All Delco Remy Generator charging systems are "A" circuit, therefore their regulator controlled how much ground was 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 the output. The further apart the points were during the vibrating function, the less the output.

The solid state generator controls work by controlling the generator field to ground through a system of electronic components rather than the vibrating points. The theory remains the same, the more ground on the generator field, the more output and vice versa.

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 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 of the "A" circuit type. These are normally found in prestolite alternator systems with two field terminals on the alternator. If neither of the fields is grounded to the frame of the altnator, then the system is "A" circuit. This simply means that the voltage regulator is controlling the alternator field to ground to control output.

The reason it is important for the technician to determine whether the system is of the "A" or "B" style is so that they can proceed to diagnose the system. A common method of diagnosing charging system problems begins with the full field test. This test is used as a method of determining whether the generator or alternator is functioning. Since the regulator controls how much battery or ground is being applied to the generator or alternator, the full field test requires bypassing the regulator and jumping full ground or battery to the field circuit. This causes the generator or alternator to charge at full output when rotated at cruise RPM.

If you are testing a Delco Remy Generator system, remove both the armature and field wires from the generator. To prevent damage, cover the wire ends with tape. Next, jump the field post on the generator to a good airframe ground. Connect a DC voltmeter to the generator armature terminal. Connect the plus side to the post and the negative side of the meter to ground. With the field grounded the generator, if good, will generate a voltage on the armature terminal. The voltmeter should follow the throttle. The more engine speed the more output. Using a digital meter in this application is sometimes not effective. An analog (needle movement) meter is much more reliable.

Should you have a Generator that is not Delco, it could possibly be of the "B" circuit, Field to Battery, type application. If so, follow the instructions in the previous paragraph, except connect the jumper from the Armature to the Field post. Then connect the meter from the armature post to ground and measure the output voltage. Normally, the generators manufactured by Bendix, General Electric and Leece Neville, adopted the "B" circuit method.

When testing general aviation alternators, the brands encountered will be Ford, usually found on Cessna and Chrysler on most older Pipers. You might also run across Delco Alternators on various models of Piper and Beech aircraft. Motorola units, in the form of STC approvals by Alcor or InterAv, must also be taken into consideration. All of these unit are "B" circuit requiring a jumper from the Battery post to the field post in order to "full field". This means you must remove the field wire, tape the terminal for protection, and connect a jumper from the large output terminal over to the field post. Connect your DC voltmeter from the output terminal to ground and the voltage should thenfollow the RPM if the alternator is good.

In the instances where you have a Prestolite charging system, you must first determine whether the system is "A" or "B" circuit. If the alternator has only one field post it is "B" circuit and requires jumping battery to field. If the unit has two field post, normally f1 and f2, disconnect both field wires and protect. Now connect one jumper from the Battery output post to one field post and a second jumper from the remaining field post to ground. This will full field the dual field models. The voltmeter is connected from the Output post to ground as in the previous example.

If the generator or alternator passes the full field test the next most likely culprit is the voltage regulator. A fairly reliable quick test is to connect your voltmeter to the wire delivering battery to the regulator from the bus. This should read battery voltage, either 12 or 24 volts depending on the system. If you are getting good input voltage to the regulator, connect your voltmeter to the field circuit exiting the regulator. You should read some amount of voltage. If the reading is zero the voltage regulator is probably defective.

Before replacing the regulator it is always a good idea to measure the resistance of the field circuit. This can be done at the regulator field wire or on the field post of the generator or alternator. The normal reading from the field post on 12 volt systems is 4 to 6 ohms. On 24 volt systems this doubles to 8 to 12 ohms. This can vary slightly and still be considered normal.

This writer has high hopes this explanation makes some sense and that the reader is not totally confused. If, however, you would rather discuss this by telephone to perhaps get a better description, please do not hesitate to call us at our toll free number (1-800-634-0190).

John and Mike Evans