Alternator - Delco 21SI Caterpillar

The test for the regulator does not cover all of the possible failures. The test verifies a few common problems.

Illustration 1	g00721753
Test circuit for regulator test (typical example)

The regulator limits the output of the alternator to a preset value by controlling the current through the field coil. The regulator electronically switches the connection between the field coil and ground. The switching occurs at rates between 10 hertz and 7000 hertz.

This test determines the two voltages that cause the regulator to switch the connection between the field coil and ground. The difference between the two voltages should be less than 0.54 VDC for 12 volt systems. 24 volt systems should have a difference less than 1.0 VDC.

This test will determine problems in the following components: Diode Assembly, Field Coil and Regulator.

1. Connect a variable power supply and two multimeters to the alternator. The setup is shown in illustration 1. Multimeter (A) is connected as an ammeter. Multimeter (V) is connected as a voltmeter. The "R" terminal and the "B+" terminal are connected together.

Note: Reduce the voltage output of the power supply to zero before applying power to the alternator.

2. Monitor the current on multimeter (A). Adjust the variable power supply until the current increases and the current begins to stabilize.

3. Make a note of the voltage that is measured. This measurement is the "Turn On" voltage.

   When the alternator is operating correctly, the voltage is 14.0 ±0.5 volts for 12 volt systems. The "Turn On"voltage for 24 volt systems should be 28.0 ±1.0 volts. If the voltage is out of specification, replace the regulator.

4. The reading on multimeter (A) represents the field current. When the alternator is operating correctly, the field current is between 6.7 and 7.1 amperes for 12 volt systems. 24 volt systems should be between 2.2 and 2.5 amperes.

   If the field current is too high, the field coil is shorted. If the field current is too low, the following conditions may exist:

   • The field coil is open.

   • The brushes are open.

   • The regulator is open.

5. Adjust the variable power supply to the "Turn On" voltage. The "Turn On" voltage was measured in step 3.

6. Adjust the voltage until multimeter (A) reads zero amperes. Make a note of the voltage that is measured. This is the "Turn Off" voltage.

7. Subtract the "Turn On" voltage from the "Turn Off" voltage. The difference between the two voltages should not be more than 0.54 VDC for 12 volt systems. 24 volt systems should have a difference less than 1.0 VDC. Replace the regulator if the difference in voltages is too great.