ENGINE COOLING SYSTEM OPERATION
Note. Cooling System Coolant Flow
Scheme 5
When the engine is running the coolant pump is driven by the ancillary drive belt. This forces coolant to circulate around the engine and heater, while the thermostat and bypass valve are shut. As the temperature and pressure increases the bypass valve is forced open allowing coolant to circulate through the bypass valve. When the temperature reaches 88°C (190°F) the main thermostat begins to open allowing coolant to circulate through the main radiator. As the thermostat progressively opens (fully open at 95°C (203°F)), the bypass valve progressively closes forcing any coolant through the heater or radiator. Coolant flows through the radiator from the top RH tank to the bottom LH tank and is cooled by air passing through the matrix. A small flow of coolant from the top of the engine is directed to the expansion tank where any 'trapped' air is separated. The expansion tank also controls the cooling system running pressures. The charge air cooling circuit constantly flows coolant around the system with the engine running or with the ignition on position II. When the engine is running the two circuits are considered to be separate systems and the coolant does not transfer between the two. The temperature of the cooling system is monitored by the ECM via the Engine Coolant temperature (ECT) sensor located in the coolant housing. The ECM uses signals from this sensor to control the cooling fan operation. For additional information, refer to: Electronic Engine Controls - Description & Operation . To control the cooling fan, the ECM sends a Pulse Width Modulated (PWM) signal to the cooling fan clutch. The frequency of the PWM signal is used by the cooling fan clutch to control the level of clutch engagement. The ECM varies the duty cycle of the PWM signal between 0 and 100% to vary the clutch engagement and thus fan speed. The ECM operates the fan in response to inputs from the ECT sensor, the transmission oil temperature sensor and the A/C switch and pressure sensor. For additional information, refer to: Auxiliary Climate Control - Description & Operation . The speed of the cooling fan is also influenced by vehicle road speed. The ECM adjusts the speed of the cooling fans to compensate for the ram effect of vehicle speed, using the Controller Area Network (CAN) road speed signal received from the Anti-lock Braking System (ABS) module.