Delco 27SI Troubleshooting – Charging System

System Operation Description:

The alternator is an electronic component that is belt driven. The alternator is used to charge the storage battery during engine operation. The alternator converts a combination of mechanical energy and magnetic energy to alternating current and voltage. This conversion is done by rotating a direct current electromagnetic field on the inside of the three-phase stator. The alternating current and voltage are generated by the stator. The current and the voltage are changed to direct current by the three-phase full wave rectifier. The rectifier uses six silicon rectifier diodes. The alternator also has a diode assembly which rectifies field current. The field current is necessary for alternator output.

Note: Use the diagnostic flow chart as a reference, while you are troubleshooting the alternator charging system.

Illustration 1 g00706351
Diagnostic flow chart

Note: Severely discharged batteries can cause low system voltage. Severely discharged batteries can occur even while the engine is running above idle, and the alternator is working properly. Proper low engine idle is also important. The alternator is self-excited. Self-excited alternators must exceed a turn-on speed before charging will begin. Alternator output can be low at idle.



  1. Before you start the machine, connect a voltmeter between the “B+” terminal and the case of the alternator. Turn OFF all electrical loads.


  1. Turn the key to the ON position but do not start the engine. Read the voltage on the voltmeter. Make a note of the voltage that was measured.

Expected Result:

This voltage should be approximately system voltage.



  • YES – The voltage is approximately system voltage. Proceed to test step 2. 
  • NO – The voltage is less than system voltage. Proceed to test step 4.

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