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 boils out the acid.

Your alternator quit 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. Since yours is not fully charged after the in flight failure of the charging system, we'll be happy with anything higher than about 10 volts at this point. If there is voltage, then your Master Switch, Master Relay, Alt Field Circuit Breaker and the associated wiring is all working well enough that you should be getting a charge from the alternator. Continue on.

Locate the "F" terminal and measure for voltage like you did at the "S" terminal. If there is within 1-2 volts of the battery voltage, then the regulator is trying to turn the alternator on, and your problem is elsewhere.

Remove the "F" wire from the regulator and check for continuity between the wire and ground. Remember to change the VOM to measure resistance, since you had it set for voltage. You should see less than 10 ohms of resistance, and usually less than 5. If you have significantly higher resistance (say 20 or more) then you have alternator field or brush problems, or possibly the wiring going to the alternator is in bad shape. If you have "good" continuity to ground on the "F" wire, then 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 prop to access the alternator.

On Travelers before 75, you can check the wiring by removing the screws on the RH lower cowling and leaning it out such that you can 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 we have already pin pointed the problem as being in that area anyway.

With the nose off, or at least the lower nose off if you have the split nose, you will need to remove the alternator belt from the pulley. The belt LOOKS OK, doesn't it? Our testing so far has not ruled out a worn, loose, or missing alternator belt. Cut the safety wire on the bolt holding the alternator to the bracket, and loosen it enough to push the alternator up, releasing the belt tension. Remove the big bolt at the bottom of the alternator that holds it to the engine mounting bracket. Support the alternator so the wiring is not stressed, and remove the first 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 the "A" or sense wire which tells the Voltage Regulator what the alternator is doing. These also break, though not as often, it seems. 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, though there is no telling how worn it might be after 30+ years in service.

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.