GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC K Sierra - 4WD 🢒 Engine 🢒 Engine Controls - 4.8L, 5.3L, and 6.0L 🢒 Powertrain Control Module Controlled Generator Diagnosis

Object Number: 348736 Size: MF
Engine Controls Component Views
Cell 21: G102, G104, Cruise Control Module
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

Circuit Description

The L-terminal circuit from the generator is a discrete circuit into the PCM. The PCM supplies the ignition voltage to the generator lamp feed. This ignition voltage is pulled low by the generator once the ignition switch supplies ignition 1 voltage. The voltage stays low until the generator turns. When the generator is up and running, the PCM detects ignition voltage. Therefore, with no failures with the vehicle's charging system, the lamp terminal circuit will go low (0 volts) when the ignition switch is powered up and then goes to the ignition voltage after the engine is started.

When the charging system detects a fault (circuit shorted to ground), the Instrument Panel Cluster ( IPC ) displays a message.

Diagnostic Aids

Important: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing/replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.

    • The following may cause an intermittent:
       - Poor connections: Check for adequate terminal tension. Refer to Body and Accessories/ Intermittents and Poor Connections Diagnosis for proper procedure.
       - Corrosion
       - Mis-routed harness
       - Rubbed through wire insulation
       - Broken wire inside the insulation
    • For an intermittent, refer to Symptoms .
    • A generator fault, such as a shorted output diode, malfunctioning regulator, open or shorted rotor may cause a low battery charge. Refer to Engine Electrical/ Charging System Check for further information and generator diagnosis.

Test Description

The numbers below refer to the step numbers on the Diagnostic Table.

  1. This step determines if a fault is present.

    2.

  2. A poor connection at the generator causes no output.

Step

Action

Value(s)

Yes

No

1

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

  1. Install a scan tool.
  2. Start the engine.
  3. Monitor the Generator L terminal parameter using a scan tool.

Does the scan tool display the Generator L terminal as Inactive?

--

Go to DTC P1637 Generator L-Terminal Circuit

Go to Step 3

3

Measure the voltage at the battery using the DMM J 39200 .

Is the battery voltage within the specified range?

13-15V

Go to Diagnostic Aids

Go to Step 4

4

Probe the Generator battery feed circuit using the Test Lamp connected to ground. Refer to Measuring Voltage in Wiring Systems.

Does the test lamp illuminate?

--

Go to Step 5

Go to Charging System Check in Engine Electrical

5
  1. Disconnect the electrical connector from the generator.
  2. Start the engine.
  3. Measure the voltage to ground at the generator harness connector terminal B using the DMM J 39200 . Refer to Measuring Voltage in Wiring Systems.

Is the voltage the same or more then the value specified?

10V

Go to Step 10

Go to Step 6

6
  1. Turn OFF the ignition.
  2. Disconnect the PCM connector C2 located on the opposite side of the manufacturer's logo. Refer to Powertrain Control Module Replacement/Programming .
  3. Test continuity from the generator circuit at the PCM harness connector to the B terminal of the generator harness connector using the DMM J 39200 . Refer to Testing for Continuity in Wiring Systems.

Does the DMM indicate continuity?

--

Go to Step 8

Go to Step 7

7

Repair the open/high resistance in the generator circuit. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

System OK

--

8
  1. Inspect the terminal contact tension at the PCM harness connector. Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems.
  2. If you find a poor connection repair the connection as necessary. Refer to Repairing Connector Terminals in Wiring Systems.

Did you find and correct the condition?

--

System OK

Go to Step 9

9

Important:: Program the replacement PCM. Refer to Powertrain Control Module Replacement/Programming .

Replace the PCM.

Is the action complete?

--

System OK

--

10

  1. Inspect the terminal contact tension at the generator electrical connector. Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems.
  2. If you find a poor connection, repair the connection as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

System OK

Go to Step 11

11

Repair or replace the generator. Refer to Charging System Check in Engine Electrical.

Is the action complete?

--

System OK

--