Charging Systems
Mechanical Difficulty:
[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. The field Rep for Kelly Aerospace, one of the largest OH shops for alternators, came to an FAA Safety meeting that I attended. He told us that 8 out of 10 alternators returned to them as cores for OH worked when tested before disassembly. He stated that we as A&P mechanics in the field were NOT doing our job properly, and that we needed to apply better troubleshooting techniques rather than repalce so many functional alternators.
If you have a Volt-Ohm meter, you can save lots of time and money. Inexpensive voltmeters are widely available from such vendors as Harbor Freight and Radio Shack, often for as little as $10.
All the troubleshooting described below is 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. Using your VOM, measure the voltage between the S terminal and a good ground such as the battery's negative terminal. The VOM should read close to 12.6 volts on a fully charged battery. In this example, however, your battery is not fully charged after the in-flight failure of the charging system, so a measurement equal to 10 volts or greater is acceptable. If an acceptable 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. Therefore, your problem is elsewhere. Continue on.
Locate the "F" terminal and measure the voltage just as you did at the "S" terminal. As before, if the measured voltage is within a couple of volts of the nominal battery voltage, then the regulator is correctly trying to "excite" the alternator field so that the alternator can produce an output. Therefore, your problem is elsewhere. Continue on.
Reset the VOM to measure resistance instead of voltage. Remove the "F" wire from the regulator and measure between the wire and ground. You should measure less than 10 ohms of resistance, and usually less than 5 ohms. If you have significantly higher resistance (say 20 ohms or more) then there are problems with the alternator field or brushes, or possibly the wiring going to the alternator is in bad condition. A reading of less than 5 ohms, would indicate a short in the Field wire or inside the alternator. Otherwise, since you have the appropriate resistance between the "F" wire and ground, 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, so turn off the Master switch.
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.
Mechanical Difficulty: 
Up to this point, we were just CHECKING electrical measurements without any actual disassembly of the plane. If you do not have the split nose bowl STC, then this is the point at which you will require A&P supervision to complete the testing, removal and replacement of the alternator, prop, mounting bracket, etc. FAR 43 Appendix A paragraph (c)(19) permits the owner/pilot to remove and install cowlings, if the prop does not have to be removed, without A&P supervision, after demonstrating the ability to do it correctly.
On Travelers before 75, you can check the wiring by removing the screws on the right-hand lower cowling and letting the cowl side tip out from the firewall and nose bowl just enough to enable you to reach under the front cylinder and touch the wires on the back of 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 nose bowl 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 and properly tensioned.
Folks tend to underestimate the amount of tension required on this belt. Just recall that the amount of power required to turn the alternator increases as the SQUARE of its output (charging) current! Lycoming's service information was updated a couple years back, and they now suggest setting belt tension by checking to see if the belt will slip before reaching a specified torque. You can check this by putting a socket on the alternator drive pulley nut and setting a torque wrench to 540 in-lb, and then holding the prop and trying to turn the alternator (CCW when viewing it from the front) with the torque wrench. The wrench should indicate 540 in-lb BEFORE the pulley slips. The older method mentioned in the Maintenance Manual, no longer favored by Lycoming, was to press in the center of the belt span between the pulleys and measure the deflection. New belts were to have less deflection than those that were worn. Refer to the maintenance manual if you don't have a torque wrench handy.
If the belt itself is in good condition and is tensioned correctly, the next step is to remove the alternator for inspection. First release the tension on the belt. To do this, cut the safety wire on the bolt holding the alternator to the bracket, then loosen the bolt enough so that you are able to push the alternator up, thus relieving the belt tension.
In the next step, you remove the alternator, which is quite heavy. If you have a portable service table or stand, now would be a good time to position it under the alternator. Now 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. The sense wire is also a common failure point, 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 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 from 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.
Prior to installing the alternator, carefully inspect the U shaped mounting bracket for cracks. They are prone to crack across the middle of the U, often into one of the bolt holes. Replacement brackets are available from any Lycoming distributor or from FletchAir.
