Movement of coolant through the system
When the engine is running, the water pump pulley is driven by the accessory drive belt. This circulates coolant through the engine water jacket and heater while the thermostat and bypass valve are closed. As the coolant temperature rises, the bypass valve opens and coolant begins to circulate through it. When the temperature reaches 82°C (180°F), the main thermostat opens, allowing coolant to circulate through the main radiator. As the main thermostat gradually opens (fully open at 95°C (203°F)) the bypass valve gradually closes, allowing all of the coolant to circulate through the heater or radiator. As coolant begins to circulate through the radiator, it also begins to flow into the transmission fluid cooler and engine oil cooler.
Excess coolant volume caused by thermal expansion enters the expansion tank through a hose from the top of the radiator. The expansion tank has a drain hose connected to the cooling system circuit. This hose returns coolant to the system when the engine cools down.
The coolant flows through the radiator from the top of the right reservoir to the bottom of the left reservoir and is cooled by air passing through the radiator core. The temperature of the coolant is monitored by the engine control unit using the coolant temperature sensor (ECT) located in the cylinder head. The signals from this sensor are used by the engine control unit to control the cooling fan and fuel delivery in accordance with engine temperature. For more information, refer to Electronic Engine Controls (303-14C Electronic Engine Controls - 2.7L Diesel)
The ECM controls the cooling fan via a pulse width modulated signal that is sent to the fan control module integrated into the engine control module. The frequency of the pulse width modulated signal is used by the fan control module to determine the supply voltage to the fan motor.
The Engine Control Module (ECM) varies the duty cycle of the pulse-width modulated pulse train within the range of 0-100%, thereby regulating the fan speed. If the signal from the ECM goes beyond the range of 0-100%, the fan control module perceives this as an open circuit or short circuit and turns the fan on at maximum speed to prevent overheating of the engine and transmission.
The engine control module controls the fan in accordance with signals from the ECT sensor, transmission fluid temperature sensor, air conditioner switch and air conditioning pressure sensor. For more information, refer to Air Conditioning (412-03A Air Conditioning - 4.0L) In addition, the fan speed depends on the vehicle speed. The ECM regulates the cooling fan speed to compensate for the oncoming air flow. The vehicle speed signal is transmitted via the CAN bus from the anti-lock brake system control unit.
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