Charging Systems

[Bob Steward writes] The Basics: The battery stores power chemically, the alternator converts the engine's rotating motion into electrical energy, the Voltage Regulator controls the alternator's output to keep the battery charged, but not over charged (which results in boiling out its acid).

Your battery stopped charging during your last flight. You shed loads by turning off unneeded equipment until you arrived at the airport and landed normally. Congrats on handling the inflight portion properly. Now how do you get it repaired?

You need to find out exactly which part failed. The usual FBO response is to replace the alternator and the voltage regulator. They don't have to do much troubleshooting and they sell lots of expensive parts.

If you have a $10 Volt-Ohm meter from Harbor Freight, you can save lots of time and money.

All these tests can be done with the engine OFF and the keys in your pocket.

Open the cowl and locate the voltage regulator. On Grumman-American singles its on the passenger side just inboard of the motor mount attach point, directly above the battery. On AA-5x you'll see a connector on the bottom with several wires. Look closely and you'll see that they are marked with letters on the regulator.

Locate the "S" terminal and turn on the Master. With the VOM, measure for voltage between the S terminal and a good ground. You should read about battery voltage, or 12.6 volts on a fully charged battery. Because your battery is not fully charged after the in flight failure of the charging system, anything equal to or greater than 10 volts is acceptable. If voltage is present, then your Master Switch, Master Relay, Alt Field Circuit Breaker and the associated wiring are all working well enough to charge the battery from the alternator. Continue on.

Locate the "F" terminal and measure for voltage just as you did at the "S" terminal. If the measured voltage is within a couple of volts of the battery voltage, then the regulator is correctly trying to stimulate the alternator into making current. Therefore, your problem is elsewhere.

Remove the "F" wire from the regulator and check for continuity between the wire and ground. Make certain to reset the VOM to measure resistance instead of voltage. You should measure less than 10 ohms of resistance, and usually less than 5 ohms. If you have significantly higher resistance (say 20 or more) then there are problems with the alternator field or brushes, or possibly the wiring going to the alternator is in bad condition. Otherwise, since you have "good" continuity to ground on the "F" wire, you are through checking the easy stuff, and are ready to pull the nose bowl.

Turn OFF the Master at this time. We are through with the powered portion of the testing.

Right about now you are wishing that you had installed the split nose bowl STC, because without it you have to remove the propeller to gains access to the alternator.

On Travelers before 75, you can check the wiring by removing the screws on the righ-hand lower cowling and leaning it out (ed: ??) just enought to enable you to reach under the front cylinder and touch the alternator. On 75 Travelers with the "Cheetah nose" and on all Cheetahs and Tigers, you have to remove the nose bowl to continue troubleshooting. That's OK, because you have already narrowed the source of the problem to that area anyway.

With the nosebowl off, or at least the lower nose off if you have the split nose, now remove the alternator belt from the pulley. Our testing so far has not ruled out a worn, loose, or missing alternator belt, so make certain that the belt is in good condition.

If the belt itself is in good condition and is tensioned correctly, the next step is to remove the alternator for inspection. The first step is to release the tension on the belt. To do this, cut the safety wire on the bolt holding the alternator to the bracket and loosen the bolt enough so that you are able to push the alternator up, thus relieving the belt tension.

Next, remove the big bolt at the bottom of the alternator (the one that holds it to the engine mounting bracket). While supporting the alternator so the wiring is not stressed, remove the bolt that was safety wired. The alternator should slide off the engine mounting bracket and be attached only by the wiring on the back.

Carefully turn the alternator around, and lift the rubber boot that hides the connection of the big wire to the back of the alternator. This is a VERY common failure point on the aging fleet. The wire breaks off the ring lug that is bolted down, and the alternator output is prevented from flowing back to the battery by the broken connection. Look also at the small wire that is bolted to the same connection. This the "A" or sense wire which tells the Voltage Regulator what the alternator is doing. These sense wires also break, though not as often as the larger output wire. Look at the other small wire which attaches to a different post. This is the "F" or field wire. If the resistance checks were normal earlier, then this wire should be fine; nevertheless, after 30+ years in service all these wires bear warrant close examination. Generally inspect the wires for large cracks in the insulation, or, where appropriate, the shielding.

If there are no obvious failures in the wiring, you need to mark them and remove them so that the alternator can be tested at a shop. Pep-Boys and AutoZone will check your alternator for free. Tell them its a 71 Ford Pickup with AC. That is the actual original application for the DOFF-10300J alternator on your plane.

After the alternator is spun up and bench tested, all but the rarest and intermittent failures will have been diagnosed, and you will be repairing JUST the wiring, or replacing the alternator or the voltage regulator, and NOT spending money on needless repairs.