turns on and off, (fluctuates) will usually be very rapid (more than once a second).

I have to say that I know some very competent aircraft mechanics whom I trust and respect very much when it comes to mechanical work. I must admit through I wouldn't let them wire the lights on a Christmas tree let alone troubleshoot (at my expense) the electrical system on my aircraft.

I am frankly amazed at some of the bizarre suggestions I hear or read about on electrical systems on light aircraft. They are about as accurate as saying that the Earth is flat and the Sun revolves around the Earth.

The Ammeter (Load Meter)

Last but not least is the Ammeter (sometimes called the Load Meter). In some airplanes (and cars) the Ammeter shows the charge and discharge state of the battery.

In Cherokees the Ammeter shows only the charging current (the alternator output). Under normal conditions this charging current is equal to the electrical load on the system and this is why the Ammeter is a Cherokee is sometimes caller the Load Meter.

Keep in mind though that the Ammeter in a Cherokee only shows the output of the alternator. If the alternator is off line (turned off or broken) the Load meter will read zero.

Obviously one must assume that there will still be an electrical load that will continue to drain the battery unless the Master switch is turned off.

The Charging System

OK, now you know what all the major components of the charging system are and how they function, lets put it together. Figure I shows the most typical aircraft electrical system and is an accurate depiction of the electrical and charging systems in the Cherokee.

Note the dashed line between the breaker panel and the voltage regulator with the Alternator switch in the middle. This line provides three fractions:

1) Power for the voltage regulator's internal circuits.

2) Voltage sense of the electrical system.

3) Power (through the voltage regulator) to the alternator field drive (rotor windings).

Poor connections in the line can wreak havoc with the charging system. Many hundreds of dollars have been spent replacing perfectly good voltage regulators, alternators, overvoltage relays and who knows what else in an attempt to cure the "fluctuating" alternator output. In 99% of the cases it is a problem in this line. Most of the time it is simply a faulty or worn out Alternator switch, typically a $10 part.

If there is a problem in this line that limits the current that can be drawn by the voltage regulator (and the alternator field drive) several things will happen. When the voltage regulator senses a low-voltage condition it attempts to provide more current to the alternator field.

The more current that flows through this line the greater the voltage drop that occurs; the more the voltage drops in the line the more the voltage regulator attempts to provide to the alternator field. This vicious cycle continues until either the alternator is turned on full (and potentially causes an overvoltage) or the voltage drops so low that the voltage regulator can no longer function and shuts down.

As soon as the voltage regulator shuts down the current flow to the alternator stops and the voltage on the line increases. When the voltage increases enough to operate the voltage regulator the whole cycle repeats. The pilot sees a rapidly (usually more than once a second) fluctuating Amp (load) meter as the charging system turns off and on like a Christmas tree bulb.

The first thing most mechanics will do is to take the system apart, suspecting a faulty alternator or voltage regulator. Unfortunately this is the beginning of the expensive route to failure.

Why suspect a switch? Or a circuit breaker?

Few people realize the thousands of times that the master and alternator switch is turned on and off in the lifetime of an aircraft. Each time that switch is turned on or off a small amount of internal arcing occurs. This arcing degrades the metal in the switch contacts and gradually increases the resistance across the contacts.

This usually does not cause a problem for the battery side of the master since the actual current is carried by a remote master relay. The increased resistance across the alternator portion of the switch will cause problems. A contact resistance of a mere one ohm will cause havoc. As the voltage regulator attempts to provide more current to the alternator field the one ohm resistance in the switch (or anywhere in the line) will cause the voltage to drop one volt for every one Amp of current drawn by the voltage regulator and causes it to provide more field drive current than is required to maintain the correct system voltage level.

In the early stages of this problem there are may be a tendency to overcharge the battery and increase the overall electrical system voltage or may even cause the overvoltage relay to trip, leading to the incorrect suspicion of a failed or failing voltage regulator.

The output of the alternator goes to the alternator output breaker and is connected to the aircraft electrical bus. The load meter is placed between the alternator output and the circuit breaker to show the current flow into the aircraft electrical bus.

One of the reasons that the load meter AD was of so much concern is that if the stud on the back broke off and the alternator output was grounded out behind the panel it would occur before the alternator output circuit breaker and could be a fire hazard.

I am sure that many members have seen the ridiculous solution from Piper for the Load meter AD. I personally feel less safe with the Piper AD solution than with the original problem.

