Alternator Service Procedures - Manual Revision: Diagnosis and Testing
On Vehicle Tests
NOTE: Before performing tests turn off all electrical accessories.
1. With the engine stopped connect an accurate voltmeter such as Rotunda Digital VoltOhmmeter 014-00407 or equivalent across the battery. Note the reading on the meter.
2. Start the engine and observe the meter reading. If the meter reading rises excessively, the charging system may be defective or may require adjustment.
3. Remove the nylon screw from the voltage regulator and with the engine at approximately 1000 RPM, carefully turn the adjusting screw clockwise to raise, or counterclockwise to lower the voltage to 14.2-14.3 volts. When turning the adjustment screw, do not force the screw beyond its stop as damage will occur. Replace the nylon screw in the regulator adjustment hole to prevent entry of foreign material into the regulator.
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DIAGNOSIS AND TESTING (Continued)
4. If the voltage is excessively high and cannot be lowered by adjustment, the regulator is at fault and must be replaced.
5. If the output voltage cannot be raised, either the alternator, regulator, or diode trio may be at fault.
6. To determine if the fault is with the regulator or the alternator, perform the following test.
7. Attach one end of the short jumper lead to the negative rectifier output terminal and connect the other end of the lead to a short stiff piece of wire at least 1-1/2 inches long. Insert this end into the small hole in the end of the brush holder so it firmly contacts the outer brush terminal.
8. If the voltmeter reading now rises with the engine at a fast idle, the alternator is OK and a faulty regulator or diode trio is indicated.
Bench Tests
Before performing these tests carefully, inspect all parts for wear, cracks, breakage, or other mechanical defects, Replace all damaged parts.
Voltage Regulator Test
The regulator circuitry contains devices connected in such a manner that parallel or "sneak" circuits exist, making it impossible to electrically test each individual component, as several will be in the circuit at the same time. For this reason, point to point resistance checks with an ohmmeter may be inconclusive or misleading. The regulator can be most accurately tested by installing it in a serviceable alternator.
Diode Test These tests may be performed on heat sink assemblies without removing them from the end housing. If they are tested in this manner, remove the stator. Be sure the diode trio has been removed from the AC terminals, and disconnect the capacitor between the lower end of the heat sinks.
Diodes are tested to insure that they only pass current in one direction. Diodes which do not allow current to flow in either direction are open while diodes passing current both ways are shorted. Diodes should be checked with a diode tester, but in emergencies where one is not available, an ohmmeter or a battery powered test light may be substituted.
Positive Heat Sink Tests
NOTE: The positive heat sink is the one to which the positive output terminal is connected. The square hole in the terminal end of the positive heat sink is larger than the terminal hole of the negative heat sink, approximately 11/32 inch square.
1. Connect the positive lead of the diode tester or ohmmeter to the positive heat sink and touch the negative test lead to each of the three diode terminals. A high resistance should be indicated. If any of the three diodes shows a low resistance, the diode is shorted.
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DIAGNOSIS AND TESTING (Continued)
2. Reverse the test leads so that the negative test lead is connected to the positive heat sink. The positive test lead should now be touched to each diode terminal and a low resistance reading should be obtained. If a high resistance reading is obtained, an open diode is indicated.
If a shorted or open diode is detected in the preceding tests, the entire heat sink assembly should be replaced.
Negative Heat Sink Tests
NOTE: The negative heat sink terminal hole is approximately 9/32 inch square.
1. Connect the negative lead of the diode tester, ohmmeter, or test lamp to the negative heat sink and touch the positive test lead to each of the three diode terminals. A high resistance should be indicated. If any of the three diodes shows a low resistance, the diode is shorted.
2. Reverse the test leads so that the positive test lead is connected to the negative heat sink. The negative test lead should now be touched to each diode terminal and a low resistance reading should be obtained. If a high resistance reading is obtained, an open diode is indicated.
If a shorted or open diode is detected in the preceding tests, the entire heat sink assembly should be replaced.
Diode Trio
The diode trio may be tested with the same equipment which was used to check the diodes in the heat sink assemblies.
1. Connect the negative lead of the tester to the output lead of the diode trio, and touch the positive tester lead to each of the three copper terminal pads. A low resistance should be indicated.
2. Connect the positive terminal of the tester to the output lead of the diode trio and touch the negative lead to each of the three copper terminal pads. A high resistance should be indicated. Discard any diode trio which does not pass all of these tests.
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DIAGNOSIS AND TESTING (Continued)
Capacitor
The capacitor connected across the heat sinks may be tested on a capacitor tester if available. Its value is .158 MFD, and 100 working volts D.C.
In the absence of a capacitor tester, the unit may be checked for shorts by means of an ohmmeter connected across the terminals. A low resistance reading indicates a shorted or leaking capacitor which should be replaced.
Rotor Test
Check rotor for grounds by connecting an ohmmeter to the shaft and to each of the two slip rings. If a zero or close to zero reading is obtained, then the rotor assembly is grounded and must be replaced.
Check rotor coil resistance by connecting an ohmmeter to the two slip rings. Resistance readings should fall between 2.6-2.9 ohms. If resistance falls outside values shown, then repeat this test by connecting the test leads to the rotor coil soldered connections to the slip rings. If readings are now within 2.6 and 2.9 ohms, then the rotor coil soldered connections must be properly re-soldered. If the resistance values are still outside figures shown when measured at the soldered connections, then the rotor assembly must be replaced.
Stator Test
Inspect the stator visually. If windings appear to be charred, burned, or if insulation is missing and bare copper is noticed, then stator must be replaced.
To test stator, remove from slip ring end housing and check with a digital ohmmeter such as Rotunda Digital Volt-Ohmmeter 014-00407, or equivalent capable of reading 1/1000 of an ohm. Check for grounds between the stator lamination and each of the three stator terminals. If continuity is present, then the stator must be replaced. Check stator phase resistance across stator terminals. If resistance is about the same for each of the three phases, then the stator phases are satisfactory and the stator may be re-used (provided the windings appear undamaged and provided the stator is not grounded).