From: "Robert L. Nuckolls, III" Subject: Re: Alternator tripping offline Date: Mon, 06 Jul 1998 21:27:36 To: "Grumman Gang" At 10:06 PM 7/6/98 -0500, you wrote: > my alternator is tripping offline every 30min now... (still had great > flight to/around NH this weekend!) Had a fluke attached to cigarette > lighter and from its 3 times a second update it looks like its NOT the > over voltage relay. (unless the OV relay of before the regulator) The > graph is exactly like one would expect, a regulated 28.5 to 28.8 when > the alternator is working with a quick drop to 24.2 and slow discharge > of the battery from there. http://www.logan.com/cht/images/volts.gif When it trips off . . how, if at all, do you get it back on line? Does it "reset" by turning the alternator field switch off and back on? If it does, then your ov relay may be geting flakey. If not, the next step is to measure the output from the field terminal of your regulator. If the field voltage goes UP when the alternator goes off, the alternator is probably bad (brushes?, wiring?). If the field voltage goes down or to zero when the alternator is off, then move your measurement point to the "BAT" or "BUS" terminal of the regulator. If the voltage falls here also, then wireing between bus and regulator is suspect including all intermediate pieces of hardware . . breaker, switch, connectors, ov relay, etc. If the voltage doesn't go to zero, then the regulator is suspect. Bob . . . //// (o o) ===========o00o=(_)=o00o========= < If you continue to do > < What you've always done > < You will continue to be > < What you've always been. > ================================= From: "Robert L. Nuckolls, III" Date: Thu, 11 Mar 1999 17:21:01 Here's an excerpt from a recent update to my book . . . Note 8. When an alternator quits alternating, good data is useful in making an accurate diagnosis of the problem. If you don't know what the alternator field voltage is doing when the system is failed or misbehaving, you're not ready to put a wrench to the airplane. There are few mechanics out there that even know about what you're going to learn here . . . and even fewer that will stand behind a running propeller to gather the needed information. So consider this: You need to attach a voltmeter to the "field" terminal of your alternator with long leads so that the meter can be in the cockpit while the engine is running. For this test, a cheapie analog voltmeter is best . . . a radio shack meter with pointer on it will do. You might have to fabricate some long leads with aligator clips on the end for attaching to alternator. (-) black lead goes to alternator case or any hand place on engine crankcase. (+) red lead goes to alternator "F" or "field" terminal. Climb in cockpit and watch the field voltage readings while the engine is running at 1200 RPM or better and alternator and battery master switches ON> (a) If the alternator field voltage is zero when the output is zero, then the regulator or associated wiring has failed. (b) If the alternator field voltage shows some fairly healthy reading on the order of 10 volts or more and alternator output is zero, the alternator has failed. (c) If the alternator has become unstable . . . loadmeter is jumpy, panel lights flicker . . . watch the field voltage and compare it with loadmeter readings. If the field voltage and loadmeter readings swing up and down together, then the regulator has become unstable. Check for increased resistance in regulator field supply wiring and components. Breakers, switches, overvoltage relays, and connectors are all contributors to regulator instability when their resistance ages upward a few milliohms in resistance. (d) If the loadmeter (or ammeter) swings UP while the field voltage is swinging DOWN, then the alternator has some unstable connections inside . . . perhaps worn brushes? (e) If field voltage is high, does not drop significantly when engine RPM increases but bus voltage seems normal under light load and sags under heavy loads, then the alternator may have one or more diodes open/shorted. (f) While operating with full system loads, carefully observe the engine RPM where alternator field voltage peaks: i.e. begin at idle RPM with all loads ON--if your bus voltage is lower than the regulator setpoint, then the alternator is turning too slow to support present loads. Now, adjust engine RPM carefully to get the highest possible reading on field voltage. At this time, the bus voltage should be at the regulator setpoint. The engine RPM is your minimum speed for regulation at full load. If your system is working properly and pulley ratios are appropriate, engine RPMs should be equal to or LESS than required to sustain flight. (g) Should your alternator suddenly become "noisy" in that alternator whine becomes markedly worse, you may have suffered a blown diode in the alternator. Before taking the alternator off the airplane, Attach a multimeter to the alternator case and the b-lead (output terminal). Set the multimeter to read AC voltage. Run the engine up and turn everything electrical ON. If the AC voltage exceeds 500 millivolts, there's a good chance that a diode is bad. If it's less than 200 millivolts, then it's more likely that the noise is getting into your audio system via a ground loop (noise is present even when radio volume is all the way down) or failed noise filter (noise goes up and down with radio volume control). The above paragraphs describe about 100 times more than most mechanics know about alternator troubleshooting but none of it is possible unless you can measure field voltage (sometimes in flight), observe a combination of effects and deduce their meaning. The parts cost a few dollars and the feature adds significantly to the efficient and safe maintenance of your airplane.