That is all there is to the Cherokee charging system. It is really quite simple. The charging system design on the Cherokee is virtually identical to most other charging systems in light aircraft, whether it is a Cessna, Mooney or Beechcraft. As simple as these charging systems are, the problems which occur are often blown way out of proportion. I have watched qualified mechanics laboriously take apart an electrical system before they even checked to make sure the Alternator field circuit breaker was working.

Of course the airplane owner is paying for their time (and the parts). I have also heard the war stories from aircraft owners who troubleshoot and service their own aircraft. Most of these expenses can be chalked up to a misunderstanding of the charging and electrical systems.

Symptom: Battery not being charged at all. No alternator output.

1) Check alternator drive belt for correct tension.

2) Verify that the Alternator switch is on.

3) Verify that Alternator Field breaker is not tripped.

4) Verify that Alternator Output breaker is not tripped.

5) Verify that alternator field drive has voltage.

6) Verify the voltage regulator input voltage.

7) Verify that the overvoltage relay is not open.

8) Verify voltage at Alternator Field circuit breaker.

Likely problems are:

1) Open circuit voltage regulator failure.

2) Faulty Alternator Field circuit breaker.

3) Stuck or worn out rotor brushes.

Symptom: Some charging is occurring, not enough to support electrical load. Battery dies after a short time.

1) Check the alternator drive belt for correct tension.

2) Remove alternator and perform output power test.

(Most reputable automotive parts shops can safely perform

this test).

Likely problems are:

1) Loose or worn drive belt.

2) Partially failed diode rectifier (One or two phases).

Symptom: Alternator shows a substantial charge rate and then trips off. Alternator Output breaker does not trip. Upon resetting the Master (or alternator) substantial charge is indicated and alternator trips off.

Likely problems:

1) Voltage regulator failure. Short circuit type.

2) Faulty Alternator Switch.

2) Faulty Alternator Field circuit breaker or connections.

Symptom: Fluctuating alternator output relatively rapidly, one or more times a second.

1) Check the alternator drive belt for correct tension.

2) Verify that no heavy loads are being switched on and off.

(Such as landing gear retraction motor, landing/nav lights, pitot heat etc.)

3) Verify all connections between the Alternator Field circuit breaker and the alternator field drive connections.

4) Check rotor brushes (if fluctuations are very rapid and vary with engine rpms).

Likely problems:

1) Faulty Alternator Field circuit breaker.

2) Loose/bad connection(s) between alternator field circuit breaker and alternator field drive connections at the alternator.

3) Faulty/intermittent overvoltage relay.

4) Faulty voltage regulator.

Symptom: Fluctuating alternator output relatively slowly, once every few seconds.

1) If retractable, verify that high pressure accumulator is not leaking. (This causes hydraulic pump to activate every few seconds to keep landing gear up).

2) Verify that no heavy loads are being switched on and off. (Such as landing gear retract motor, landing/nav lights, pitot heat etc.)

3) Verify all connections between the Alternator Field circuit breaker and the alternator field drive connections.

4) Remove alternator and perform output power test. (Most reputable automotive parts shops can safely perform this test.)

Likely problems:

1) Heavy load switching on/off (landing light, pitot heat, landing gear retract motor).

2) Faulty Alternator switch (right side of master switch).

3) Partially failed diode rectifier. (Temperature sensitive).

4) Faulty over-voltage relay.

5) Loose connection between Alternator Field circuit breaker and alternator field drive connections.

6) Faulty Alternator output circuit breaker.

7) Loose/bad connection between alternator output and alternator output circuit breaker.

The above steps should be performed in the order shown in order to examine the simplest items first. Keep in mind that simple parts can and do fail. In fact if your airplane is more than ten years old or has more than 2,000 hours on the airframe I would suspect the switches and circuit breakers before any of the active components. Switches and circuit breakers that are old sometimes just fall apart internally.

Finding the problem

So now that you have all of this wonderful information, how can you check your charging system? Actually you can check out 80% of the system quite easily with one piece of common test equipment; a hand held voltmeter (sometimes called a VOM or a Digital Multimeter, a DMM.)

All of the voltage measurements can be made without the engine running, just with both sides of the master switch on. When the engine is not running the alternator is not producing any output, therefore to test the alternator output I recommend that it be removed from the aircraft and bench tested. This is the most effective method for troubleshooting the alternator.

