Engine Beat

Alternator maintenance: Don't let the smoke out.

That’s the BAT or B+ post on the left and the FLD or field tang to the right.

Alternators are converters. Through the magic of electromagnetism they transform mechanical energy from the engine into electrical energy used to start engines, power navigators, communicators and entertainers, and assist the pilot with tasks such as illumination and comfort. Alternators are compact, have few parts that wear and pack a lot of power per pound. Today kit builders are fortunate to have access to new production aircraft-specific charging system components from companies such as B&C Specialty Products, Plane Power and Zeftronics. Systems based on these companies products are dependable, but alternators do fail. Lets look at some tricks for understanding and troubleshooting alternator system problems.

Where the battery and alternator are proximate, its easy to compare B+ voltage with true battery voltage. You’re looking for resistance in the B+ line.

Type A, Type B

There are two types of alternator charging systems: Type A (sometimes called grounded regulator) and Type B (grounded field), which is the most common. Type B systems control field current before the alternator; Type A systems control the field current after the alternator.

Installation drawings from B&C and Plane Power detail Type B circuits. Type B alternators have one wire connected to the field (F) post on the alternator case. If a two-field-post alternator-with posts labeled F1 and F2-is used in a B circuit, the second field post is grounded to the alternator case with a short jumper wire. Some alternators used in B circuits have one field post; some have two.

The case of all Type B alternators must be grounded. If vibration-isolating or rubber mounts isolate the alternator from the bracketry, a hefty ground strap must be installed between the alternator and the engine. The amount of current through the field coils controls the alternator current output.

All alternator charging systems have voltage regulator circuits to control alternator output. Today, most charging systems also include over-voltage circuits to protect avionics and other circuits in the event of an over-voltage, low-voltage sensing circuits to warn of a loss of alternator function and in some cases ground-fault detection circuits, which guard against shorts in the field circuit.

Sniff & See If Smoke Got Out

Lets start with a couple of low-tech checks at the alternator. First, the smell test. Stick your nose down by that alternator and take a strong sniff. If it smells burnt, the alternator has overheated and probably needs to be removed, disassembled and inspected for internal damage.

Second, is a good visual inspection. Check for a loose or worn drive belt. Grab the alternator and give it a push and pull. Is it tight on its mountings? Look closely at the places where the different sections of the alternator body mate. Is there any evidence of movement between the sections or trails of black aluminum oxide at these joints? Look closely at the wiring. Does it look new and strong? Blackish or burnt-looking insulation means the wire has gotten hot. Grab the wires at the terminal ends and try to move them. Are the terminals tight? Look over the wires as they run back to the firewall. Are they clamped and tied securely, or are they abrading?

Alternator mechanical problems such as worn or corroded bearings are rare but relatively easy to detect. Rotate the alternator and listen for noisy or scratchy bearings by putting one end of a sound conductor such as a wood dowel or a screwdriver against the alternator case and the other end against your head just forward of the ear. If you hear what sounds like rocks being swirled in the bottom of a tin can, you have bearing problems and the alternator must be disassembled.

Quick Watson, the Meter

If these simple tests don’t reveal any faults, its time to move on to some electrical checks. Typical electrical problems boil down to a faulty field circuit due to extremely worn brushes, or alternators that don’t produce rated current or don’t work at all.

Testing is done with a multimeter. Sturdy, reliable, affordable digital multimeters are readily available and work well for these tests. They measure alternating current voltage (ACV), direct current voltage (DCV), resistance (in ohms Ω), current flow and in some cases diodes. Analog (moving needle) multimeters work for these tests, too.

DC Voltage Checks at the Alternator

DC voltage checks are easy because all thats required is to connect the (-) lead of the multimeter to a good ground, which is almost anything metal on the airframe or engine, and touch the (+) lead to the point you want to read. In actuality you cant hurt a digital meter if you connect it backward, but its good practice to always be aware of polarity.

Set the multimeter to a DCV scale that is higher than your aircraft system voltage (approximately 15 for a 12-volt system, or 30 for a 24-volt system), and turn on the aircraft master switch. Take a voltage reading at the big terminal (called B+) on the back of the alternator. It should be the same or within a few tenths of a volt as a voltage reading taken at the positive post of the battery. Next, turn on the alternator (or alternator field) switch, and read the voltage on the small terminal (labeled F or F1) on the back of the alternator. It should read between 0.5 and 2.0 volts lower than the battery. These tests determine if the problem is the alternator or the airplane circuitry. If the values at the B+ post and the F post are correct, this means the alternator is at fault. If either of these values is too low, then the wiring in the airframe part of the charging system is faulty.

Homebuilders get the benefit of using non-certified parts, such as this B&C alternator.

Alternator Field and Rotor Winding Check

The field circuit in the alternator runs from the F post input on the alternator case through one of two soft carbon brushes that are spring-loaded to bear against surfaces called slip rings on the rotor shaft, to the windings in the rotating coil. After passing through the windings in the rotating coil, the field current exits the windings through the second brush and terminates at F2 in the case of a Type A circuit system, or is terminated at the alternator case, which acts as a ground in the Type B circuit system.

