Diagnostic Aids

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DTCs P1637 and P1638 may set at the same time depending on the failure.
Use a scan tool in order to diagnosis the generator circuits. The scan tool should display Inactive for the L terminal and 10-40 percent for the F terminal when the ignition is ON. When the engine is operating, the display should indicate that the L terminal is Active and the F terminal is more than 10 percent.
A generator fault, such as a shorted output diode, malfunctioning regulator, open or shorted rotor, or open sense lead or battery feed may cause a low battery charge. Refer to TEST A: CHARGING SYSTEM TEST (CS/AD/SI GENERATORS) for further information and generator diagnosis.

Fig 1: DTC P1638

Courtesy of GENERAL MOTORS CORP.