Current Output Test
The condition of the charging system can be checked by testing AC generator current output. The exact test procedures vary according to the vehicle and AC generator model. Some test procedures call for connecting an ammeter in series between the AC generator output terminal and the positive battery terminal. A carbon pile regulator is then connected across the battery terminals. The engine is then operated at moderate speeds and the carbon pile adjusted to obtain maximum current output. This reading is compared against the rated output. Normally, readings more than 10 amperes out of specifications indicate a problem.
When performing a current output test, the use of an inductive ammeter is highly recommended. an inductive ammeter eliminates the need to break battery connections. if you must break connections to hook up an ammeter, make sure you do not create an open circuit any time the ac generator is generating current.
Other current output tests can be done by hooking up a voltmeter, ammeter, and carbon pile.
Ammeter, voltmeter, and carbon pile hook-up for a current output test on an AC generator with an internal voltage regulator.
With the engine running at a set speed, the carbon pile is adjusted to obtain the specified voltage reading. The ammeter will then read the system's current output. Always follow the appropriate instructions when conducting a current output test.
Regulator By-Pass Test
If no or low voltage or current output is indicated, bypassing the voltage regulator by full-fielding the AC generator will tell you if the generator is bad. The exact procedure for full-fielding an AC generator, which makes it produce maximum output current, varies according to the application. On AC generators where the terminals are not very accessible, jumper wires may be needed to make the connections. Many new model AC generators cannot be full fielded at all.
On some internal regulator AC generators, full-fielding can be done by inserting a screwdriver through a special hole on the back of the unit.
Grounding the field terminal with a screwdriver inserted into a special test hole on the back of the AC generator.
The normal procedure for full-fielding an A-circuit AC generator is to ground the field terminal. If the A-circuit generator has two field terminals, ground one and feed battery positive voltage to the other to full-field the AC generator. With B-circuit AC generators, the generator is full-fielded by connecting the field terminal to battery positive.
An A-circuit AC generator has one carbon brush connected to positive battery voltage and the voltage regulator switches between field and negative to control output. With B-circuit systems, one brush is connected to the negative of the battery and the regulator switches between field and positive to control output. If an A-circuit regulator loses positive voltage, the AC generator will overcharge if the field coils still have power. If the A-circuit regulator loses its ground, the system will go dead.
In B-circuit AC generators, the opposite is true. If a B-circuit system loses ground, it will overcharge. If it loses positive voltage, it will go dead.
If full-fielding an AC generator suddenly brings charging system voltage or current output up to specifications, the voltage regulator, not the generator is at fault. If charging output is still low or nonexistent when full-fielding, the generator is bad.
A faulty AC generator can be the result of many different types of internal problems. Diodes, stator windings, and field circuits may be open, shorted, or improperly grounded. The brushes or slip rings can become worn. The rotor shaft can become bent and the pulley can work loose or bend out of proper alignment.
To test the components of an ac generator it must be removed and disassembled.
Using an ohmmeter to test a diode trio.
Testing a stator for opens.
Testing a rotor for opens.
Testing a stator for a short to ground.
Testing a rotor for a short to ground.