Operation

Operation

General
When the engine is running, the coolant pump draws coolant through the pipe connected to the thermostat housing and pumps it through the outlet at the right front comer of the cylinder block and through the cooling jackets In the cylinder block, cylinder heads and Inlet manifolds. From the outlet at the right front comer of the cylinder block, the coolant flows through the hoses and the coolant rail to the IRD cooler, the engine oil cooler and the transmission fluid cooler. From the top of the cylinder block, coolant flows through the outlet elbow and the top hose to the heater matrix. Coolant returning to the thermostat housing, from the cylinder block, the coolers and the heater matrix, bypasses the thermostat and flows back to the coolant pump inlet to begin the cycle again.

Coolant also flows through the expansion pipes to the expansion tank, from where it is drawn back into the system through the expansion tank outlet hose connected to the coolant rail. The increase in coolant volume, created by heat expansion, reduces the air space In the expansion tank, which pressurizes the system and raises the boiling point of the coolant.

Coolant flows through the radiator, from the top of the RH end tank to the bottom of the LH end tank, and is cooled by air passing through the radiator. When required, the two cooling fans provide additional air flow through the radiator, particularly when the vehicle is stationary. Operation of the cooling fans is controlled by the ECM via the cooling fan ECU. The temperature of the cooling system is monitored by the ECM using inputs from the thermostat monitoring sensor (NAS only) and the Engine Coolant Temperature (ECT) sensor located in the cylinder block, to the left of the thermostat housing.

Pressure Relief Thermostat (PRT)
The thermostat is exposed to 85% hot coolant from the engine on one side and 15% cold coolant returning from the radiator bottom hose on the other side. This allows the thermostat to react to the ambient conditions and provide coolant control for both winter and summer use. Hot coolant from the engine passes via holes in the bypass flow valve Into a tube which surrounds 85% of the thermostat sensitive area. Cold coolant from the radiator conducts through the remaining 15% of the sensitive area. In cold ambient conditions, the engine temperature is raised by approximately 100°C (50°F) to compensate for the heat loss of 15% exposure to the cold coolant returning from the bottom hose. This improves heater performance and engine warm-up.

The by-pass flow valve is held closed by a light spring and operates to further assists engine and heater warm-up. When the main valve is closed and the engine speed is at idle, the coolant pump does not produce sufficient flow and pressure to overcome the spring and open the valve. In this condition the valve prevents coolant circulating through the by-pass circuit and directs coolant through the heater matrix only. This provides a higher flow of coolant through the heater matrix improving passenger comfort In cold conditions.

When the engine speed increases above idle, the coolant pump produces a greater flow and pressure than the heater circuit can accommodate. The build up of pressure acts on the flow valve, overcoming the spring pressure, opening the valve and relieving the pressure in the heater circuit. The valve then modulates to provide maximum coolant flow through the heater matrix and allowing excess coolant to flow Into the by-pass circuit to provide the engine's cooling requirements at higher engine speeds. The thermostat then regulates the flow through the radiator to maintain the engine at the optimum temperature. Maximum opening of the thermostat, and therefore maximum flow through the radiator, occurs if the coolant temperature reaches 102°C (216°F).

On NAS vehicles, operation of the thermostat is monitored by the Engine Control Module (ECM) using an input from the thermostat monitoring sensor in the bottom hose.

Heater Diverter Valve (NAS and Gulf States only) - From 2003 Model Year
At low engine speeds the heater diverter valve is closed, allowing only a bleed flow of coolant through the engine oil and transmission fluid coolers. This increases the coolant flow through the heater matrix improving heater performance.

As the engine speed increases, the Increased output from the coolant pump opens the heater diverter valve allowing the full flow of coolant through the engine oil and transmission fluid coolers and the heater matrix

Cooling Fan Control
The ECM controls the operation of the variable speed cooling fans via a Pulse Width Modulated (PWM) signal to the cooling fan ECU. The cooling fan ECU regulates the voltage of a common power feed to the two fans, and relates the voltage to the duty cycle of the PWM signal.

All Except NAS
The ECM varies the duty cycle of the PWM signal between 10 and 90% to vary the fan speed. If the PWM signal is outside the 10 to 90% range, the cooling fan ECU Interprets the signal as an open or short circuit and runs the fans at maximum speed to ensure the engine and gearbox do not overheat.

