Diagnostic Aids

WARNING: This page is about a different car, the 2004 GMC Sierra, 2004 GMC Cab & Chassis Sierra, 2004 Chevrolet Silverado, and 2004 Chevrolet Cab & Chassis Silverado. 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 (CS/AD/SI Generators) for further information and generator diagnosis.

DTC P1638

Step	Action	Value(s)	Yes	No
Schematic Reference: Starting and Charging Schematics (Except HP2) or Starting and Charging Schematics (HP2) Connector End View Reference: Engine Electrical Connector End Views
1	Did you perform the Engine Electrical Diagnostic System Check?	-	Go to Step 2	Go to Diagnostic System Check - Engine Electrical
2	Install a scan tool. Start the engine. With a scan tool, observe the GEN - F Terminal parameter in the PCM data list. Does the scan tool indicate that the GEN - F Terminal parameter is within the specified range?	5-95%	Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems	Go to Step 3
3	Turn ON the ignition, leaving the engine OFF. Connect a test lamp to battery positive voltage and repeatedly probe the generator field duty cycle signal circuit in the harness connector while monitoring the Generator PWM on the scan tool. Is the Generator PWM display affected?	-	Go to Step 5	Go to Step 4
4	Test the generator field duty cycle signal circuit for a short or open. Refer to Circuit Testing andWiring Repairs in Wiring Systems. Did you find and correct the condition?	-	Go to Step 8	Go to Step 6
5	Inspect for poor connections at the harness connector of the generator. Refer to Connector Repairs in Wiring Systems. Did you find and correct the condition?	-	Go to Step 8	Go to Charging System Test (CS/AD/SI Generators)
6	Inspect for poor connections at the harness connector of the PCM. Refer to Connector Repairs in Wiring Systems. Did you find and correct the condition?	-	Go to Step 8	Go to Step 7
7	IMPORTANT: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module (PCM) Replacement in Engine Controls-4.3L, Powertrain Control Module (PCM) Replacement in Engine Controls-4.8L, 5.3L, 6.0L, or Powertrain Control Module (PCM) Replacement in Engine Controls-8.1L.Did you complete the repair?	-	Go to Step 8	-
8	Review and record the scan tool Fail Records data. Clear any DTCs. Operate the vehicle within the Fail Records conditions as noted. Using a scan tool, monitor the Specific DTC info for this DTC. Does the scan tool indicate that this DTC failed this ignition?	-	Go to Step 2	System OK