The engine cooling fan system is controlled by the body control module (BCM) and the powertrain control module (PCM) or engine control module (ECM). The BCM performs the calculations as to how long, when and what speed the cooling fans should turn on. The BCM then sends a class 2 message to the PCM/ECM to engage the cooling fan relays. If there is a malfunction with the BCM, the PCM/ECM will control the engine cooling fans independently. The engine cooling system consists of 2 electrical cooling fans and 3 fan relays. The relays are arranged in a series/parallel configuration that allows the PCM/ECM to operate both fans together at low or high speeds. The cooling fans and fan relays receive battery positive voltage from the underhood accessory wiring junction block. The ground path is provided at G100.
During low speed operation, the PCM/ECM supplies the ground path for the low speed fan relay through the low speed cooling fan relay control circuit. This energizes the cooling fan 1 relay coil, closes the relay contacts, and supplies battery positive voltage from the cool fan 1 fuse through the cooling fan motor supply voltage circuit to the left cooling fan. The ground path for the left cooling fan is through the cooling fan 2 relay and the right cooling fan. The result is a series circuit with both fans running at low speed.
During high speed operation the PCM/ECM supplies the ground path for the cooling fan 1 relay through the low speed cooling fan relay control circuit. After a 3 second delay, the PCM/ECM supplies a ground path for the cooling fan 3 or S/P relay and the cooling fan 2 relay through the high speed cooling fan relay control circuit. This energizes the cooling fan 2 relay coil, closes the relay contacts, and provides a ground path for the left cooling fan. At the same time the cooling fan 2 relay coil is energized closing the relay contacts and provides battery positive voltage from the cool fan 2 fuse on the cooling fan motor supply voltage circuit to the right cooling fan. During high speed fan operation, both engine cooling fans have there own ground path. The result is a parallel circuit with both fans running at high speed.
The BCM and PCM/ECM command Low Speed Fans on under the following conditions:
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Engine coolant temperature exceeds approximately 106?Ā°C (223?Ā°F).
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A/C refrigerant pressure exceeds 1 310 kPa (190 psi).
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After the vehicle is shut off if the engine coolant temperature at key-off is greater than 140?Ā°C (284?Ā°F) and system voltage is more than 12 volts. The fans will stay on for approximately 3 minutes.
The BCM and PCM/ECM command High Speed Fans on under the following conditions:
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Engine coolant temperature reaches 110?Ā°C (230?Ā°F).
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A/C refrigerant pressure exceeds 1 655 kPa (240 psi).
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When certain DTCs set.
The three relays are located in the under hood fuse / relay box ! You need to have it checked for DTC'S -diagnostic trouble codes ! An you need a GM Tech 2 scan tool !
This diagnostic procedure supports the following DTCs:
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DTC P0480 Cooling Fan 1 Relay Control Circuit
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DTC P0481 Cooling Fan 2 and 3 or S/P Relay Control Circuit
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DTC P0691 Cooling Fan 1 Relay Control Circuit Low Voltage
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DTC P0692 Cooling Fan 1 Relay Control Circuit High Voltage
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DTC P0693 Cooling Fan 2 and 3 or S/P Relay Control Circuit Low Voltage
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DTC P0694 Cooling Fan 2 and 3 or S/P Relay Control Circuit High Voltage
- Install a scan tool.
- Turn ON the ignition, with the engine OFF.
- With a scan tool, command the Fans Low Speed ON and OFF.
Do the low speed engine cooling fans turn ON and OFF with each command?
Go to Step 3
Go to Step 4
3
Important:: A 3 second delay occurs before the powertrain control module (PCM)/engine control module (ECM) changes the cooling fan speed.
With a scan tool, command the Fans High Speed ON and OFF.
Do the high speed engine cooling fans turn ON and OFF with each command?
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