Taking Charge of Alternator Problems

Taking Charge of Alternator Problems Single Engine Alternator Charging System Troubleshooting By Winston Greer and Mike McCluskey April 2001 If you've ever had an "out-of-box failure" of a replacement alternator then you need no description...


Pulsing ammeter
Another "first scenario" common complaint is an oscillating or pulsing ammeter needle. Like all other troubleshooting problems, knowing where to start looking for solutions will conserve your time and temper. Usually this problem is caused either by the field circuit breaker, the alternator switch, or both. However, the regulator over voltage sensor might also be bad. To diagnose the problem, turn the electrical system on without starting the engine and measure the voltage coming off the output terminal of the alternator. Next, measure the voltage on the input (power) wire of the regulator and compare the two readings. If there is more than one-half (0.5) volt difference between these readings, then, as you recall from our earlier comments, either (1) the circuit breaker is defective; (2) the alternator switch is defective; or (3) the over voltage sensor is bad. Note: You could have any combination of the three. At this point, check the input voltage of the circuit breaker and compare it with the output voltage. Again, if there is more than one-half volt difference, the circuit breaker must be replaced. Then perform the same input-voltage versus output-voltage test on the alternator switch. There cannot be more than one-half volt difference or the switch must be replaced. Voltage differences inside the switch or circuit breaker originate at the contacts. When they lose the dielectric grease they are packed with, they arc and pit and oxidize on the contact surface. These compromised surfaces eventually lose the ability to properly conduct current. The outcome is very much like magneto breaker points that have been run a long time. Although it is rare, one last possible cause of an oscillating or pulsing ammeter is the regulator itself. There are some early regulator designs that operate on such a low frequency that they will cause the ammeter needle to pulse at low RPM with a moderate load. If you have one of these regulators you have two choices: (1) live with it; or (2) upgrade to a newer, high frequency regulator.

"Nuisance tripping"
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figure 3

The final problem we’ll discuss could fall in either of our two scenarios: an alternator that drops off line for no apparent reason. A simple cycling of the alternator switch temporarily corrects the problem. For obvious reasons, this is frequently referred to as "nuisance tripping." The first thing to check for is a poor connection in the remote sense or field wires. Some regulators are very sensitive to ambient electrical noise and any intermittent condition will cause the system to trip off line. If connections are confirmed as good and the problem persists, then look at the alternator itself. Even the slightest scratch on the slip ring of the rotor can cause nuisance tripping. Inspect the brushes for excessive wear; especially note if the copper wire is showing, and if it is then the brushes must be replaced (see Figure 3, page 32). When the brushes are too short, the brush spring may no longer maintain proper pressure for brushes-to-slip ring contact. Before reassembly, the alternator slip ring must be resurfaced. Don’t ruin your good work Image

figure 5

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figure 4

on the slip ring (see Figure 4, page 33); use a piece of plastic or plastic-insulated wire (you can even use a toothpick)as a brush retainer in the brush block (see Figure 5, opposite page). Bare metal wire can scratch the slip ring when it’s pulled out of the brush block to release the brushes once reassembly is complete. It doesn’t take very much arcing between the brushes and the slip ring to trip some systems off line and even a tiny scratch can initiate or perpetuate the "dropping off line" problem. If you’ve looked at the alternator and are confident that it’s not the source of the problem, it is very probable that the regulator overvoltage circuit or separate overvoltage sensor is failing. Some systems have no overvoltage protection at all, some have regulators with overvoltage protection built in, and others have a separate overvoltage sensor. Make sure you know which you have before replacing them.
ImageThis has been a very basic survey of single engine electrical system troubleshooting. Just remember one thing: if you can’t figure out what’s wrong don’t hesitate to call a manufacturer’s Technical Service Representative. They probably can save your time and frustration, your customer’s money and both you and your customer a few gray hairs!

"Radio Noise"

It’s well known that charging system components can affect radio communication in the form of noise. Failing diodes or a bad stator will normally generate a whine in the headset that will vary in pitch with the R.P.M. of the engine. Radio noise falsely attributed to leaking diodes or a partial short in the stator can be one of the hardest problems to track down and solve. Given a new manifestation of radio noise, one of the easier checks is for alternator wires situated too close to the antenna cables. Alternators inherently make some electrical noise and if wiring has recently been re-routed too close to antenna cables, the radio will pick it up. If you have shielded wiring going to or from the alternator, make sure both ends of the shield are properly grounded and that you have good continuity to the airframe. It’s also advisable to inspect the connections at the battery. These connections should be perfectly clean, bright and tight. Often overlooked is the connection of the ground strap from the engine to the engine mount. These connections tend to corrode over time and you cannot see the corrosion without taking the ground strap off. If both ends of the connection are clean and tight and the problem persists, then unfortunately everything else has to be checked. Be certain to look closely before deciding all is OK; the problem of corrosion can be very subtle. It only takes about 0.2 ohms resistance on the ground plane to cause radio noise (and a variety of other problems). A friend had an older airplane with the battery mounted in the rear. He had radio noise, slow cranking R.P.M., apparent alternator problems, and several other electrical problems. It was recommended that he run a dedicated ground wire from the battery to the firewall. When he did, all his electrical problems went away. Over the years, the aluminum skin of the aircraft had oxidized where it was overlapped and riveted, putting too much resistance in his ground plane. Regrettably, all solutions are not this simple. Once all potential sources of radio noise have been investigated and eliminated, you may have a genuine avionics problem. Your local avionics shop will have to be consulted for further investigation and diagnosis.

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