Al:
I flew my night requirements the other night and when I started out plane cranked up fine. Amps looked good. Flew 3 TO and LNDS full stop and hangered it. I will have to admit I did not look at the amp gage when I shut the engine down. I went to go fly today and no juice. I jumped it off with a 12 volt battery and it started up fine but my amp gage was not showing a charge like normal even at higher RPMs. My A&P is out of town and I am at an airport with no A&P. Does anyone know where I could start to check the voltage regulator and or alternator. I have an meter but not sure where to check and for the outputs. I am trying to narrow down the problem until I can get an A&P.
Bob Steward, A&P IA
The basic troubleshooting with a VOM or DMM meter is to begin by making sure you have power at the VR “S” terminal (S = Switch, as in power coming from the ignition switch on a car). If so, then you have a good field CB and wiring. Next check for output on the “F” terminal (F= Field coils in the alternator). With the engine off, Master and Alternator switches ON, the F terminal ought to read nearly bus voltage ~12.6 volts on a fully charged battery, or whatever you read at the “S” terminal. 11+ volts is OK). If you have voltage at “F”, remove the “F” wire and read RESISTANCE to ground between the wire going to the alternator and a good airframe ground. Ought to be 4-10 ohms. Less than that means possible shorted field or wire going TO alternator. High resistance is an open wire or field (bad brushes).
If all that checks out, and you have a helper, try “flashing the field” with the engine at idle, all expensive radios OFF. Briefly touch the “F” wire to 12V. Either the “S” terminal or some other wire you’ve rigged up to give you 12V to test with. Ought to INSTANTLY load the engine and possibly even make the belt slip with a shrieking sound. DO NOT leave the wire touching for more than a couple seconds to just SEE if there is an immediate load on the engine, and DO NOT do this at above idle. Its possible for the alternator in this “full fielded” state to reach HUNDREDS of volts at higher RPM.
OK, if you got this far and still didn’t get anything, you have a broken lead from the alternator to the battery or a seriously fried alternator and the nose bowl has to come off either way. Drop the alternator off, inspect wiring, and then for a free sanity check, swing by Pep Boys or Autozone and have them check your “pickup truck” alternator.
Many of the Beech alternators are DOFF-10300J Ford. This is a 71 pickup with AC 61 amp alternator. They will be able to tell you if the alternator works normally or is dead.
If you had no power to the “S” terminal on the VR, follow wires back to alternator master and alternator field CB.
If you had power at “S” but not at “F” and the alternator full fielded and loaded down the engine at idle, you need a VR.
This is the “hammer and screwdriver” method of troubleshooting a generic alternator charging system. There are differences on some models, but the Baby Beech are all pretty much like this with only minor differences in part numbers and over voltage relay protection.
If yours has had the OVR kit installed, and its not working, you’ll know because you’ll have no voltage at the “S” terminal.
Power comes from the battery, through the Master Solenoid, to the Bus bar with the CBs, into the field CB, to the alternator “master” switch, into the OV module (if installed), then to the “S” terminal on the regulator and out the “F” terminal as needed to excite the alternator field.
Output of the alternator is determined by the input voltage at the field circuit and the RPM. If the voltage is low at the F terminal and the RPM is low, the alternator output will also be low. If the voltage is high (12V) at the F terminal and the RPM is high, the output voltage will be over 100 volts! So at high RPM the voltage to the F terminal will be less. Those with a EE understand that the speed the lines of magnetic force are cut with affects the output, as does the strength of the magnetic field.
For the rest of us, its enough to know that the Voltage Regulator varies the field voltage to maintain the desired output voltage.
John Waterman:
Unless you are particularly electrically inclined, probably best idea is to put the battery on a charger and then fly it to your home base where you can get somebody to look at it. Might want to check the basics. Like, do you still have a fan belt? Are the connections to the alternator all intact? Is the alternator switch functional? More advanced techniques would be to ground the field (this will cause a high rate of charge and will tell you if the problem is with the alternator or with the regulator. Sometimes, you can get a flat spot on the potentiometer. By putting a small screwdriver on the voltage adjustment and moving it both ways and then returning to approximately the same place, you may be able to bring it back to life. Also, a digital voltmeter on the output (anywhere on the bus or the cigar lighter) should read 14.8 volts or so .
Bob Steward, A&P IA
Some Beech (Bonanza) are wired with the VR grounding the alternator field. I’ve not seen this on the Musketeer series, but then I’ve not seen every Musketeer made. Grounding the field on a system with the VR providing 12V to the Field will result in sparks and a blown Field CB, and possibly damage to the VR. Best to take a look at the factory wiring diagram before trying that.
Normal charging voltages for flooded electrolyte batteries of nominal “12V” rating is 13.8-14.2. Outside this range either fails to keep the battery fully charged or boils the acid out.