Chevrolet Silverado / GMC Sierra. Service manual — part 1696

Circuit/System Description

The engine control module (ECM) uses the generator turn ON signal circuit to control the load of
the generator on the engine. A high side driver in the ECM applies a voltage to the voltage
regulator. This signals the voltage regulator to turn the field circuit ON and OFF. The ECM
monitors the state of the generator turn ON signal circuit. The ECM should detect low voltage on
generator turn on signal circuit when the ignition is ON and the engine is OFF, or when the
charging system malfunctions. With the engine running, the ECM should detect high voltage on
the generator turn on signal circuit. The ECM performs key ON and RUN tests to determine the
status of the generator turn on signal circuit.

Conditions for Running the DTC

Key ON Test

No generator, crankshaft position (CKP) sensors, or camshaft position (CMP) sensor DTCs
are set.

The ignition is in RUN position.

The engine is not running.

RUN Test

No generator, CKP sensors, CMP sensor DTCs are set.

The engine is running.

Conditions for Setting the DTC

During the key ON test, the ECM detects high voltage on the generator turn on signal circuit
for 5 seconds.

During the RUN test, the ECM detects low voltage on the generator turn on signal circuit
for 15 seconds.

Action Taken When the DTC Sets

The ECM will command the charge indicator and or warning message to be illuminated on
the instrument panel cluster (IPC) and the driver information center (DIC), if equipped.

The ECM will not illuminate the malfunction indicator lamp (MIL).

The ECM will store conditions, which were present when the DTC set as Fail Records data
only.

Conditions for Clearing the DTC

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Electrical - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

A current DTC will clear when the conditions for setting the DTC are no longer met.

A history DTC will clear after 40 consecutive warm-up cycles have occurred without a
malfunction.

Reference Information

Schematic Reference

Starting and Charging Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Charging System Description and Operation

Electrical Information Reference

Circuit Testing

Connector Repairs

Testing for Intermittent Conditions and Poor Connections

Wiring Repairs

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the generator.

2. Ignition ON, test for less than 1 volt between the signal circuit terminal 1 and ground.

If greater than the specified range, test the signal circuit for a short to voltage. If the
circuit tests normal, replace the ECM.

3. Engine running, test for greater than 3.5 volts between the signal circuit terminal 1 and

ground.

If less than the specified range, test the signal circuit for a short to ground, an open or a
high resistance. If the circuit tests normal, replace the ECM.

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Electrical - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

4. If all circuit tests normal, test or replace the generator.

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Generator Replacement (4.3L) or Generator Replacement (4.8L, 5.3L, 6.0L, and
6.2L) or Generator Replacement (6.6L)

Control Module References for ECM replacement, setup, and programming

DTC P0622

Diagnostic Instructions

Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

Review Strategy Based Diagnosis for an overview of the diagnostic approach.

Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

DTC P0622

Generator F-Terminal Circuit

Diagnostic Fault Information

Circuit/System Description

The engine control module (ECM) uses the generator field duty cycle signal circuit, or F-terminal
circuit, to monitor the duty cycle of the generator. The generator field duty cycle signal circuit
connects to high side of the field windings in the generator. A pulse width modulated (PWM)
high side driver in the voltage regulator turns the field windings ON and OFF. The ECM uses the
PWM signal input to determine the generator load on the engine. This allows the ECM to adjust
the idle speed to compensate for high electrical loads. The ECM monitors the status of the
generator field duty cycle signal circuit. When the key is in the RUN position and the engine is
OFF, the ECM should detect a duty cycle near 0 percent. However, when the engine is running,

Circuit

Short to

Ground

Open/High

Resistance

Short to

Voltage

Signal

Performance

Generator Field Duty Cycle
Signal

P0622

P0622

P0622

-

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Electrical - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

the duty cycle should be between 5-95 percent.

Conditions for Running the DTC

The vehicle speed is greater than 8 km/h (5 mph).

The system voltage is between 9.5-18 volts.

Conditions for Setting the DTC

The ECM detects a PWM signal greater than 65 percent for 5 seconds during the KEY ON
test.

The ECM detects a PWM signal less than 5 percent for 15 seconds during the RUN test.

Action Taken When the DTC Sets

The ECM will command the charge indicator to be illuminated on the instrument panel
cluster (IPC).

The ECM will not illuminate the malfunction indicator lamp (MIL).

The ECM will store conditions, which were present when the DTC set as Fail Records data
only.

Conditions for Clearing the DTC

The ECM will command the charge indicator OFF after one trip in which the diagnostic test
has been run and passed.

The history DTC will clear after 40 consecutive warm-up cycles have occurred without a
malfunction.

Reference Information

Schematic Reference

Starting and Charging Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Charging System Description and Operation

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Electrical - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado