26SI Series Alternator – T3 Charging System – Test

  1. Verify that the alternator “B+” terminal nut is tight and verify that the wire has a good connection to the “B+” terminal.
  1. Many Caterpillar machines are equipped with a connector for the 6V-2150 Starting/Charging Analyzer. Use of this tester replaces the testing below.
  1. Start the engine and set the throttle to at least 75 percent. Turn ON all electrical accessories for all test steps below. Allow the engine to run for at least 3 minutes before continuing to Step 4. The following table will assist in making calculations during this test.

    Table 1
    Test Step     Voltage Reading     Voltage should be below this for 12 volt system     Voltage should be below this for 24 volt system    
    4        
    5        
    4 minus 5         1.0 volt     2.0 volts    
    6                
    7        
    6 minus 7         0.5 volt     1.0 volt    
    8         0.5 volt     1.0 volt    

  1. Measure the voltage between the alternator “B+” terminal and the alternator case ground.
  1. Measure the voltage across the battery. Put the red lead on the + battery terminal, and put the black lead on the negative battery terminal. Step 5 should be completed as quickly as possible after Step 4.

    Expected Result: On a 12 volt system, the voltage should not be greater than the voltage that was recorded in Step 4. The voltage must also be within one volt. On 24 volt systems, the voltage is within 2 volts.

    Results:

    • YES – If the voltage in Step 4 is not more than 1 volt for 12 volt systems (2 volts for 24 volt systems) higher than the voltage in Step 5, this test is complete and the related wiring is correct at this time. Replace the alternator or go to Testing and Adjusting, “Component – Test” in order to test the internal parts of the alternator.

    • NO – If the voltage in Step 4 is more than 1 volt for 12 volt systems (2 volts for 24 volt systems) higher than the voltage in Step 5, there is high circuit resistance: corrosion, loose connections and damaged wiring. Continue to Step 6.

  1. Check the voltage between the machine frame and the alternator “B+” terminal. Record the voltage.
  1. Check the voltage between the machine frame and the + battery post. Step 7 should be completed as quickly as possible after Step 6.

    Expected Result: The voltage difference between Step 6 and Step 7 does not exceed 1 volt on 24 volt systems or 0.5 volts on 12 volt systems.

    Results:

    • YES – The voltage difference does not exceed the tolerance. The charging circuit is good. Go to Step 8.

    • NO – The voltage difference exceeds the tolerance. There is high resistance in the charging circuit: loose cables, corroded cables, damaged cables, faulty circuit breaker and faulty main relay. Correct the problem and retest the system.

  1. Check the voltage between the negative battery post and the alternator case ground.

    Expected Result: The voltage does not exceed 1 volt on 24 volt systems or 0.5 volts on 12 volt systems.

    Results:

    • YES – The voltage difference does not exceed the tolerance. The ground circuit is good. There is an internal problem with the alternator. Replace the alternator or go to Testing and Adjusting, “Component – Test” in order to test the internal parts of the alternator.

    • NO – The voltage difference exceeds the tolerance. There is high resistance in the ground circuit: loose cables, corroded cables, loose alternator mounting and poor engine ground. Correct the problem and retest the system.

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