The ECM operates the fans in response to Inputs from:
^ The ECT sensor, for engine cooling. With the engine running, the fans come on at maximum speed If the coolant temperature increases to 105°C (221°F). The fans go off when the coolant temperature decreases to 102°C (216°F).
^ The instrument pack and the A/C pressure sensor, for refrigerant system cooling.
^ The EAT ECU, for gearbox cooling.

If there is a conflict between requested fan speeds from the different inputs, the ECM adopts the highest requested speed.

As part of the power down routine, when the ignition is switched off the ECM samples the engine coolant temperature. If the coolant temperature is below 102°C (216), the fans remain off. If the coolant temperature is between 102 and 105°C (216 and 211°F) the fans run at 8% of full speed for 5 minutes and then go off. If the coolant temperature is 105°C (221°F) or above, the fans run at 20% of full speed for 5 minutes and then go off.

As the engine cools after the ignition is switched off, the coolant volume decreases and coolant is drawn from the expansion tank through the outlet hose connected to the coolant rail. The air space in the expansion tank increases and the pressure in the coolant system decreases to approximately ambient pressure.

NAS
The ECM varies the duty cycle of the PWM signal between 3 and 90% to vary the cooling fan speed. At duty cycles between 3 and 9% the cooling fans are off. When the duty cycle goes above 9% the cooling fans come on at minimum speed, then Increase in speed, In proportion to the increase In the duty cycle, up to maximum speed at a duty cycle of 90%. If the duty cycle is less than 3% or more than 90%, the cooling fan ECU interprets the signal as an open or short circuit and runs the cooling fans at maximum speed to ensure the engine and gearbox do not overheat.

The speed of the cooling fans varies between a minimum of 750 rev/min, at 6 volts and a maximum of 4000 rev/min at nominal battery voltage. To reduce the noise from the cooling fans they are driven at slightly different speeds, except when running at minimum and maximum speeds. Stepped speed changes occur at 1500 rev/min (RH cooling fan), 1450 and 2600 rev/min (LH cooling fan) to improve refinement.

Control Inputs
While the engine is running, the ECM adjusts the speed of the cooling fans in response to inputs from:
^ The thermostat monitoring sensor, for engine cooling. The fans come on at minimum speed if the coolant temperature goes above 90°C (194°F), and progressively Increase to maximum speed at a coolant temperature of 102°C (216°F).
^ The A/C system, via the instrument pack and the CAN bus, for refrigerant system cooling.
^ The EAT ECU, via the CAN bus, for gearbox cooling.
If there is a conflict between requested cooling fan speeds from the different inputs, the ECM adopts the highest requested speed.

As part of the power down routine, when the ignition is switched off, If the ambient air temperature Is more than 15°C (59°F) the ECM samples the coolant temperature using the Input from the Engine Coolant Temperature (ECT) sensor. If the coolant temperature is more than 106°C (223°F), the ECM signals for the cooling fans to come on. The speed of the cooling fans is in proportion to the coolant temperature, from minimum speed at >106°C (223°F), to maximum speed at 115°C (239°F). The cooling fans are switched off after 5 minutes or when the coolant temperature decreases to 106°C (223°F), whichever occurs first.

Motor Protection
The cooling fan ECU monitors the speed of the cooling fans, from the current draw of the motors, and incorporates strategies to protect the motors from electrical overload if the fans are seized or heavily loaded (e.g. by debris or during wading).

When the duty cycle of the PWM signal indicates the cooling fans should be switched on, the cooling fan ECU Initially outputs 2.5 volts to the motors to produce a 'soft' start. When the cooling fan ECU detects the motors are running satisfactorily, it then increases the outputs to the appropriate voltages for the required cooling fan speeds. If a motor falls to start within 3 seconds, the cooling fan ECU switches off the output to the affected motor, s for 5 seconds and then tries another soft start. If the second soft start falls, after a further wait of 5 seconds the cooling fan ECU outputs 6 volts to the motor in an attempt to get it started. If the motor starts, the cooling fan ECU then increases the output to the appropriate voltage for the required cooling fan speed. If the motor falls to start within 4 seconds, the cooling fan ECU switches the output off, then, provided the cooling fans are still requested on, periodically invokes the start routine in an attempt to get the motor running.

If a cooling fan is already running and then seizes or becomes heavily loaded, the cooling fan ECU switches off the output to the affected motor, then periodically invokes the start routine, while the cooling fans are still requested on, in an attempt to get the motor running again.

When one of the cooling fans is not running, the cooling fan ECU runs the other cooling fan at maximum speed.