Keeping an engine cool is no easy task. Nearly a third of the heat released by combustion ends up as waste heat in the engine block and cylinder heads. All that heat has to go somewhere to prevent a meltdown, so coolant circulating through the block and heads soaks up the heat and carries it to the radiator where airflow absorbs the heat and carries it away.
Managing the heat also requires maintaining a relatively stable operating temperature as engine speeds and loads change. This is necessary for emissions control, fuel economy and performance.
As long as the cooling system does its job and keeps up with the heat generated by the engine, nothing bad happens. But if the cooling system falls behind and fails to keep up with the heat generated by the engine, the engine can quickly overheat. That’s why cooling system failures are one of the leading causes of vehicle breakdowns on the highway according to the American Automobile Association (AAA).
The most common cause of engine overheating is loss of coolant, which is often the result of an external or internal engine coolant leak. But overheating can also occur if a thermostat sticks shut, a water pump fails, a mechanical or electric cooling fan fails to move enough air through the radiator at low speed, if the radiator becomes clogged with bugs or debris externally or sludge internally, if a hose is pinched or collapses, or other factors cause the engine to run hotter than normal (such as a partially plugged catalytic converter or other exhaust restriction).
In the days when most vehicles had a simple belt-driven mechanical fan (without a clutch), heat management was handled almost exclusively by the thermostat. The fan’s job was to pull air through the radiator, and the faster the vehicle was driven, the more air the fan pulled. The fan really wasn’t needed at higher vehicle speeds so the energy expended spinning it just wasted power and fuel. Fan clutches were introduced that allowed the fan to slip at higher speeds (and thus save power and fuel), or to vary the speed of the fan in response to changes in engine heat (thermostatically controlled fan clutches). But the primary job of regulating the engine’s operating temperature was left up to the thermostat.
A thermostat speeds engine warm-up by remaining closed after a cold engine is first started. This traps the coolant in the block and prevents it from circulating back to the radiator. Heat builds up and pretty soon the coolant is hot enough (typically 195 degrees F or higher) to cause the thermostat to open. The little wax pellet inside the bottom of the thermostat expands and pushes open the valve that allows the coolant to pass.
As the coolant temperature drops, the wax inside the thermostat contracts. This allows the spring on the top of the thermostat to push the valve shut to restrict or block the flow of coolant until the temperature goes back up. This simple open/close cycling of the thermostat creates a yo-yo effect that more or less keeps the engine’s operating temperature within limits, but not with a high degree of precision.
If a thermostat fails and sticks shut, it blocks the flow of coolant and will quickly cause the engine to overheat. If it sticks open, the engine runs too cold and never reaches its normal operating temperature. On engines with computer controls, this can prevent the engine management system from going into closed loop, causing the engine to run rich, waste fuel and pollute.
Some thermostats have a “fail-safe” design so that even if the temperature sensing element fails, it will remain partially open to prevent the engine from overheating.
A common quick check for a stuck thermostat is to carefully feel the upper radiator hose when the engine is hot and running. If the vehicle has been overheating and the hose is not hot to the touch, the thermostat is not opening and no coolant is flowing from the block to the radiator. A handheld infrared thermometer can also be used to read the temperature of the upper hose and thermostat housing.
Nowadays, temperature control is a more complex process. The thermostat still opens and cycles in response to temperature changes in the coolant (and can still stick open or shut causing the same problems as before), but now we have electric cooling fans and electronic engine controls that also play an active role in managing engine cooling.
The Powertrain Control Module (PCM) monitors coolant temperature via a coolant sensor and cycles the electric cooling fan(s) on and off as needed to fine-tune engine cooling. The result is less temperature fluctuation and a more consistent operating temperature for lower emissions and more consistent performance.
Electric cooling fans are typically cycled on only when the coolant reaches a certain temperature, or when the A/C is turned on to increase airflow through the A/C condenser and radiator. The PCM’s operating strategy may also turn the fan(s) on or off depending on vehicle speed, engine load, throttle position, gear position and the temperature of the transmission fluid.
