The modern automotive heater provides comfort in the coldest situation. When it fails to provide heat, finding the cause is easier with a basic understanding.
Several hundred designs are used in automotive heating systems today. Most heating systems function similarly, with subtle variations. In the past different sources of heat have been used. Today, most vehicles use engine coolant as the source of heat.
Most engines are thermostatically controlled to run around 200 degrees Fahrenheit. The hot coolant is circulated, by the water pump, through a heater core. The heater core is much like a tiny radiator. Air flowing through the heater core is warmed and used to heat the passenger compartment.
Because coolant is so hot, the temperature of the air flow must be regulated. In older vehicles this was often accomplished by valves that restricted flow to the heater core. On modern vehicles, temperature is typically controlled by directing air flow either through or around the heater core.
Air flow, from the blower fan, is controlled by a series of doors within the blower case. The position of the door is controlled by an actuator, in response to commands from the driver. This allows precise temperature control and dual-zone temperature, with multiple doors.
Three primary failures cause most heater malfunctions
Lack of proper engine temperature
Lack of coolant flow through the heater core
Lack of air flow through the heater core
Lack of proper engine temperature
When the engine thermostat fails to close fully, the engine coolant temperature may fall. Low engine temperature causes several problems and may cause the heater to blow cool air. Engine coolant temperature is easy to verify and should be around 190 to 200 degrees Fahrenheit. A mechanical thermometer may be placed in the coolant flow, and the engine is run until it warms to temperature. Readings below specifications usually show a bad thermostat. Replacing the engine thermostat will normally restore the proper temperature and heater function.
Lack of coolant flow through the heater core
On vehicles that use coolant-flow control valves, problems may develop with the valve. These valves may be a mechanical, vacuum or electrical control. With the mechanical valves, a cable connected to the temperature control is used. Moving the temperature selector up and down should produce movement of the cable and valve.
Vacuum controlled valves will have a vacuum line attached. Applying and releasing a vacuum, causes the valve to open and close. Electrical valves are operated by the flow of current.
Checking the temperature of the hoses, before and after the valve helps in diagnosing a valve that is not working. When the temperature is commanded up, the hose leaving the valve should be nearly the same temperature as the input hose. Removing the valve and testing the flow, will verify the diagnosis.
Many systems today no longer use flow control valves. On such systems, a large temperature difference between the input and output hoses, may mean a restricted heater core. In the heater core, the coolant passages are very small and can become blocked.
Stop-leak added to the system is a common cause of heater core restriction. Lack of cooling system maintenance is another cause. Depleted coolant becomes corrosive and can easily eat through the heater core. Unfortunately, repair involves removal of the dash in most cases.
With the heater hoses removed, flow through the heater core is easy to check. Minor restrictions can sometimes be cleared with low air-pressure. Care must be taken, as excess pressure may rupture the core and require replacement.
These actuators are frequently under the dash and difficult to reach. Diagnosis involves determining if the command is reaching the actuator, and if it is responding. This can be very complex, especially with electronic systems and automatic temperature control.
With electronic systems, current flow is NOT necessarily off and on. Most use a digital signal and varying the signal causes a stepper motor to change the door position. The signal must then return to the computer. Professional scan tools, and digital lab scopes are many times needed to test such signals.
If the signal is present, and correct, the actuator may be bad or the blend door may be broken. This is a particularly common problem on vehicles built by Ford and Chrysler. Repair of this nature normally involves removing the dash of the vehicle.
Proper cooling system maintenance, helps prevent many problems with heaters. When actuators, blend doors or temperature control modules fail, the professionals at AGCO Automotive can help. AGCO can repair your automatic temperature control problems. AGCO, it’s the place to go!