Removing the alternator on most aircraft requires a significant amount of work, therefore I recommend that you perform as much testing as possible with the alternator in place.

Since a voltage regulator is set to maintain the electrical system voltage at a higher voltage than can be produced by the battery when the electrical system (both master and alternator switched on) is on and the engine is not running, the charging system is fooled into thinking that a low-voltage condition is present.

As a result the voltage regulator will usually be turned full on (sourcing the maximum current to the alternator rotor field). This condition will benefit your troubleshooting efforts since it represents the worst case scenario of maximum charge rate (if the engine were running).

Check the Battery

Before measuring any voltages first make sure that the aircraft battery has been fully charged, has the correct amount of fluid in each of the cells and has clean battery posts with properly secured connections to the posts.

To measure the voltages at the various points in the charging system, connect your VOM or DMM negative (black) lead to a secure ground point on the airframe. A secure ground point should be a structural member where a connection to bare metal can be found. Ideally you would like to connect your VOM negative lead directly to your battery negative post, but this is usually not practical (especially in most Cherokees).

Everything is relative. Establish a reference.

The first step is to measure the reference voltage to which you will compare all of your other measured voltages. With the electrical system on measure and record the voltage on the buss bar behind the circuit breaker panel.

The buss bar is a piece of metal that is found behind the circuit breaker panel in your aircraft. All of the circuit breakers are usually mounted to one leg of this bar.

This voltage will be your reference voltage and should be 12 (or 24) volts, plus or minus 0.5 volts. If this voltage is too low there may be a problem with the battery or master switch.

Check the Voltage Regulator sense line

Measure the voltages at all of the connections in the line between the Alternator Field circuit breaker and the voltage regulator. The measured voltages should be, plus or minus 0.1 volt of your reference voltage.

If a lower voltage is measured check the part and or connection in question. For instance if you measure 12.0 volts at the input side of the alternator switch and 11.2 volts on the output side there is no question that the switch is faulty and should be replaced.

Verify that the Voltage Regulator is grounded.

Once the voltages on the line that feeds the voltage regulator have been measured the next step is to verify that the voltage regulator is properly grounded. This step is usually overlooked.

An improperly (or poorly) grounded voltage regulator will have you chasing a flock of wild geese. Checking the grounding of an electronic voltage regulator is simple. With the electrical and charging system on and your VOM negative lead connected to a secure ground, simply measure the voltage between the case of the regulator and ground.

Do not use the transistor (usually mounted on a heat sink on the outside of the case) as a measurement point. There should be no voltage measured. If there is any measurable voltage the regulator in not properly grounded.

For the old mechanical style regulators, simply select the Ohms ® x I if your meter has it) scale and measure the resistance between the case of the regulator and the airframe. There should be no resistance.

Of course you should check to verify that the regulator is properly secured and that there are no loose mounting screws.

Check the Alternator Field drive

Next task is to measure the voltage from the regulator to the alternator field. The engine should not be running. Measure the output voltage from the regulator. Be careful not to short the voltage regulator output to the airframe as this will destroy the regulator! The output voltage should be approximately one volt less than the input voltage. If there is no output voltage the regulator is no good. If the output voltage is very low (only 1 to 5 volts) the regulator is marginal and should be bench tested.

Next measure the voltage at the alternator field connection. There is usually only one wire running to the field of the alternator. The other side of the alternator field is usually grounded to the case.

The large wire on alternator is the output line. Measure the voltage on the small wire going to the alternator field. Again, be careful not to ground the field drive line as this will destroy your voltage regulator! This voltage should be within 0.1 volts of the output voltage measured at the regulator.

If the alternator field drive voltage is low a faulty line or connections should be suspected.

Check the Alternator Output

Next measure the voltage at the output line of the alternator. This voltage should be within 0.1 volts of the reference voltage measured at the buss bar. If the voltage is low a faulty line or connections or circuit breaker should be suspected.

When everything checks out OK ... but isn't

There are bound to be those systems that check out perfectly when measuring all of the voltages as I have described but still don't work right. In these cases I recommend taking the time to remove the alternator and have it bench tested to ensure that it is operating properly.

Once this check has been performed satisfactorily you have eliminated one of three active parts in your system (and usually the most expensive) from being the cause of the problem.

