26SI Series Alternator – T8 Alternator Overcharging – Test

  1. Verify that the nut for the “B+” terminal is tight. 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. See Systems Operation, SENR2947, “Starting and Charging Systems”. Use of this analyzer replaces the testing that is listed below. If the analyzer finds a problem, correct the problem and retest the system. Go to Troubleshooting, “Initial Troubleshooting Procedure”.
  1. Run the engine at 75 percent. Turn ON all electrical accessories for all test steps below. Allow the engine to run for at least 3 minutes before you continue to 4. The following table will assist in making calculations during the 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    

  1. Measure the voltage between the “B+” terminal and 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 12 volt systems, the voltage is within one volt of the voltage that was recorded in Step 4. 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 step is complete and the related wiring is correct at this time. There is an internal malfunction in the alternator. Refer to Testing and Adjusting, “Component – Test”.

    • 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. Go to Step 7.

  1. Check the voltage between the machine’s frame and the “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. There is an internal malfunction in the alternator. Refer to Testing and Adjusting, “Component – Test”.

    • 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. Go to Troubleshooting, “Initial Troubleshooting Procedure” and retest the system.

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