Cooling system - Td4 (Operation): Description and operation

Operation

With the engine running, the coolant pump pulley is driven by the ancillary drive belt. When the engine is cold, the thermostat is closed and coolant is prevented from circulating through the radiator. Coolant is able to circulate through the bypass, heater matrix and coolers.

As the temperature increases, at 88 °C (190 °F) the thermostat begins to open, bleeding cool fluid from the radiator bottom hose through the pump and into the cylinder block. This allows hot coolant to flow from the cylinder block to the radiator through the top hose, balancing the flow of hot and cold fluid to maintain the optimum operating temperature. When the thermostat opens fully, the full flow of coolant passes through the radiator.

On automatic gearbox models with a liquid cooled transmission fluid cooler, the coolant from the radiator bottom hose is diverted through the cooler and back into the bottom hose. The bottom hose incorporates a valve which opens and closes dependent on system pressure to divert the flow through the cooler when applicable.

The increased coolant volume, created by heat expansion, is directed to the expansion tank through a bleed hose from the top of the radiator. The expansion tank has an outlet hose which is connected into the coolant circuit. This outlet hose returns the coolant to the system when the engine cools.

Coolant flows through the radiator from the top right hand tank to the bottom left hand tank and is cooled by air passing through the matrix. The temperature of the cooling system is monitored by the ECM via a temperature sensor located in the cylinder head. The ECM uses signals from this sensor to control the cooling fan operation and adjust fuelling according to engine temperature. ENGINE MANAGEMENT SYSTEM - EDC, DESCRIPTION AND OPERATION, Engine Management Component Layout.

To control the cooling fans, the ECM sends a Pulse Width Modulated (PWM) signal to the cooling fan ECU. The frequency of the PWM signal, which is varied by the ECM, is used by the ECU to determine the output voltage supplied to the fan motors.

The ECM operates the fans in response to inputs from the ECT sensor, the A/C switch and the A/C pressure sensor. Refer to the A/C system for details. AIR CONDITIONING, DESCRIPTION AND OPERATION, Description.

The speed of the cooling fans 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 CAN road speed signal received from the ABS ECU.

When the engine is switched off, the ECM maintains the cooling fans in an active condition for up to 8 minutes. If the temperature does not reach a predetermined value within 4 minutes, the ECM will terminate the active period. If the fans are active and the temperature falls below a predetermined value, the ECM will terminate further fan operation.