The last two remaining active parts are the voltage regulator and the overvoltage relay. How can you check them? Well earlier we measured the input and output voltages without the engine running and were satisfied with the readings. Now we will perform the same measurement with the engine running. Secure the aircraft to a tie down and set the brake.

Connect your VOM negative to a secure ground (airframe) and the positive lead to the voltage regulator input. Start the engine and set it to approximately 800 rpm. Note the voltage reading on your DVM. If it is 13.8 to 14.2 volts and is steady your first step is complete.

Now measure the voltage at the main buss bar (the same place the reference voltage was measured earlier). Verify that it is within 0.1 volts of the voltage reading at the input to the voltage regulator and should be within 13.8 to 14.2 volts.

If this is not true suspect a faulty connection between the buss bar and the voltage regulator input, most probably a high resistance connection or switch.

If your system is not charging but the alternator bench test checks out OK perform the following tests. Measure the voltage at the Alternator Output circuit breaker. Make sure that the voltage reading on both sides of the circuit breaker is the same. If there is more than a 0.1 volt difference the circuit breaker should be replaced.

Measure the voltage at the output of the voltage regulator. Turn on the landing lights and pitot heat. Verify that the voltage regulator output voltage increases more than 1 volt.

Measure the voltage at the input side of the regulator. Verify that the voltage is still 13.8 to 14.2 volts. If not then suspect a faulty regulator (this assumes that your alternator has passed a bench test).

If your alternator output is fluctuating check for a fluctuating input voltage at the regulator. If the input voltage is going below ten volts suspect a faulty regulator sense line, switches or connections.

One way to verify a fluctuating charging system is to use small jumper lines and jump across the Alternator circuit breaker (only temporarily for ground testing!) and Master Alternator switch.

If the fluctuating stops, your problem is in either the switch or circuit breaker or perhaps both. And of course, check the connections. If when the Master Alternator switch and circuit breaker are jumped the fluctuations don't stop make sure that there is no load being switched on and off.

Finally check the overvoltage relay connections and grounding. Make sure that the relay is properly secured. Measure the voltage across the relay connections. The reading should be 0 volts. If there is more than 0.2 volts across the overvoltage relay terminals or it is fluctuating, consider it to be defective and replace it.

Generally speaking overvoltage relays are inexpensive items. They are also very reliable, but they do deteriorate with age.

Fluctuating Current Demands

You may be surprised at the number of things that may turn on and off unexpectedly causing a fluctuating ammeter, yet the charging system is working fine. In fact it is doing the job it was designed to do.

Rotating beacons (that are near the end of their lives), some transponders, landing lights (with bad connections) and, yes, pitot tube heaters and landing gear systems can all turn on and off unexpectedly.

The most impressive case I have heard about (and perhaps the most common to retractable gear aircraft) is the landing gear pump problem.

A friend of mine with an Arrow was chasing the elusive fluctuating alternator, he tried everything. I tried to slow him down, but he had it figured out each time. I must admit I didn't quite know what the problem was myself but I did suspect a heavy load switching on and off... I just didn't know what it could be.

The tip-off should have been when he said "... everything checks out fine on the ground.. It doesn't start acting up until after I take off." The problem was ultimately found during the next annual.

With the gear retracted after about five minutes the gear pump would reactivate for a second or two. Thereafter, every minute or two this would occur.

The problem turned out to be a bad 0-ring in the high pressure accumulator, however the total cost was a new alternator, regulator, over-voltage relay and hours of frustration.

Remember Murphy likes airplanes too.

Making your system last...

So you don't have any problems and hope you never will. Well, perhaps if you follow these two simple suggestions you may never have a problem, or at least you will reduce substantially the chances of having one.

Have you ever asked yourself why the charging systems in aircraft fail so often. In reality the charging systems in aircraft should last longer than their automotive counterparts.

Why, you ask? Well consider that the alternator in a car is subjected to far more engine starts (requiring a heavy charge at low rpm), often has to sustain a heavy load during engine idle on hot days while you're stuck in traffic with the air conditioner running full blast. They have to operate in heat conditions that are well above 200 degrees F with poor ventilation.

Usually the alternator is mounted near the bottom of an engine and is subjected to repeated water drowning (complete with road grime and dirt) when driving in the rain. The alternator (and regulator) in your car is subjected to all of these things and yet easily survives ten years and over 100,000 miles.

