Monday, May 20, 2024 Detailed Auto Topics
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A modern automotive heater provides warmth in the coldest situation. When it fails, things can get cold in a hurry. Finding the cause is important and easier with a basic understanding of how the system works.

Why does the heater not blow hot air?

How the heater works 

To warm the passenger compartment of vehicles, designers have tried different sources of heat. Heat from exhaust gases work, but are corrosive and dangerous. The electrical current needed is excessive. Today, most vehicles use engine coolant as the source of heat. Engineers use many designs for automotive heating system but they function similarly, with only subtle variations.

The vehicle’s cooling system removes excess engine heat. A water pump pushes the coolant around the cylinder head, through hoses and to a radiator, where it is cooled. This removes excess heat and keeps the engine operating at the proper temperature. The coolant also flows through a small heat exchanger called the heater core. A fan circulates air through this core when we need heat to warm the passenger compartment.

typical automotive heater core

Most engines are thermostatically controlled to run around 200 degrees Fahrenheit. The heat of this coolant easily warms the heater core. Air flowing through the heater core become very hot and we use this to heat the passenger compartment. Beyond operating the fan motor, such a heater requires no energy to work. With the heater, we recycle waste heat and put it to good use.

Basic automotive heater operation

Regulating the temperature in the vehicle  

Because coolant is so hot, the temperature in the passenger compartment might become uncomfortable. We cannot readily change the temperature of the coolant. To control the temperature in the vehicle, we must regulate the coolant flow or the air flow. In older vehicles valves that restricted the flow of coolant to the heater core often accomplished this. On modern vehicles, directing air flow typically controls temperature. Control devices change air flow allowing it to 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. When heat is needed, air flows through the heater core.  Once the passenger compartment is warm, air flow is diverted around the core. The heater core remains hot, but air flowing around it is not heated.  We can also blend the flow with air conditioned air to produce almost any temperature required.  

Most modern systems first cool and dehumidify the air and then reheat it to the desired temperature.  This makes the passenger compartment more comfortable. This reheating of dehumidified air is also very useful for defrosting the windshield.

Three primary failures cause most heater malfunctions

  1. Lack of proper engine temperature
  2. Lack of coolant flow through the heater core
  3. Lack of air flow through the heater core

Lack of proper engine temperature

Testing radiator temperature with a mechanical gauge

A thermostat is used to block coolant flow to the radiator until it warms to the proper temperature.  When the engine thermostat fails to close fully, the engine coolant temperature may fail to rise enough. Low engine temperature causes several problems and may cause the heater to blow cool air. We can easily verify this problem. Engine coolant temperature should be around 190 to 200 degrees Fahrenheit. A thermometer may be placed in the coolant flow, and the engine 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.

Another symptom of a partially failed thermostat is a heater that is warm when on low blower speed, but cools at higher speed. The heater core is much like a small radiator. Air flow, from the blower lowers the coolant temperature. If the temperature is insufficient, increased air flow from the heater's blower may cool it below the point that produces heat.

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 controls the valve position. Moving the temperature selector up and down should produce movement of the cable and valve.  We can visually check these valves for movement, by having someone operate the controls.  A valve that does not move suggest a cable or control panel problem.  With the valve open, a hose that is hot before the valve and cool to the touch after, shows a valve that is not working.

Heater control valve styles

Vacuum controlled valves will have a vacuum line attached. Applying and releasing a vacuum, causes the valve to open and close.  These may be checked visually and by checking the temperature of the hose before and after the valve.  The flow of current operates electrical valves. This is checked with a voltmeter and current flow should open the valve.  If back-probing the terminals at the valve shows current flow, the controls are working.  Lack of coolant flow with current present, means the valve is bad.

With any of these designs, checking the temperature of the hoses, before and after the valve helps in diagnosing a valve that is not working. When we command the temperature up, the hose leaving the valve should be nearly the same temperature as the input hose. Hoses that are hot to the touch, before the valve and cool after suggest a bad valve. Temporarily bypassing the valve with a connector, will verify the diagnosis.

Leaks and restricted flow are typical heater core problems

Most systems today no longer use flow control valves. On such systems, the inlet and outlet hoses going to the heater core, should be nearly the same temperature. A large temperature difference between the input and output hoses, may mean a restricted 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. Blowing through the inlet with very low air pressure should offer little resistance to the air flow. Minor restrictions can sometimes be cleared with low air-pressure. We must take care, as excess pressure may rupture the core and require replacement.

Water pump impeller eaten away by galvanic corrosion

Far less common, water pumps may fail to provide enough pressure to push coolant through the heater core. This happens when corrosion eats the impeller away and sometimes when a plastic impeller breaks. Lack of coolant flow can be difficult to find, especially when partial flow is present. One symptom is often a heater that does not heat.

A hot thermostat outlet-housing, combined with a cool radiator return hose, is a sign of insufficient flow. Another symptom of diminished flow is a vehicle that overheats when driven but is okay at idle. The pump may produce enough flow when demand is low, but fail to keep up when demand is high. Often the only way to confirm a water pump problem is by removing the pump.

Lack of air flow through the heater core

Heater not blowing hot when turned up

Vent temperature on a working system will normally be 115 degrees Fahrenheit or more. Vent temperature under 100 degrees Fahrenheit, when the system is set to the maximum, represents a problem. If the engine temperature is correct and coolant is flowing through the heater core, likely an air flow problem exists. Electrical and sometimes vacuum actuators open and close blend doors. The blend door controls the flow of air through the evaporator or heater core.  Improper flow can cause lack of heat or lack of air conditioning when requested. 

Electronic actuators operated heater and air conditioner blend doors 

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.

Broken Ford temperature blend door for air conditioner or heating

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!

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