ASE Parts Specialist Test Preparation: Engine Cooling & Climate Control Systems

Although today’s computerized vehicles use sensors in the cooling system to properly control fuel and ignition functions, the basic functioning of this system has remained unchanged for many years.

The Antifreeze/Coolant is the lifeblood of a vehicle’s cooling system.

The coolant’s most important function is to remove the heat that is produced during combustion and the friction of the reciprocating parts (the oil helps remove heat too). A variety of formulations are available but most are based on propylene glycol or ethylene glycol mixed about half and half with water for use in the engine (often the “best” ratio is 60/40). Additives in the coolant combat the effects of water on the metal components in the engine, radiator, hoses and other components which include corrosion, erosion, and electrochemical degradation. Without these additives, the continually circulating water/coolant mixture would quickly erode and corrode the internal passages of the engine and cooling/heating components.

The Radiator is designed to dissipate the heat that has been picked up by the coolant as circulated through the engine.

Coolant is circulated to and from the radiator via Rubber Hoses. These can suffer from a variety of ailments, but contrary to the “old days”, modern hoses don’t show outward signs of imminent failure. Some bulging or softness may signal a problem, but it can’t be counted on as a sure check. Cracks that work their way from the inside out can be a hidden threat to the integrity of the cooling system.

Belts are also a critical part of every cooling system, as they are often still called upon for a variety of accessory driving functions. With Serpentine Belt systems, a single belt may be charged with a variety of responsibilities: water pump, alternator, air conditioning compressor. To perform these functions, the Tensioners must be operating properly. A professional counterperson must be vigilant on this point: a belt may have been replaced multiple times without a tensioner replacement, which can serve to reduce its life and the life of the components it drives. Tensioners should be inspected whenever a serpentine belt is replaced.

Under moderate conditions, the ram air effect forces air through the rad, carrying away heat. When the coolant or engine bay temperature rises above a preset point the Cooling Fan will be activated in order to provide the air circulation required.

A Thermostatic Fan Clutch may be employed for this. In this case, the clutch is designed to lock up when the engine heat and load demand require it. In such systems, the fan clutch is linked via a belt to the engine’s rotation via a pulley or system of pulleys. It slips until the temperature rises to the point where the fan must be operated fully.

Many systems, notably front wheel drive cars with transversely mounted engines, do not employ a clutched fan system but use a Fan Motor instead. This is activated by the Coolant Temperature Sensor which is usually a simple bi-metallic switch that completes the circuit, activating the fan. On these systems it is important that technicians use caution when working in the engine bay as coolant temperature can rise even after the engine is shut off, causing the fan to activate. This sensor should not be confused with the Engine Coolant Temperature (ECT) sensor which is linked to the engine management and fuel system but does not control the fan activation.

The Thermostat controls coolant flow to the radiator. It is designed to restrict coolant flow from the engine to the radiator until a preset temperature has been reached. Restricting coolant flow allows the engine to heat up more quickly, arriving at its proper operating temperature for good emissions and driveability.

The Water Pump is the heart of the system. Driven by a belt, it rotates and provides coolant flow whenever the engine is running.

The Coolant Tank (or Reservoir) is used differently on different systems. On some it is used to accept excess coolant that would otherwise spill out as it expands and is not part of the normal circulation path of the coolant. On others, it is part of the circulation system, with coolant running continually through it.

It both cases, it is important for the pressure in the cooling system to be retained.

This may be through a Radiator Cap or through the Coolant Tank Cap if the tank is part of the pressurized system. These pressure-limiting devices allow the coolant’s boiling point to be raised above what it would normally be if it was heated at normal atmospheric pressure. When coolant boils, its cooling capabilities are lost. In both cases, these devices are designed to release pressure if it gets above a certain point so as to prevent damage to other components. A system that has lost its ability to cool may have suffered a failure of this component.

Essentially, these are the components of the engine cooling system but, as mentioned, the hot coolant has the added benefit of providing heat to the passenger compartment. We won’t deal with a/c functioning here, just the heating system.

The Heater Core is the key component of the passenger compartment heating system. Coolant is circulated through it and it releases heat to the air in the same way as the radiator does.

The circulation of coolant through the heater core is controlled from the passenger compartment by either levers or switches hooked up to the Heater Valve. The heater valve may be either a mechanical valve hooked up directly to the lever, by a cable for example, or it may be a vacuum operated type, opening and closing under the command of preset temperature settings. There are also electromechanical versions, which may be referred to as actuators.

In Automatic Temperature Control Systems the passenger compartment temperature can be preset with the heater valve, air conditioning, blower motor and blend doors located in the plenum, the overall name for the contained passages where air is directed to the passenger compartment, being controlled automatically.

Blend doors determine whether the passenger compartment receives heating or air conditioning by directing air flow either through the heater core or around it. The evaporator is generally located in such a way that it always receives air flow. Blend doors are controlled directly by levers or by actuators (electric or vacuum operated) in automatic temperature control systems. The Blower Motor provides the forced air through the heater core and on to the passenger compartment. It may be activated by a switch in the cabin or by the temperature control system.

Although the function of the temperature control system is straightforward, when problems arise a proper diagnostic approach is still necessary. For example, if the fan does not blow warm air it could be due to low coolant — none is reaching the heater core — a stuck blend door, or, on ATC systems, a faulty relay or switch. Patience is important when handling queries from consumers or technicians.