DIODE TESTS:

Due to the Manner in which the three stator windings are connected, two windings are always being used at any one time. Only two diodes are being used at any one time. If any diode is defective, causing one phase of the 3 phase winding to be missing, alternator output is decreased by approximately two-thirds because any single missing phase undesirably influences both of the other phases.

The diodes on some makes of alternators can be tested with the alternator assembled and on the engine. Other makes have to be disassembled for diode testing.

Diodes can be readily checked using a diode tester. When using this test instrument follow the manufacturer's instructions.

Diodes may also be tested with the 12 volt test lamp. Touch the prods of the test lamp leads to the diode case and diode lead and then reverse the test lamp prods as previously explained. If the diode is good, the test lamp will light in only one test. If the lamp fails to light in both tests or lights in both tests, the diode is defective.

Another method of checking diodes is with an ohmmeter. After the stator leads have been disconnected, each diode can be tested for shorts and opens. Touch one ohmmeter prod to the diode case and the other prod to the diode lead and observe the ohmmeter reading. Then reverse the ohmmeter prods and again observe the meter readings. A diode in good condition will have one high reading and one low reading. If both readings are very low, or if both readings are very high, the diode is defective. Push and pull the diode leads gently while testing it to detect loose connections. Test all six diodes in the same manner. The readings of the diodes in the insulated heat sink will be opposite from those in the grounded heat Sink or end frame.

When diode is found defective, it is advisable to replace all 3 diodes in the end frame or heat sink using the correct procedure and the proper removal and installation tools. Diode must never be hammered into position as the impact can easily crack the Silicon wafer. Be sure to test the condenser as it may also be damaged.