ROTOR TESTING.- To test the rotor for grounds, shorts, and opens, perform the following:

To check for grounds, connect a test lamp or ohmmeter from one of the slip rings to the rotor shaft (fig. 2-29). A low ohmmeter reading or the

Figure 2-29.- Testing rotor for grounds.

lighting of the test lamp indicates that the rotor winding is grounded.

To check the rotor for shorts and opens, connect the ohmmeter to both slip rings, as shown in figure 2-30. An ohmmeter reading below the manufacturer's specified resistance value indicates a short. A reading above the specified resistance value indicates an open. If a test lamp does not light when connected to both slip rings, the winding is open.

STATOR TESTING.- The stator winding can be tested for opens and grounds after it has been disconnected from the alternator end frame.

If the ohmmeter reading is low or the test lamp lights when connected between each pair of stator leads (fig. 2-31), the stator winding is electrically good.

A high ohmmeter reading or failure of the test lamp to light when connected from any one of the leads to

Figure 2-30.- Testing the rotor for opens and shorts.

Figure 2-31.- Testing a stator for opens.

the stator frame (fig. 2-32) indicates the windings are not grounded. It is not practical to test the stator for shorts due to the very low resistance of the winding.

DIODE TESTING.- With the stator windings disconnected, each diode may be tested with an ohmmeter or with a test light. To perform the test with an ohmmeter, proceed as follows:

Connect one ohmmeter test lead to the diode lead and the other to the diode case (fig. 2-33). Note the reading. Then reverse the ohmmeters leads to the diode and again note the reading. If both readings are very low or very high, the diode is defective. A good diode will give one low and one high reading.

An alternate method of testing each diode is to use a test lamp with a 12-volt battery. To perform a test with a test lamp, proceed as follows:

Connect one of the test leads to the diode lead and the other test lead as shown in figure 2-34. Then reverse the lead connections. If the lamp lights in both checks, the diode is defective. Or, if the lamp fails to light in either direction, the

Figure 2-32.- Testing a stator for grounds.

Figure 2-33.- Testing diodes with an ohmmeter.

Figure 2-34.- Testing diodes with a test lamp.

diode is defective. When a good diode is being tested, the lamp will light in only one of the two checks.

After completing the required test and making any necessary repairs or replacement of parts, reassemble the alternator and install it on the vehicle. After installation, start the engine and check that the charging system is functioning properly. NEVER ATTEMPT TO POLARIZE AN ALTERNATOR. Attempts to do so serves no purpose and may damage the diodes, wiring, and other charging circuit components.