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Chevrolet Silverado / GMC Sierra. Manual - part 1608
Testing for Intermittent Conditions and Poor Connections .
Symptom List
Refer to a symptom diagnostic procedure from the following list in order to diagnose the
symptom:
Cooling Fan Always On
Cooling Fan Inoperative
Engine Overheating
Loss of Coolant
Thermostat Diagnosis
Engine Fails To Reach Normal Operating Temperature
LOW ENGINE COOLANT INDICATOR ALWAYS ON
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.
Diagnostic Fault Information
Circuit/System Description
The low engine coolant level indicator is controlled by the HVAC control module. The normal
state of the engine coolant level switch is closed when in contact with coolant.
Reference Information
Schematic Reference
Engine Cooling Schematics
Circuit
Short to
Ground
Open/High
Resistance
Short to
Voltage
Signal
Performance
Coolant Level Switch Signal
Indicator
OFF
Indicator ON
-
-
Ground
-
Indicator ON
-
-
2008 Chevrolet Silverado 1500
2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado
Connector End View Reference
Component Connector End Views
Description and Operation
Cooling System Description and Operation
Electrical Information Reference
Circuit Testing
Connector Repairs
Testing for Intermittent Conditions and Poor Connections
Wiring Repairs
Circuit/System Verification
The low coolant indicator should be OFF with the engine coolant at the proper level.
Circuit/System Testing
1. Ignition OFF, disconnect the harness connector at the coolant level switch.
2. Install a 3A fused jumper wire between the signal circuit terminal and ground.
3. Engine running, verify the low coolant indicator turns OFF.
If the indicator does not turn OFF, test the signal circuit for an open/high resistance.
If the indicator turns OFF, test the ground circuit for an open/high resistance.
4. If all circuits test normal, replace the coolant level switch.
Repair Procedures
Radiator Surge Tank Replacement (Non-HP2)
Control Module References
Instrument Cluster Replacement (without RPO SLT)
Repair Verification
Verify that the low coolant indicator is OFF with the engine coolant at the proper level.
IMPORTANT: Circuit/System Verification must be performed before performing
the Circuit/System Testing.
2008 Chevrolet Silverado 1500
2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado
COOLING FAN ALWAYS ON
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.
Circuit/System Description
The engine control module (ECM) commands the fans ON in either high speed or low speed,
depending on cooling requirements. In low speed, both fans are turned ON at a reduced speed.
High speed has both fans turned ON at full speed.
In low speed operation, the ECM applies ground to the coil side of the fan low relay. This
energizes the coil and applies voltage directly to the left cooling fan through the switch side of
the fan low relay, which is fed by the fan 1 fuse. The right fan is connected in series to the left fan
through the de-energized fan control relay so that both operate at low speed.
In high speed operation, the ECM applies a ground to the coil side of the fan low relay, the fan
control relay, and the fan high relay. On the fan high relay, the energized coil closes the switch
side of the relay and applies voltage directly to the right cooling fan through the switch side of the
relay, which is fed by the fan 2 fuse. At the same time, the ECM energizes fan control relay
pulling the switch side over, providing a direct path to ground for the left cooling fan, which has
voltage applied through the energized fan low relay. In high speed mode, the fans are operated as
a parallel circuit with full voltage applied to each.
Reference Information
Schematic Reference
Engine Cooling Schematics
Connector End View Reference
Component Connector End Views
Description and Operation
Cooling Fan Description and Operation (w/o LFA)
Electrical Information Reference
2008 Chevrolet Silverado 1500
2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado
Circuit Testing
Connector Repairs
Testing for Intermittent Conditions and Poor Connections
Wiring Repairs
Circuit/System Verification
1. If DTCs P0480 or P0481 are set, perform those diagnostics first.
2. Ignition ON, verify with a scan tool that the control module is not commanding fan
activation.
3. Ignition ON, observe that the fans are not activated.
Circuit/System Testing
1. Ignition OFF, disconnect the fan high relay.
2. Ignition ON, observe that both fans are now OFF.
If both fans are OFF, replace the fan high relay. If either fan is still activated, remove
the fan low relay. If both fans are now OFF replace the fan low relay. If either fan is
still activated, test for a short to voltage in the fan voltage supply circuit.
Repair Verification
1. Ignition ON, verify with a scan tool that the control module is not commanding fan
activation.
2. Ignition ON, observe that the fans are not activated.
COOLING FAN INOPERATIVE
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.
Circuit/System Description
The engine control module (ECM) commands the fans ON in either high speed or low speed,
depending on cooling requirements. In low speed, both fans are turned ON at a reduced speed.
High speed has both fans turned ON at full speed.
2008 Chevrolet Silverado 1500
2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado
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