To repair the alternator, observe the following procedure:
To disassemble the alternator, take out the four through-bolts, and separate the drive end frame and rotor assembly from the stator assembly by prying apart with a screwdriver at the stator slot. A scribe mark will help locate the parts in the same position during assembly. After disassembly, place a piece of tape over the slip ring end frame bearing to prevent the entry of dirt and other foreign material, and also place a piece of tape over the shaft on the slip ring end. If brushes are to be reused, clean with a soft dry cloth. Caution: Use pressure sensitive tape and not friction tape which would leave a gummy deposit on the shaft.
To remove the drive end frame from the rotor, place the rotor in a vise and tighten only enough to permit the removal of the shaft nut. Caution: Avoid excessive tightening as this may cause distortion of the rotor. Remove the shaft nut, washer, pulley, fan, and the collar, and then separate the drive end frame from the rotor shaft.
To check for opens, connect the test lamp or ohmmeter to each slip ring. If the lamp fails to light, or if the ohmmeter reading is high (infinite), the winding is open (Fig. 6). Connect test lamp or ohmmeter from one slip ring to shaft. If lamp lights, or if reading is low, the rotor winding is grounded (not illustrated).
The winding is checked for short-circuits or excessive resistance by connecting a battery and ammeter in the series with the edges of the two slip rings. Note the ammeter reading and refer to our service specifications page. An ammeter reading above the specified value indicates shorted windings; a reading blow the specified value indicates excessive resistance. An alternate method is to check the resistance of the field be connecting an ohmmeter to the two slip rings (Fig. 6). If the resistance reading is below the specified value, the winding is shorted; if above the specified value the winding has excessive resistance. The specified resistance value can be determined by dividing the voltage by the current given on our specifications page.
Remember that the winding resistance and ammeter readings will vary slightly with winding temperature changes. If the rotor in not defective, but the alternator fails to supply rated output, the defect is in the diode trio, rectifier bridge, stator, or regulator.
The diode trio is identified in Figure 7. To check the diode trio, remove it from the end frame assembly by detaching the three nuts, the attaching screw, and removing the stator assembly. Note that the insulating washer on the screw is assembled over the top of the diode trio connector. Connect an ohmmeter having a 1 1/2 volt cell, and using the lowest range scale, to the single connector and to one of the three connectors (Fig. 8). Observe the reading. Then reverse th ohmmeter leads to the same two connectors. If both readings are the same, replace the diode trio. A good diode trio will give one high and one low reading. Repeat this same test between the single connector and each of the other two connectors. Also, connect the ohmmeter to each pair of the three connectors (not illustrated). If any reading is zero, replace the diode trio.