Chevrolet Silverado / GMC Sierra. Service 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