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Chevrolet Silverado / GMC Sierra. Manual - part 1609
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 series/parallel relay, which is the 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.
Diagnostic Aids
The ECM has the capability of providing command to the fan relays even while a scan tool output
control is being used. Always refer to the fan control command parameters on the scan tool to
know which fans are being commanded ON by the ECM.
Reference Information
Schematic Reference
Engine Cooling Schematics
Description and Operation
Cooling Fan Description and Operation (w/o LFA)
Connector End View Reference
Electrical Center Identification Views
Component Connector End Views
Electrical Information Reference
Circuit Testing
Connector Repairs
Testing for Intermittent Conditions and Poor Connections
Wiring Repairs
2008 Chevrolet Silverado 1500
2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado
Scan Tool Reference
Control Module References
Circuit/System Verification
1. Verify that the following DTCs are not set: P0480 or P0481.
If either of the DTCs is set, repair the DTC first.
2. Ignition ON, command high and low speed fans ON and OFF with a scan tool. Observe to
verify that the fans turn ON and OFF when changing between the commanded states.
Circuit/System Testing
1. Ignition OFF, disconnect the series/parallel fan relay.
2. Ignition ON, activate the cooling fan relay 1 with a scan tool. The fan should be OFF.
If the fan is activated, test the left fan ground circuit for a short to ground.
3. Ignition OFF, disconnect the remaining fan relays.
4. Connect a 30A fused jumper between the normally-open switch contact terminals at the fan
control relay connector in order to complete the left fan ground circuit. Leave this jumper in
place for the remainder of this procedure.
5. One at a time, connect a 30A fused jumper between the positive terminal at the battery and
the fan voltage supply circuit terminal at each relay connection and verify the appropriate
fan activation.
If the appropriate fan does not activate, test the fan voltage supply circuit for a short to
ground or open/high resistance. If the circuit tests normal, test the fan ground circuit
for open/high resistance. If the circuits test normal, replace the fan.
6. Ignition ON, connect a 30A fused jumper between the relay switch voltage supply circuit
terminal and the fan voltage supply circuit terminal at the low speed and then the high speed
relay connections and verify the appropriate fan activation.
If the appropriate fan does not activate, test the relay switch voltage supply circuit for
a short to ground or high resistance.
7. If the circuits and fans test normal, replace the relay.
Repair Procedures
Engine Coolant Fan Motor Replacement (Non-HP2)
IMPORTANT: You must perform the Circuit/System Verification before
proceeding with Circuit/System Testing.
2008 Chevrolet Silverado 1500
2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado
Relay Replacement (Attached to Wire Harness) or Relay Replacement (Within an
Electrical Center)
Repair Verification
Ignition ON, command the fan relay ON and OFF with a scan tool. Observe to verify that the
appropriate fan turns ON and OFF with each command.
ENGINE OVERHEATING
Step
Action
Yes
No
1
Inspect for a missing or damaged radiator side
or upper baffle and/or radiator air deflector.
Is the baffle and/or deflector missing or
damaged?
Go to Step 8
Go to Step 2
2
Inspect for a loss of coolant.
Is there a loss of coolant?
Go to Step 3
Go to Step 4
3
Fill the system to the specified level.
Does the engine overheat?
Go to Step 4
System OK
4
Inspect for low temperature protection.
Is the coolant to the correct concentration?
Go to Step 5
Go to Step 8
5
Inspect for a loss of cooling system pressure.
Is there a loss of system pressure?
Go to Step 8
Go to Step 6
6
Inspect for a faulty engine coolant temperature
(ECT) sensor. Refer to DTC P0117 or P0118 .
Is the sensor operating properly?
Go to Step 7
Go to Step 8
7
Inspect for the following:
Damaged coolant surge tank
Leaking hose
Bad/incorrect surge tank or radiator cap
Were any of the above found?
Go to Step 8
Go to Step 3
8
Repair or install new parts as necessary, then
retest.
Does the engine overheat?
Go to Step 9
System OK
9
Inspect for incorrect drive belt tension.
Is the belt tension correct?
Go to Step 10
Go to Step 8
2008 Chevrolet Silverado 1500
2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado
LOSS OF COOLANT
10
1. Remove the water pump. Refer to Water
Pump Replacement (LU3) or Water
Pump Replacement (LY6, L76 and
L92) or Water Pump Replacement
(LH6, LY2, LY5, and LMG) or Water
Pump Replacement (LMM).
2. Inspect for a damaged water pump
shaft/hub.
Is the water pump driveshaft damaged or is the
seal leaking?
Go to Step 8
Go to Step 11
11
Inspect for obstructed radiator air flow or bent
radiator fins.
Is the radiator air flow obstructed?
Go to Step 8
Go to Step 12
12
Inspect for blocked cooling system passages.
Are the cooling system passages blocked?
Go to Step 8
Go to Step 13
13
Inspect the thermostat. Refer to Thermostat
Diagnosis.
Is the thermostat stuck in the closed position?
Go to Step 15
Go to Step 16
14
Replace the thermostat. Refer to Engine
Coolant Thermostat Housing Replacement
(LY2, LH6, LY5, LMG, LY6).
Does the engine overheat?
Go to Step 16
System OK
15
Inspect the radiator cooling capacity.
Is the proper sized radiator being used on the
vehicle?
Go to Step 3
Go to Step 17
16
Consult the current parts catalog and replace the
radiator. Refer to Radiator Replacement (4.3,
4.8, 5.3, and 6.0L) or Radiator Replacement
(LLM).
Is the repair complete?
System OK
-
Step
Action
Yes
No
DEFINITION: The cooling system is loosing coolant either internally or externally.
1
Were you sent here from Symptoms, or another
diagnostic table?
Go to Symptoms
- Engine
2008 Chevrolet Silverado 1500
2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado
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