Diagnostic Aids

WARNING: This page is about a different car, the 2003 Oldsmobile Bravada, 2003 GMC Envoy XL, 2003 GMC Envoy, and 2003 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.

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 CHARGING SYSTEM TEST for further information and generator diagnosis.

Fig 1: DTC P1638 - Diagnostic Chart (1 Of 2)

Courtesy of GENERAL MOTORS CORP.

Fig 2: DTC P1638 - Diagnostic Chart (2 Of 2)

Courtesy of GENERAL MOTORS CORP.