ALTERNATOR GP-CHARGING 6N2740 - Caterpillar

Table 1
Test Equipment Chart    
Qty     Symbol     Description    
1     VPS     0 to 40V, 6A Variable Power Supply    
2     V.A     6V-7070 Digital Multimeter or Equivalent    
Illustration 1 g03231379
Circuit for regulator test (K1)The regulator test can be performed to determine an alternator or regulator malfunction. The regulator test does not cover all of the failure possibilities, but verifies a few common problems. Refer to Table 1 for the equipment needed for the regulator test.Alternator output voltage is regulated by controlling field coil current. The regulator senses output voltage. If the voltage is low, the regulator allows field current flow and the voltage builds. When output voltage exceeds the upper limit, field current is turned OFF. The output voltage of a properly functioning alternator and regulator is:12V System ... 14.0 0.5V24V System ... 27.5 1.0VThe voltage should not vary more than approximately 0.3V during this process. The test measures voltage when the regulator turns field current ON and OFF, and determines if there is an open or short in the diode trio (exciter diodes), field coil, or regulator.
Connect a variable power supply (VPS) positive (+) lead to the alternator B+ and D+ terminals as shown in Illustration 1. Connect VPS negative (-) lead to the alternator B- terminal or frame ground.
Adjust the voltage of the VPS until ammeter (A) first indicates current draw. Measure voltage (V) and record the measurement as "Turn On" voltage. When correctly operating, the "Turn On" voltage is:12V System ... 14.0 0.5V24V System ... 27.5 1.0V
Ammeter (A) reads field current. When operating properly, the field current is: Refer to Specifications for the K1, 24V alternators. 3E-7295 Charging Alternator Gp, and 9X-0341 Charging Alternator Gp ... 5.1 to 5.7A at 14.0V 8C-5535 Charging Alternator Gp, and 7T-2876 Charging Alternator Gp ... 3.8 to 4.1A at 14.0V 9W-3043 Charging Alternator Gp ... 2.8 to 3.1A at 27.5V 2Y-4212 Charging Alternator Gp ... 1.3 to 1.5A at 27.5V 2Y-8310 Charging Alternator Gp ... .8 to .9A at 27.5VNote: Divide the voltage that first indicates field current by rotor field winding resistance as shown in specifications. This will give the correct field current that should be seen on ammeter (A) .
If ammeter (A) indication is zero amps, the rectifier or field coil or regulator are open. If ammeter (A) indication is too high (Refer to Step 2), the field coil may be shorted. The regulator is also bad if the "Turn On" voltage is not:12V System ... 14.0 0.5V24V System ... 27.5 1.0V
If the measurements in Steps 3 and 4 are correct, go to Step 6. If they are not correct, the alternator and/or regulator are faulty.
Adjust VPS to the "Turn On" voltage as measured in Step 2. Increase the voltage slowly until ammeter (A) indicates zero amps. Record the voltage as "Turn OFF" voltage.
The difference between the"Turn OFF" and "Turn ON" voltages must be not more than 0.3V. A larger value indicates a regulator malfunction.