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Cooling system, description and function, E39

Cooling system, description and function, E39


Engine coolant, indicators

• In the instrument panel (IPC), the engine temperature is shown on the temperature gauge. This value is sent from the engine control module (ECM) via a bus. If the coolant temperature is above 128°C (262°F), the IPC receives a signal from the ECM which requests that the indicator be switched on.

• The IPC carries out the display test when every ignition cycle begins. The IPC switches on the TEMP indicator.


Coolant level regulation

In the engine's cooling system, there is a level switch for coolant which warns the driver if the coolant level is low. If the coolant level in the expansion tank falls below a certain level, the circuit is broken in the level switch. If the body control module (BCM) detects an interruption or a high voltage level on the coolant level control circuit for more than 10 seconds, a GM LAN message is sent to the driver's information centre (DIC) and requests that the low coolant level message be displayed. There is a delay of about 10 seconds before the BCM sends the GM LAN message in order to prevent the message being sent due to the coolant splashing about in the expansion tank.

Block heater

The block heater is run on external alternating current and is designed to heat the coolant in the engine block in order to make the engine easier to start in cold weather. The block heater helps to reduce fuel consumption while a cold engine is warming up. This equipment has a removable mains cable (alternating current). There is a guard on the cable to protect the contact when it is not in use.

Cooling system

The job of the cooling system is to maintain an efficient working temperature for the engine at all engine speeds and under all operating conditions. The cooling system is designed to dissipate about a third of the heat arising when the air-fuel mixture is burnt. When the engine is cold, the coolant does not flow to the radiator until the thermostat opens. This allows the engine to heat up quickly.

Cooling cycle

The coolant is fed from the radiator outlet to the water pump inlet. Some of the coolant passes from the water pump to the heating element and then back to the water pump. This provides the passenger compartment with heat and permits defrosting when the coolant is heated up.

Coolant also passes from the water pump outlet to the engine block. In the engine block, the coolant circulates in the cooling ducts which surround the cylinders, where the coolant absorbs heat.

The coolant then flows through the openings in the cylinder head gasket and to the top cover. There, the coolant flows through the water ducts which surround the combustion chambers and valve seats and absorbs more heat.

The coolant is also passed to the throttle body. Here, the coolant circulates through openings in the casting. Immediately after starting, the coolant helps to heat up the throttle body.

From the top cover, the coolant is fed to the thermostat. The coolant flow is stopped either at the thermostat until the engine reaches normal working temperature, or else it passes through the thermostat and on to the radiator, where the coolant is cooled. This is where the coolant flow cycle ends.

For the cooling system to work efficiently, all components have to work as designed. The cooling system consists of the following components.

Coolant

The coolant is a solution consisting of half pure water and half antifreeze. The coolant dissipates excess heat from the engine to the radiator, where the heat is transferred to the atmosphere.

Radiator

The radiator is a heat exchanger consisting of a core and two reservoirs. The aluminium core is a cross-flow structure made up of pipes and cooling flanges which extends from the inlet reservoir to the outlet reservoir. The cooling flanges are located on the outside of the pipes so as to improve heat transfer to the atmosphere.

The inlet and outlet reservoirs are made of cast, nylon reinforced, high temperature plastic. A high temperature rubber gasket seals between the reservoir flange and the aluminium core. The reservoirs are secured to the core by means of clamp lugs. These lugs are included in the aluminium head at every end of the core.

The radiator also has a drain cock at the bottom of the left reservoir. The cock unit consists of the cock itself and a seal.

Heat dissipates from the coolant when it passes through the radiator. The cooling flanges in the core transfer heat from the coolant which is fed through the pipes. Air passing between the cooling flanges absorbs heat and cools the coolant.

Pressure cap

The pressure cap seals and pressurises the cooling system. In the cap is a relief valve and a vacuum or atmosphere valve:


• The relief valve is held against the seat by a spring which protects the radiator by releasing pressure which exceeds 1.03 bar (15 psi).

• The thermal vacuum valve is held against the seat by a spring which allows the valve to open in order to reduce the vacuum which occurs when the cooling system cools. This vacuum could cause the radiator to collapse if it were not relieved.


