Cooling System: Description and Operation
Engine Cooling
COMPONENT LOCATION
INTRODUCTION
The engine cooling system maintains the engine within an optimum temperature range under changing ambient and engine operating conditions. The system is a pressurized expansion tank system with continuous bleeds to separate air from the coolant and prevent the formation of air locks. The engine cooling system also provides:
- Heating for:
- The passenger compartment.
- The throttle body.
- Cooling for:
- The engine oil cooler.
- The transmission fluid cooler.
The primary components of the engine cooling system are the:
- Coolant pump.
- Thermostat.
- Radiator.
- Cooling fan.
- Expansion tank.
- Outlet tube and heater manifold.
- Connecting hoses and pipes.
COOLANT PUMP
The body of the coolant pump contains an impeller attached to a shaft supported in a bearing assembly. The impeller is driven by a pulley, pressed on to the front of the shaft, which is driven by the accessory drive belt. For additional information, refer to Accessory Drive Description and Operation
Two coolant outlet flanges attach the coolant pump to the front of the cylinder heads. A pipe connects a further coolant outlet to a pipe from the engine oil cooler. A bleed connector is installed in the front of the coolant pump, adjacent to the coolant inlet connection from the thermostat. A check valve is incorporated into the bleed connection.
THERMOSTAT
The thermostat is a multi-stage device located in the coolant pump inlet to provide fast response and control of the engine outlet temperature.
The thermostat allows rapid engine warm-up by preventing coolant flow through the radiator and by limiting coolant flow through the cylinder block when the engine is cold. During warm-up and at engines speeds above approximately 1800 rev/min, a by-pass valve opens to control the coolant flow and pressure, to protect the engine components. When the thermostat opening reaches 6 mm (0.24 in.), the by-pass flow is shut-off. When the thermostat opening exceeds 6 mm (0.24 in.), the radiator coolant flow is further controlled up to the point where the thermostat is fully open. At this point maximum radiator coolant flow is achieved to provide maximum cooling.
The thermostat begins to open at 88 - 90 °C (190 - 194 °F) and is fully open at 102 °C (216 °F).
RADIATOR
The radiator is an aluminum cross flow type with plastic end tanks. Upper and lower supports locate the radiator in the radiator support assembly and the front crush siderails respectively.
Connections are incorporated into the end tanks for the upper and lower hoses, the supply hose of the transmission fluid cooler and a bleed hose.
COOLING FAN
For additional airflow through the radiator, particularly when the vehicle is stationary or moving slowly, there is an engine driven electro-viscous cooling fan. The cooling fan functions as a normal viscous fan, but with electronic control over the level of engagement of the viscous clutch. The ECM (engine control module) controls the level of engagement to optimize fan speed for all operating conditions.
A securing nut attaches the cooling fan to a drive pulley, which is mounted on the front of the engine and driven by the accessory drive system.
NOTE:
The securing nut has a LH (left-hand) thread.
The blades of the cooling fan are located in a fan cowl attached to the rear of the radiator frame. Brushes around the circumference of the aperture in the fan cowl provide a seal with the blade shroud. An electrical connector in the top left corner of the fan cowl provides the interface between the cooling fan harness and the vehicle wiring.
EXPANSION TANK
The expansion tank is installed on the LH (left-hand) side of the cooling module, attached to the radiator support assembly and the floor sidemember. A filler cap, bleed screw and level sensor are incorporated into the expansion tank. MAX and MIN level markings are molded onto the exterior of the tank.
The expansion tank provides the following functions:
- Service fill.
- Coolant expansion during warm-up.
- Air separation during operation.
- System pressurization by the filler cap.
The expansion tank has an air space of approximately 1.1 liters (1.16 US quarts), above the MAX level, to allow for coolant expansion.
OUTLET TUBE AND HEATER MANIFOLD
Outlet Tube
Heater Manifold
ENGINE COOLANT
The engine coolant is formulated to last for ten years or 240,000 km (150,000 miles). The coolant is silicate free and must not be mixed with conventional engine coolant.
CONTROL DIAGRAM
NOTE:
A = Hardwired; N = Medium speed CAN (controller area network).
OPERATION
Engine Cooling Flow Diagram
When the engine is running, the coolant is circulated around the engine cooling system by the coolant pump. From the coolant pump, coolant flows through the cylinder heads and the engine oil cooler into the cylinder block and the heater manifold.
In the cylinder block, the coolant flows forwards to the outlet tube. When the coolant is cold, the thermostat is closed and the coolant flows direct from the outlet tube back to the coolant pump. Once the coolant reaches operating temperature the thermostat begins to open, to control system temperature, and coolant flows from the outlet tube to the coolant pump via the radiator. When the thermostat is open, the coolant flow through the radiator also generates a coolant flow through the transmission fluid cooler.
From the heater manifold the coolant flows through the electronic throttle and the heater core, in parallel circuits that are unaffected by the position of the thermostat. From the electronic throttle, the coolant merges with bleed coolant from the coolant pump and flows to the expansion tank. From the heater system, the coolant flows back to the inlet of the coolant pump.
Expansion and contraction of the coolant is accommodated by an air space in the expansion tank and the compliance of the flexible hoses.
If the coolant level in the expansion tank decreases below a predetermined value, the coolant level sensor connects a ground to the CJB (central junction box), which sends a message to the instrument cluster on the medium speed CAN (controller area network) bus to display the message COOLANT LEVEL LOW in the message center. For additional information, refer to Information and Message Center Description and Operation
To control the cooling fan, the ECM (engine control module) sends a PWM (pulse width modulation) signal to the cooling fan drive unit. The ECM (engine control module) varies the duty cycle of the PWM (pulse width modulation) signal between 0 and 100% to vary the clutch engagement and thus fan speed. The ECM (engine control module) determines the required fan speed from:
- Coolant, ambient air and transmission fluid temperatures
- A/C (air conditioning) system condenser cooling fan demand
- Road speed
- Terrain optimization mode.
If the electrical connections to the viscous fan are disconnected the fan will idle and the engine may overheat. If the ECM (engine control module) detects a cooling fan fault it stores the appropriate DTC (diagnostic trouble code) and signals the instrument cluster on the medium speed CAN (controller area network) bus to display the message COOLING SYSTEM FAULT MONITOR GAUGE in the message center.