Set the meter to a low ohms scale. Remove the wire connected to F1 (and F2 if you have a Type A system), and touch one meter lead to F1 and the other lead to the alternator case if you have a Type B system, or to F2 after you have removed the wire on this post if you have a Type A system. If the brushes, slip rings and rotating coil windings are in good shape, you should get a reading of between 3.5 and 6 ohms for 12-volt systems-double these values for 24-volt systems. More than 6 ohms means the slip rings are dirty or worn. Under normal conditions slip ring brushes last a long time, but dusty or dirty operating conditions will shorten brush life. Plane Power, a Texas company that manufactures a line of high-quality alternators for both certified and Experimental airplanes, told KITPLANES that engine oil will cause brushes to wear rapidly, so keep your alternator clean. Finally, a reading of less than 3 ohms may mean the windings in the rotating coil have shorted together.

Easy First

Airframe-based charging system problems are almost always caused by high resistance at one or more of the many terminals, switches, circuit breakers or connections in the system. Fortunately, these kinds of troubles are fairly easy to find with a voltmeter.

First, take a voltage reading at the positive terminal of the battery, then turn on the master switch and take a reading at the aircraft electrical bus. If the bus voltage is lower than the battery voltage, there’s resistance to current flow between the two points.

The resistance is most often in the return, or ground leg of the circuit. This resistance may be at the clamps to the positive and negative posts of the battery. It may also be at the battery box where the battery ground strap attaches to the airframe. Disconnect those and clean them. Reconnect them and test again. Experienced aircraft electricians know that a series of seemingly unrelated electrical system problems are often resolved by first looking for corrosion in the ground path of a circuit.

The voltage at the battery is the benchmark. Lets walk through a series of voltage checks used to locate resistance in a typical alternator field circuit. One of the rules about troubleshooting is to always do whats easiest first. Lets say the voltage at F1 is outside the range (0.5 to 2.0 volts less than bus) specified above.

Using our easiest first rule, as the cowling is already off, the next step would be to do voltage checks at points between the voltage regulator and F1 on the alternator. There shouldn’t be any resistance between these points. If the voltage drop isn’t in the connections in that leg of the circuit, then its time to remove the pilots and maybe copilots seats, rig a trouble light, and scrunch under the panel for access to the circuit breakers and other connections aft of the firewall.

Every Step You Take

With the negative meter lead grounded, touch the positive lead to the aircraft bus with the master switch on. Then touch it to the leg of the alternator field circuit breaker that is screwed to the bus. There shouldn’t be any resistance at that connection. Next, touch the probe to the other side of the circuit breaker and compare that reading to the bus voltage.

Checking the field wire for voltage: It will be less than bus voltage, but should be present even when the engine is stopped. No field voltage would explain why the alternator is not putting out.

Lets say the readings are the same, so we know there’s no internal resistance across the circuit breaker. Touch the probe to the next connection between the bus and the regulator. This will likely be the closest side (electrically) of the alternator (field) switch. If there’s a voltage drop-make sure the switch is on-then we know there’s resistance in the wire connecting the circuit breaker and the switch. Maybe the connection is loose (tighten it) or there’s corrosion or a poor crimp on the terminal fitting. Repair the wire and test again.

Step-by-step voltage checks at every connection and across every component from the battery to the alternator are the keys to success. There shouldn’t be more than a 0.1-volt drop between the bus and the input at the voltage regulator. There will be a drop across the regulator-thats normal. Plane Power alternators have internally mounted voltage regulators and over-voltage protection. This eliminates the potential for resistance buildups in the circuit between the alternator and externally mounted voltage regulators and over-voltage protection circuits.

Raspy Rectifier

If your battery is always weak, and you hear a high-pitched whine in your headsets that changes pitch with changes in engine rpm, you’ve probably lost a diode in the alternator bridge rectifier. The raw output of alternators is three-phase alternating current (AC). When a single diode fails, the alternator output (in watts) drops by about half. A 70-amp, 12-volt alternator will be able to support a load of approximately 30 amps and still maintain a 14-volt output. If the load is increased to 40 amps-by turning on a landing light, for example-the output voltage will drop by 1 to 2 volts, which causes lights to dim and an incomplete battery charge.

The loss of a diode cuts back on system output, and it lets more of the raw AC pass through the rectifier to the alternator output and on to the aircraft bus. This AC component causes the headset whine. The VAC scale of the multimeter can be used to detect the AC. Put the negative lead to ground (any bare metal thats part of the airframe) and the positive lead to the aircraft bus with the engine running. If there’s more than 0.5 volts AC on the bus, you have lost a diode and the alternator must be repaired or replaced.

A Couple More Hints

Occasionally the whine in the headset is caused by a fault in the voltage regulator. This whine rises in pitch with increased electrical load and can be detected by turning on a landing light at a steady engine rpm.

Gear-driven alternators are connected to engine gearing through clutch-type devices called elastomer drive couplings. V-belts and drive couplings are used to prevent damage to the engine in case the alternator rotating parts seize. If the alternator tests out well, all of the voltage checks are good and your alternator is still not putting out rated current, inspect the drive coupling and V-belt. If the drive coupling can be turned by hand while holding the alternator, the drive coupling should be replaced.


Please enter your comment!
Please enter your name here

This site uses Akismet to reduce spam. Learn how your comment data is processed.