The pressure cap allows the pressure in the cooling system to increase when the temperature rises. When the pressure rises, the boiling point of the coolant rises. Therefore, the coolant can be used safely at a much higher temperature than its boiling point at barometric pressure. The hotter the coolant becomes, the faster heat is transferred from the radiator to the cooler air.

The pressure in the cooling system may become too high. When the pressure exceeds the power of the spring the relief valve is lifted and releases the excess pressure.

When the engine cools, the temperature of the coolant falls and a vacuum is created in the cooling system. This causes the thermal vacuum valve to open. This equalises the pressure in the cooling system in relation to barometric pressure and prevents the collapse of the radiator.

Coolant expansion system

The coolant expansion system consists of a coolant reservoir made of plastic and an overflow pipe. The coolant reservoir is also known as the expansion tank. The reservoir is partly filled with coolant and is connected to the filler pipe of the radiator via the overflow pipe. The coolant can flow back and forth between the radiator and the reservoir.

In practice, a cooling system with an expansion tank is a closed system. When the pressure in the cooling system becomes too high, the relief valve in the pressure cap opens. This allows the coolant - which has expanded on account of the heat - to flow through the overflow pipe to the expansion tank.

When the engine cools, the temperature of the coolant falls and a vacuum is created in the cooling system. This vacuum opens the pressure cap vacuum valve and allows some of the coolant in the expansion tank to be sucked back to the radiator.

Under normal operating conditions, no coolant is lost. Even if the coolant level in the expansion tank rises and falls, the radiator and cooling system remain full. One advantage of an expansion tank is that almost all air bubbles in the cooling system disappear. Coolant without bubbles absorbs heat much more effectively than coolant containing bubbles.

Air deflectors and seals

In the cooling system, there are diverters, air deflectors and air seals which improve the capacity of the cooling system. There are diverters underneath the car which lead the air flow under the car and through the radiator to increase the cooling effect. Air deflectors are also used to direct the air flow through the radiator and increase its capacity. Air seals prevent air moving past the radiator and the air conditioning condenser and prevent hot air recirculating, which results in better AC condenser cooling and capacity in hot weather.

Water pump

The water pump is a centrifugal eccentric pump with a pump rotor. The pump consists of a housing with an inlet and an outlet for the coolant and a pump rotor. The pump rotor is located on the pump shaft and is a flat plate with a number of flat or curved blades or vanes. When the pump rotor rotates, the coolant is thrown between the vanes and outwards by centrifugal force. The pump rotor shaft is supported by one or more sealed bearings which never need to be lubricated. Grease cannot leak out of a sealed bearing, and dirt or water cannot enter.

The water pump circulates the coolant through the cooling system. The pump is driven by the crankshaft via the drive belt.

Thermostat

The thermostat is a regulating component in the coolant flow and its job is to regulate the working temperature of the engine. A temperature sensitive wax element which is connected to a valve via a piston is used in the thermostat. Heat makes the element expand and exert pressure on a rubber diaphragm. This pressure forces the valve to open. Cooling causes the element to contract. This contraction allows a spring to compress the valve.

If the coolant temperature is lower than 91°C (195°F), the thermostat valve is closed. This prevents the coolant flowing to the radiator and allows the engine to warm up quickly. When the coolant temperature reaches 91°C (195°F), the thermostat valve opens.

The switching point varies slightly depending on the engine. The coolant is then allowed to circulate through the thermostat to the radiator, where the heat from the engine is output to the atmosphere. The thermostat also constitutes a restriction in the cooling system, even when it is open. This restriction creates a pressure difference, which prevents cavitation at the water pump and forces the coolant to circulate through the engine block.

In some engines, the circulation is opened and closed by a solenoid thermostat which is controlled by the ECM.

Oil cooler, transmission

The oil cooler for the gearbox is a heat exchanger which is located inside the right-hand end reservoir of the radiator. The gearbox oil temperature is regulated by the surrounding engine coolant temperature when the gearbox oil flows through the radiator.

The gearbox oil pump circulates the oil through the feed line to the oil cooler. The oil then goes back through the cooler while the engine coolant absorbs heat from the oil. The oil is then pumped via the return line and back to the gearbox.