FITTING 3B8989 - Caterpillar

Check the exciter armature. Ensure that the rotating rectifier is tight. If a failure of a rectifier is suspected, proceed to the "Testing a Three-Diode Rectifier Block" section.Testing Diodes (Rectifiers)
Illustration 1 g03723139
Rotating rectifier
Illustration 2 g03723147
Diodes Schematic
(1) reverse polarity diodes
(2) standard polarity diodes
(3) red lead
(4) surge suppressor
(5) black lead Diodes perform the function of an electrical check valve. Diodes conduct current in one direction only, and are used to rectify AC current into DC current.Use the following procedure to test proper function of the diodes:
Remove all leads from the rectifier block.
Measure the resistance first in one direction, and then reverse the leads and test in the opposite direction.
The resistance should be low in the forward direction. All readings on the meter should be between 0.4 and 1.0.
The resistance should be infinite in the reverse direction. In all cases, the meter should read "OL" (overload).
A shorted diode can damage the exciter rotor. If a diode is shorted, check the exciter rotor. Refer to the Testing and Adjusting, "Winding - Test" and Testing and Adjusting, "Insulation - Test". Perform these tests. Note: Two different polarities of diodes are used. The only difference is in the way the device is mechanically placed in the case. When changing diodes, be sure that the correct polarity is used. Refer to illustration 2Note: The internal batteries of some meters have insufficient voltage output to turn on the diode (about 0.6 V is required). The voltage required can change with different range settings. Consult the OEM manual for your meter.Note: This rectifier block also contains surge suppressor "CR7". "CR7" can be checked by measuring the resistance across the surge suppressor in both the forward and reverse directions. The measurement should essentially read like an open circuit .