System similarities & differences
There are three basic types of alternators used on general aviation aircraft: the Ford style, the Chrysler style, and the Prestolite style. The basic principle remains the same for all three models: the alternator’s job is to produce AC and convert it to direct electrical current before leaving the device. In so doing, the alternator provides the direct current required by the aircraft instrumentation and equipment.
Distinctions between these models are minor. The greatest difference lies in the wiring configuration of the voltage regulator and the alternator. In some installations, the current-controlling element of the voltage regulator is in series between the A/C bus (direct current) and the alternator field (F1). Only one field terminal will appear on the back of the alternator, with another being internally grounded. Alternators with single-lead brush racks will always be wired in this manner. In other installations, the current-controlling element is located between the alternator field (F2) and ground. In these installations, the alternator will have two field terminal connections available. Alternators with two-lead brush racks can be wired either way.
Grounding of field terminals
One common technical assistance request is diagnosing a newly installed alternator that doesn’t work. Most Prestolite-style alternators have two field terminals: one must be grounded directly or through the regulator. Some airframe manufacturers install a very small metal tab going from the F2 terminal to the brush holder screw. Better look close, though; given a well-used alternator, if you don’t know it’s there, you’ll never see it (see Figure 2).
Identifying the failure mode/isolating the cause
Now back to the first scenario: "The alternator was working OK and now it isn’t." Presuming that an alternator bearing failure is not involved, that connections all check OK, and correct belt tension has been confirmed for those belt-driven units, then use your multimeter to check and see if there is resistance on the alternator field. If the field is open, then the culprit is a bad rotor or brushes. If the field checks OK (generally 3 to 25 ohms), the next step would be to make sure voltage is getting to the field. If not, then it’s the regulator or the wiring. Determine whether or not voltage is getting to the regulator; if so, then the regulator most probably is the culprit. If everything is checking as it should, by default, things continue pointing to the alternator as the source of the trouble. There is one more test to make before you remove it from the engine — use an analog ohmmeter to check the resistance between the output terminal and ground (a digital ohmmeter won’t work). This is a reverse polarity test so you have to ground the positive probe and contact the negative probe on the terminal. The reading should be between 30 and 50 ohms; a lower reading than this indicates the stator or diodes are gone, and the alternator must be repaired or replaced.
Low current output
The next most common variant of the "It was working OK" scenario is low current output (this can cause a technician to consider changing careers!). As with most other problems, there can be several reasons for the manifestation of low current output — the most frequent one being a shorted or burned stator.
A failed diode is the next most-common suspect immediately following a suspected stator. With a failed diode, you will likely experience radio noise. Modern diodes are much more reliable and durable than those used even just ten years ago, having a much higher mean-time-to-failure life expectancy. Core units returned to our shop have had the stator shorted and burned with fully functional diodes; however, this is the exception instead of the rule. The point is that diodes today can handle the stress of stator failure better than ever. Still, it’s not advisable to reuse diodes that have survived a stator failure due to induced stress, coupled with a high probability of damage and subsequent loss of durability.
Another possible cause of low current output is a partially shorted rotor. In this case, the wires in the rotor coil short to each other but not to ground. This lowers the resistance of the coil, thus lowering the magnetic flux of the rotor and the output of the alternator.
Brushes are yet another culprit that can certainly contribute to low current output. Be certain to inspect the brushes. Are they worn? Are they making positive contact with the slip ring on the rotor? Most alternator manuals give minimum lengths for brushes. If you happen to disassemble an alternator for any reason and it’s been in service for a while, always measure the brushes.
For gear-driven alternators, the last and certainly ugliest suspect for low output is the one that your customer really doesn't want to hear about: "The coupling gear is slipping." As you may already know, the reason your customer doesn’t want to hear about this is that in many cases, the replacement cost of a coupling gear is two or three times the replacement cost of an alternator. Coupling gears have rubber inserts that act as a torsional buffer; the inserts are designed to shear to prevent damage to the engine in case the alternator stops suddenly (for example, if something gets inside the alternator and locks it up). The rubber material of the insert tends to harden with age and heat and will eventually allow the two halves of the gear to slip. A good indication that thecoupling gear is responsible for low alternator output is when the system works fine with light electrical loads but the amperage and/or the voltage starts dropping as you add additional load. Engine manufacturers’ service bulletins explain how to test coupling gears for minimum slip torque. Also, if you are experiencing high voltage with low output, this can be caused by leaky switches and circuit breakers. If these components have never been replaced, then now is the time to consider doing so.
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