Alternator Undercharges

1. Check alternator output current as follows:
a. Connect suitable charging system analyzer to system, as per analyzer manufacturers instructions.
b. Start engine and allow to idle.
c. Turn on all lights and accessories.
d. Note analyzer reading at an engine speed of 2500 to 3000 RPM. If reading is not within 10 percent of rated alternator output, proceed to step 2. If reading is within 10 percent of rated alternator output, the alternator is satisfactory.
2. Check charging system voltage output as follows:
a. Turn all electrical loads off.
b. Charge battery until charging rate indication becomes less than 5 amps or replace with new battery.
c. Connect suitable charging system analyzer to system, as per analyzer manufacturers instructions.
d. Note reading at an engine speed of approximately 2500 RPM. If reading is less than 5 amps, proceed to step e. If reading is more than 5 amps, check battery, alternator and engine ground cable for proper connections and correct as necessary.

Fig. 1 Alternator voltage output test connections:

e. Using a suitable voltmeter, connect voltmeter positive lead to back of L terminal on two wire electrical connector at rear of alternator, Fig. 1. Connect voltmeter negative lead to alternator case. The L terminal electrical connector must remain connected to the alternator during this test.



f. If voltmeter reading at L terminal is less than 14.4 volts at 68 degrees F, proceed to step 3. If reading is 14.4 to 15 volts at 68 degrees F, a faulty alternator stator or rectifier may be indicated.
3. Check regulator power source as follows:
a. With ignition switch in the On position, engine not operating, disconnect alternator terminal R connector.

Fig. 2 Voltage regulator test connections:

b. Connect voltmeter positive lead to R terminal of electrical connector and voltmeter negative lead to alternator case, Fig. 2. If battery voltage is present, proceed step 4. If meter reads less than battery voltage, check for defective wiring harness between battery and R terminal.

Fig. 3 Alternator rotor field coil test connections:

4. Check rotor field coil as follows:
a. Disconnect battery ground cable, then disconnect electrical lead at alternator B terminal. Ensure electrical lead does not contact alternator output terminal.
b. Disconnect alternator two wire connector from alternator, then measure resistance between alternator L and F terminals at rear of alternator, Fig. 3. If reading is 3-6 ohms, proceed to step 5. If reading is other than 3-6 ohms, a faulty field rotor, slip ring or brushes is indicated.

Fig. 1 Alternator voltage output test connections:

5. Check alternator L terminal voltage as follows:
a. Connect alternator B terminal connector to rear of alternator, then reconnect battery ground cable.
b. With ignition switch in the On position, connect voltmeter positive lead to back of L terminal on two wire electrical connector and negative lead to alternator case, Fig. 1.
c. If reading is 1-3 volts, faulty stator coil and diodes is indicated. If reading is over 3 volts, a faulty regulator is indicated.