On a Toyota Prius hybrid electric vehicle, there is a separate cooling system for the hybrid power inverter with its own electric pump. The PCM may also turn on the engine’s cooling fan and A/C if the temperature of the main battery pack gets too high. The battery relies on A/C cooling through special ducts in the passenger compartment.
Cooling Fan Circuits
The typical fan control circuit contains a temperature sensor or switch (which may be separate from the main engine coolant temperature the PCM uses), a power relay, a fan motor and a control module or the PCM itself. The fan may be commanded on when the coolant reaches a predetermined temperature, when the climate control system calls for A/C, or when the vehicle is traveling below a certain speed (typically 25 mph or so).
Technicians and motorists have to be careful when poking around under the hood on many vehicles because the electric cooling fan(s) may come on unexpectedly whether the engine is running or not.
If the cooling fan fails to come on when it is commanded to do so by the PCM, there may not be enough airflow through the radiator and condenser to provide adequate engine cooling especially during hot weather. This may cause the engine to overheat and boil over, and/or it may cause the A/C compressor to run dangerously hot increasing the risk of seizure and failure. That’s why the fan control circuit should always be checked as a possible cause of engine overheating or premature A/C compressor failure.
A fan motor can be checked by using a pair of fused jumper wires. The fan should spin normally when battery voltage and ground are attached to the fan motor connector. No motion or very slow motion would tell you the motor is bad and needs to be replaced. A motor that spins slowly can also be checked with an amp probe. If the motor draws more than about 15 amps, the bearings are dragging or the brushes are worn. Either way, the motor should be replaced.
Most problems with cooling fans are not the fan motor, however, but the fan relay(s). There may be one or more relays in the fan circuit. Relays typically handle high-amperage currents, which means eventually they wear out and fail and when a relay fails, it will stop the fan cold.
Mechanical relays use a magnetic coil and contacts to turn the fan current on and off. The typical relay coil has a resistance value of 40 to 80 ohms, which can be checked with an ohmmeter. If resistance is higher than normal, the coil is failing and the relay should be replaced. If there is no resistance, the coil is open and the relay has failed.
Most relays are the normally open type. The armature closes the contacts when the coil is energized. But there are also normally closed relays. On these, the armature contacts are normally closed and are pulled open when the relay is energized. There are also “dual” relays that switch the current one way when they are energized, and another way when they are deenergized. On some applications, all three types of relays may be used in the cooling fan and A/C fan circuits.
If a bad relay is suspected of preventing power from reaching the fan motor, it can be bypassed with a fused jumper cable to see if the fan spins when it receives battery voltage. If the fan works when the relay is bypassed, it doesn’t always mean the relay is the problem because there may be a wiring, connector or ground fault in the circuit.
Another quick check for a bad relay is to remove it and shake it. If it rattles, the armature is broken and your customer needs a new relay. A “known good relay” can also be swapped for a questionable relay to see if changing relays restores normal fan operation.
On OBD II vehicles, a scan tool that displays data PIDs can be used to check the operation of the fan circuit. Check the line that reads cooling fan status. There should be a value listed saying ON or OFF. When the engine is started and the A/C is turned on, the cooling fan command line should change to ON, and the fan should spin. No change in the command status would indicate a communication problem between the climate control system and PCM, or a fault within the PCM or control module. If the fan is commanded ON but fails to come on, the vehicle has a bad relay or a wiring problem that will require further diagnosis.
On older vehicles that have a mechanical fan with a clutch, the clutch reduces fan speed at high speed and when cooling demands are light to reduce fan noise and drag on the engine. A “thermal” fan clutch has a heat-sensing bimetal spring on the front that reacts to the temperature of the air passing through the radiator. The clutch increases slippage when less cooling is needed, and decreases slippage when more cooling is needed.
