I just got off the phone with my mechanic. He says T&W claims that the alternator we sent to them -- my original alternator, the Kelly that suddenly failed to put out enough to even charge the battery two weeks ago -- is perfectly good and putting out its 70 amps rated current.
Other than the "bad stator" (or was it rotor?) comment I don't think I ever heard any evidence that the original alternator was bad. One thing that's pretty certain is that T&W can test an alternator pretty darn thoroughly. An intermittent (e.g. heat sensitive) problem might go unnoticed but IIRC your original problem wasn't intermittent.
Now we're in a total quandary and have no idea what to try next. Obviously we're not going to put that one back in my airplane! But what else could possibly be wrong?
Here were my mechanic's thoughts:
1. Belt slippage could still be a possibility -- he suggested that he could sand alternator pulley a little. I hesitate to do this for fear that it might accelerate belt wear. Is this a realistic concern?
I'm no expert on belts and pulleys but roughing up the pulley's surface doesn't sound like a good idea to me. People have been depending on power transmission via v-belts for about a hundred years and roughened pulleys are not the norm. A belt can become glazed by repeated slipping but given that you said that turning the alternator pulley turns the engine, belt slipping seems unlikely. One possible simple test: a belt slipping under load will get very hot (and probably smell bad). There is a more complicated test (easy if you have the right tools) that involves measuring the frequency of the ripple (variations in the output voltage) which should be proportional to the engine RPM (double the RPM and double the ripple frequency). The ripple frequency can be measured with a dedicated alternator tester or with an oscilloscope, I'm guessing your mechanic has neither. Third option is to paint a white mark on a black pulley or vice versa and use an optical tach on the pulley to compare the proportionality with your engine tach. A lot of mechanics have optical tachs for checking prop RPM during annuals. Some require shining a bright light on the rotating part, some don't.
2. Another thought he had is that my electrical cabling, which is bundled together in salmon pink tape, might have some hidden fraying internally. This makes no sense to me. He measured 12.5 volts max, under peak load, at the output terminal of the alternator. I can't think of any way that frayed cable downstream could account for that. Besides, even at maximum load there is not much drop between the alternator output terminal and the bus -- at most 0.5 volts. He did measure the actual output wire current with his inductive ammeter under maximum load and found ~25 A.
You're getting better at this than your mechanic, perhaps he should pay you for the education. You are absolutely correct that the low voltage on the alternator's Bat terminal (measured at the stud, not the wire) as well as the minimal drop from there to the main bus indicates that the wire is carrying the load just fine. Same goes for the crimped terminals (the metal thing on the end of the wire that has a ring that goes on the alternator stud or the CB screw terminal), if there was a bad crimp the voltage on the alternator would be normal or high, not low.
There's no magic here, if you have good field voltage, no slippage, and weak output, there is a problem in the alternator. It might not be the only problem but it does need to be fixed. The only other alternative is that you aren't measuring things correctly. I mentioned grounds before, the alternator is normally grounded to the engine by it's mounting bracketry and the engine is grounded to the airframe with a woven strap. Chances are that if the engine grounding strap was resistive you would have problems with the starter turning slow, and it's pretty unlikely that the alternator isn't making good contact with the engine. But just to cover all the bases, you could measure the voltage between the alternator case and the airframe, it should be well under half a volt. Another redundant test would be to measure the field current, it should be in the vicinity of 3-4 amps with the engine stopped (master and alternator on) or with the engine running and the alternator loaded enough to drop the bus voltage.
One thing I can't tell from here is whether or not your alternator field connections are what's typical (one end grounded the other powered through the regulator) but since you said there were only two terminals on the alternator that almost has to be correct. My alternator has four terminals, two are for the field (one is connected by wire to ground) and in addition to the output (Bat) terminal there's one called "aux" which feeds my undervoltage detector. If yours had a second field terminal (for the ground side) it's possible that connection is resistive and there's a slim chance the regulator controls the ground side of the field with the positive side always at bus voltage. Checking the field current would put those (unlikely) concerns to rest.
Any thoughts from the gallery would be more than welcome! I am going crazy here trying to decide what to tell him to try next. My instincts tell me it has to be either the alternator is bad or the belt is slipping since the alternator is not putting out its rated current and that chasing bad wiring downstream is a waste of time.
Good instincts.
If these alternators are really good then it has to be the belt. But are they good? If T&W turns out one bad alternator after another, can I really trust their bench testing? Yet if they say they got 70A out of that alternator, it's hard to imagine how that can be in error.
It's hard (and wrong) to trust anything completely you don't witness
yourself but I expect they are giving you correct information. Are they also saying they did nothing to repair the alternator? What happened to the "bad stator/rotor"? Was T&W the source of that diagnosis? And don't forget you could have two problems at the same time meaning there's something wrong in addition to the currently bad alternator.
There is one other really unlikely possibility, the alternator pulley could be slipping on it's shaft. Some if not all are keyed but the key could be missing. But like I said, that's a really long shot.
Does that help? Anyway I'm nearly certain mine has only one field wire -- my mechanic pointed out the two wires coming from it, output and field. If it's grounded via its mounting bracket as you say, that's all there should be.
). I gather they did not repair the alternator -- I didn't ask specifically, but I had the impression that they had no reason to, it bench tested up to spec. But I have no idea how extensively they tested it.
