DC charging systems can be identified as type "A" or "B" circuit. It is necessary for the technician to determine which system they have in order to properly troubleshoot. The "A" type system controls the output by regulating the field circuit to ground. The "B" type system controls the output by regulating the battery to field.
All Delco Remy aircraft generator charging systems are "A" circuit therefore their regulator controls how much ground is placed on the generator field circuit. This was originally accomplished by a vibrating point system in the voltage regulator. The more the points were vibrating toward the closed position the more output. The further apart the points were during the vibrating function the less the output.
Most general aviation alternators are "B" circuit. In order for them to output current from the alternator, battery must be applied to the field. The more battery to the field the higher the output. The regulator is then assigned the task of controlling the amount of battery to field. There are some general aviation charging systems that are "A" circuit or field to ground. These can readily be identified if the voltage regulator part number has "VSF" at the front of a four digit number. When this regulator is used the alternator will not have one of the two field terminals attached to ground. The "VSF" series regulators control field to ground in order to control alternator output.
The reason it is important for the technician to determine whether the system is "A" or "B" circuit is so they can proceed to diagnose the charging system. A common method used is called the "full field" test. This test is used by technicians to determine if the alternator or generator is functioning. Since the regulator controls how much battery or ground is applied to the field circuit the "full field" test requires bypassing the regulator. This is done by jumping full battery or full ground to the generator or the alternator field post. This causes the alternator or generator to charge at full output when rotated.
The aircraft engine will be used to rotate the alternator or generator similar to using a test bench. Since this is being done on the aircraft caution must be taken to not damage anything in the electrical system. You will, for a very short period of time, be raising the system voltage. All other electrical devices should be in the off position while performing this test. Only about 4 to 6 seconds is needed to perform the "full field" test.
When the "full field" test is being done on an aircraft with a Delco Remy generator system the technician will need one jumper and a DC voltmeter. Remove both the armature and field wire from the generator. Tape the wire ends for protection. Connect a jumper from the field post to airframe ground. Connect your voltmeter from the armature post to airframe ground. Start the engine and gradually bring the rpm up from idle to approximately 1500 rpm. The voltage on the armature post should follow the throttle. If it does the generator is o.k. Check the two wires from the generator to the voltage regulator. Make sure that when the master switch is on that you have battery voltage of at least 12 or 24 volts on the regulator battery terminal. If so, the trouble is most likely the voltage regulator.
If the technician encounters a generator manufactured by Bendix, Eclipse, Leece Neville or General Electric there is a very good possibility that it is a "B" circuit. In that event the field would be jumped to a battery source.
When performing a "full field" test on an aircraft equipped with an alternator the same cautions must be taken. Be sure that all electrical devices are in the off position. On most general aviation aircraft equipped with Chrysler, Ford or Delco alternators they have only a single field. The other side of the field is connected to ground making it a "B" circuit. To "full field" these systems the technician should remove the field wire from the alternator and tape the wire end to protect the voltage regulator. Place a jumper from the battery terminal on the alternator to the field post. The best way to check the output when the alternator is configured for "full field" is to use a clamp around inductance dc meter. These are very accurate and will measure the amp output within one tenth of one amp. Run the engine and monitor the amp meter. At 2000 rpm the amperage should be very close to the amp rating of the alternator.
Most prestolite alternators are double field, f1 and f2. Disconnect and tape the wire ends from both of these terminals. Connect either field post to airframe ground and the other to the large battery post. This unit is now connected full field and the aircraft engine should be run and the amps measured as in the previous paragraph.
For additional questions please do not hesitate to contact me toll free at 800-634-0190.
Thanks, John Evans