I can honestly say that I have never seen a Chrysler alternator fail electrically in any vehicle or marine application either in my own vehicle or somebody else's. The one in my car (a 1964 Plymouth Valiant) has over 200,000 miles on it and is over ten years old. Ditto for the pickup and the station wagon.

This is the identical 60-Amp alternator installed on many Cherokees, the only difference being a little sticker that says "FAA-PMA Approved."

I have seen Chrysler alternators fail mechanically, the diode packs have fallen out or the bearings have seized but not electrically. So why does everyone curse their Chrysler alternator?

Probably because it was the alternator that was installed in the airplane. Talk to a Beech or Cessna owner and they probably don't have much good to say for Delco or Prestolite Alternators either.

The alternator in an aircraft is operating in a nearly perfect environment, this being relatively constant speed, constant load, low overall load, high rpm (best cooling and efficiency) in a fairly cool and relatively well ventilated space that rarely if ever gets wet and never sees road grime. The big question is why do they fail so much more often? The one main reason is overloading and the second is simply the design of the charging systems in light aircraft.

Often the alternator and voltage regulator have not failed, simply a switch or circuit breaker has become faulty. Why does this not happen in a car? Read on and find out.

Don't Overload the System!

You might say, "I can't overload if." And I will say "You do it every time you start your airplane!"

Yes folks that's the sad truth and probably the main reason aircraft charging systems have such a high failure rate! Yet there is a very simple thing that you can do that does not require any modifications to your aircraft whatsoever.

The two simple steps are:

1) Turn the Alternator switch OFF BEFORE starting the engine!!

2) Turn the Alternator switch ON AFTER the engine is running!!

What few people realize is that this same process occurs automatically when you start your car or boat or virtually any other machine with a charging system. This function is performed automatically by the ignition switch in your car if your car has an automatic transmission or manual transmission with a neutral safety switch).

Set the hand brake (or put your foot on the brake). Put your transmission in gear (any gear). Turn your ignition on, turn the radio or heater blower on (an accessory that works only when the ignition is on). Now, try to start the engine.

No, the engine does not start (the starter will not engage on those cars equipped with a neutral safety switch). Notice however that the radio and other accessories that were active with the ignition switch on turn off when the ignition switch is in the start position!

The switch is also cutting off the power to the voltage regulator (and shutting down the alternator) while the car is being started.

You may not have noticed all of this happening when you normally start your car and that is why I suggested that you place the car in gear to prevent it from starting so that you can take notice of what happens when the car is being started.

In an aircraft the alternator is usually turned on at the same time the Master switch is turned on. This applies full power to the alternator field since the voltage regulator is sensing the battery voltage (which is less than 13. 8 volts).

You then start the engine using the ignition switch. During the start power is available to the alternator through the voltage regulator.

As the engine is being started the alternator is turning fast enough to produce some power, however at the same time the starter is drawing 200 to 300 Amps.

Most alternators can only sustain 60 to 70 Amps, 200 to 300 Amps is equivalent to a dead short on the output of the alternator. In this situation the alternator is being substantially overloaded. The part most likely to eventually fail will be the diode rectifier.

Listening to the Radio...

Have you ever noticed how fast an aircraft battery seems to run down when you are just sitting in the airplane listening to the radio. All you have turned on is the Master and one or two radios and in an hour or two you end up with a low battery, right?

Sure the turn coordinator is running but electric gyros don't draw that much current, the radios certainly don't either.

Most radios draw less than one Amp in receive mode. If you have a typical 35 amp-hour battery you should have plenty of reserve. Yes all of this is true except one thing is missing - usually when you turn on the Master switch you turn on both sides of the Master (the battery master and the alternator switch).

So now the voltage regulator is on and providing up to four or five Amps to the alternator field in a vain attempt to generate some power and raise the electrical system voltage to 13.8 volts, but that won't happen because the engine is not running... right?

So if you want to sit in your airplane and listen to the radio for a while, or anytime you turn on the Master, don't turn the alternator on with the engine not running! It won't do you any good, it will drain the battery and, if you subsequently start the engine, it will overload the alternator during the engine start.

Chances are if you follow these two simple steps your charging system will last much, much longer, plus you will have the added benefit of not inadvertently running your battery down.

One last thing to remember: if you do adopt these suggestions remember to turn the alternator on after you start or you may find yourself with a dead battery in flight.

Above everything else, have a safe flight...

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