Circuit Description
The electric cooling fan is controlled by the powertrain control module (PCM) through the fan relay based on inputs from the engine coolant temperature sensor and intake air temperature sensors, the A/C selector switch, A/C refrigerant pressure sensor and the vehicle speed sensor. The PCM controls the cooling fan by grounding the cooling fan control circuit which turns ON the cooling fan relay.
The fan relay will be commanded ON when the following conditions are met:
| • | The engine is running in or out of gear with no vehicle speed for 10 minutes. |
| • | The engine coolant temperature reaches 106°C (223°F) or more. |
| • | The A/C clutch is requested. |
| • | The vehicle speed is less than 38 MPH. |
| The cooling fan relay will be commanded ON regardless of vehicle speed when any of the following conditions exist: |
| • | Any DTC is set that causes the malfunction indictor lamp (MIL) to be illuminated. |
| • | The engine coolant temperature is 151°C (304°F) or more. |
| • | The A/C refrigerant pressure is high. |
The cooling fan may be commanded ON when the engine is not running under the fan run-on conditions described previously in this section.
Cooling Fan Run-On
The PCM usually powers down 10-30 seconds after the ignition is turned OFF. Since the cooling fan is PCM controlled, the PCM may remain energized for as long as 7 minutes after the ignition is turned OFF. This allows the PCM to ground and energize the cooling fan relay.
This is called a fan run-on and will occur if any of the following combinations of conditions exist when the ignition is turned OFF.
| • | The engine coolant temperature is more than 100°C (212°F) and the intake air temperature is more than 80°C (176°F). |
| • | The engine coolant temperature is more than 112°C (234°F) and the intake air temperature is more than 70°C (158°F). |
| • | The engine coolant temperature is more than 116°C (241°F). |
The PCM and cooling fan will remain ON until these combinations of conditions no longer exist, or for a maximum of 7 minutes.
If the cooling fan is OFF when the PCM powers down, it will remain OFF regardless of any temperatures until the ignition is turned ON again.
Diagnostic Aids
| • | If the owner complained of an overheating problem, determine if the engine actually boiled over, or if the temperature lamp, or temperature gauge, indicated overheating. |
| • | Check for the proper amount of coolant in the system. |
| • | If the gauge, or light, indicates overheating, but no boil over is detected, the gauge or lamp circuit should be checked. The gauge accuracy can also be checked by using a scan tool to compare the engine coolant temperature sensor reading with the gauge reading. |
| • | If the engine is actually overheating, and the gauge indicates overheating, but the cooling fan is not coming ON, the engine coolant temperature (ECT) sensor may have shifted out of calibration and should be replaced. Refer to Temperature Versus Resistance for the table. |
| • | If the engine is overheating, and the cooling fan is ON, the cooling system should be checked. Refer to Diagnostic System Check - HVAC Systems - Manual in HVAC Systems - Manual. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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The cooling system operation and level should be checked before determining if any electrical conditions are present causing a cooling system failure.
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With the engine OFF, the cooling fan should not be commanded ON by the PCM.
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If the cooling fan turns OFF after the PCM connectors are disconnected, the PCM was causing the fan to be ON constantly.
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A test lamp that illuminates indicates that the cooling fan battery feed circuit, which is circuit 409, is shorted to battery voltage. The cooling fan will also remain engaged when the cooling fan relay electrical connector is disconnected.
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Connecting circuits 542 and 409 together bypasses PCM control of the cooling fan. If the cooling fan fails to engage at this point, the malfunction is in circuit 409, the cooling fan or cooling fan ground circuit.
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Both battery feed circuits are supplied to the cooling fan relay through the same fuse. No voltage on both circuits indicates that the fuse is open. Before replacing the fuse, locate and repair any shorts that may have caused the fuse to open.
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With the engine stopped and the cooling fan relay commanded ON, the PCM should ground the A/C relay circuit. This will also illuminate the test lamp.
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This step determines if the reason the cooling fan did not come ON was an open in the battery feed circuit to the cooling fan.
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Reprogram the replacement PCM. Perform the crankshaft position system variation procedure. Refer to the latest Techline procedures for PCM reprogramming.
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If no malfunctions have been found at this point, refer to Diagnostic Aids for additional checks and information.
Step | Action | Values | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||||||||
2 |
Are any DTCs set? | -- | ||||||||
Did you find and correct the condition? | -- | |||||||||
Is the cooling fan OFF? | 98°C (209°F) | |||||||||
5 | Command ON the cooling fan relay with the scan tool. Is the cooling fan ON? | -- | ||||||||
Is the cooling fan OFF? | -- | |||||||||
7 |
Does the test lamp illuminate for both terminals? | -- | ||||||||
Does the test lamp illuminate? | -- | |||||||||
Connect a fused jumper wire between the following circuits:
Is the cooling fan ON? | -- | |||||||||
Did you find and correct the condition? | -- | |||||||||
11 |
Does the lamp illuminate? | -- | ||||||||
12 | Repair the short to B+ in the cooling fan motor battery feed circuit (409). Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | -- | |||||||
13 |
Does the test lamp illuminate? | -- | ||||||||
Does the test lamp illuminate? | -- | |||||||||
Does the test lamp illuminate? | -- | |||||||||
16 |
Did you complete the repair? | -- | -- | |||||||
17 |
Did you find and correct the condition? | -- | ||||||||
18 |
Did you find and correct the condition? | -- | ||||||||
19 |
Does the test lamp illuminate? | -- | ||||||||
20 |
Did you find and correct the condition? | -- | -- | |||||||
21 |
Did you find and correct the condition? | -- | ||||||||
22 |
Did you find and correct the condition? | -- | -- | |||||||
23 | Replace the cooling fan motor. Refer to Engine Cooling Fan Replacement in Engine Cooling. Did you complete the replacement? | -- | -- | |||||||
24 |
Did you complete the replacement? | -- | -- | |||||||
25 | Repair the short to ground in the cooling fan motor battery feed circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | -- | |||||||
Important:: The replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming . Did you complete the replacement? | -- | -- | ||||||||
Does the cooling fan turn ON when commanded with the scan tool? | -- | System OK |
