The movement of coolant through the system
When the engine is running, the coolant pump pulley is driven by the accessory drive belt. In this case, the coolant circulates through the engine cooling jacket and the heater, while the thermostat and bypass valve are closed. When the temperature of the coolant rises, the bypass valve opens and the 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 (full opening at 95°C (203°F)) the bypass valve gradually closes, causing all 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 due to thermal expansion enters the expansion tank through a hose from the top of the radiator. The expansion tank has an outlet hose connected to the cooling circuit. This hose returns coolant to the system when the engine is cold.
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 core of the radiator. Fluid temperature is monitored by the engine control unit using a coolant temperature sensor (EATING), located in the cylinder head. The signals from this sensor are used by the engine control unit to control the cooling fan and fuel supply in accordance with the 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 in the engine control module. The PWM frequency is used by the fan control unit to determine the supply voltage to the fan motor.
The engine control unit (ECM) changes the duty cycle of the pulse-width modulated pulse sequence within the range of 0-100%, thereby adjusting the fan speed. If the signal from the engine control unit is outside the range of 0-100%, the fan control unit perceives this as an open circuit or short circuit and turns on the fan at maximum speed to prevent overheating of the engine and transmission.
The engine control module controls the fan in response to signals from the ECT sensor, transmission fluid temperature sensor, A/C switch, and A/C 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 speed of the cooling fans to compensate for the oncoming air flow. The speed signal is transmitted via the CAN bus from the anti-lock brake system control unit.
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