The clutch is filled with silicone fluid. The shear characteristics of the fluid gradually diminish over time, which may increase slippage to the point where the fan doesn’t spin fast enough to keep the engine cool. Fluid can also leak out of the clutch causing it to fail.
The service life of the typical fan clutch is about the same as the typical water pump, so if a customer is replacing a water pump, recommend a new fan clutch too.
Some import cars (such as Toyota and Lexus) use a hydraulic cooling fan on certain models. On these applications, fluid from the power steering pump is used to drive the fan. A separate control module controls the operation of the system. It uses a duty-cycle solenoid in the power steering circuit to route fluid to the pump motor. On the Toyota applications, the fan can operate at speeds of 600 to 3,000 RPM, at a pressure of 140 to 280 PSI. The control module looks at throttle position, A/C status, engine speed and coolant temperature to vary fan speed. The main advantage of a hydraulic fan is quieter operation and improved reliability (no drive belts, no clutches, no electric motors or relays to fail).
The radiator’s job is to cool the coolant. Air flowing through the radiator carries away heat and lowers the temperature of the coolant by a hundred degrees or more. To cool efficiently, the radiator must be clean, in good condition and receive adequate airflow. The radiator’s front-mounted location ensures good airflow when the vehicle is in motion. At low speeds and when the vehicle is stopped, the cooling fan may come on to boost airflow.
When a vehicle has air conditioning, the condenser is usually mounted in front of the radiator. This increases the heat load on the radiator and makes the fan’s job even more important.
Aluminum radiators typically go eight to 10 years or more without requiring any repairs provided there are no coolant contamination issues. But their frontal location makes them vulnerable to damage by stones and other road hazards. They can also become clogged with dirt and debris. In cold climates, road salt can also attack the metal causing corrosion that may eventually cause the radiator to leak. But the most common cause of radiator failure is internal corrosion caused by coolant neglect. If the coolant isn’t changed at the recommended service intervals (5 years or 150,000 miles on most late-model vehicles), the coolant may become acidic and attack the radiator.
The plastic end tanks on a radiator can sometimes be damaged by steam erosion. The underlying cause is usually a low coolant level, which may be due to coolant leaks or a bad radiator cap that doesn’t hold its rated pressure.
Too much pressure inside the cooling system can also damage a radiator by blowing out the seam along an end tank. The underlying cause is often a leaky head gasket that allows combustion gases to escape into the cooling system.
If a radiator is leaking, it must be repaired or replaced to stop the loss of coolant. Cooling system sealer can slow down or temporarily seal small leaks and buy a motorist some time, but even a tiny leak that seeps only a few drops a day will eventually allow enough coolant loss to make the engine overheat. Aftermarket replacement radiators are usually price-competitive with what it often costs to have an old radiator repaired or recored. Replacement radiators should be the same width, height and thickness as the original to fit properly, and have the same hose connections. Cooling capacities should also be the same. Some “economy” radiators may be thinner or have fewer tubes than the original radiator, which may increase the risk of overheating in some driving situations.
On some late model import vehicles, the radiator is part of a factory assembled “module” that includes the A/C condenser and fan. Some of these can be a challenge to disassemble, and others are designed to be replaced as an entire unit (even if only one of the components has failed).
If a customer is buying a new radiator or water pump, you should also recommend a new radiator cap (if used), hoses and thermostat for preventive maintenance. These parts don’t last forever, and when time and mileage cause one part to fail chances are many other components in the cooling system are nearing the end of the road, too.
The cooling system should also be flushed to remove any contaminants, and refilled with fresh antifreeze and clean distilled water (never softened water or tap water). Premixed 50/50 coolant is probably the best option as it is ready to use and requires no mixing.
You can recommend the type of coolant specified by the vehicle manufacturer, or a universal coolant that is compatible with all makes and models. Either will provide proper cooling system protection and performance.