Heater and blower problems

How does heat get from the engine into the cabin?

OnAllCylinders describes a heater core as part of a vehicle's climate control system that helps fill the passenger compartment with warm air. It says the heater core is fed with engine coolant through two heater hoses that plug into it on the other side of the firewall. In its words, the heater core absorbs the heat from the engine coolant, and then air is pushed through the heater core by the HVAC blower motor, heating the air on its way to the cabin.

Wikipedia gives a matching description. It says hot coolant from the engine passes through a winding tube in the core, that fins are attached to the tube, and that a fan forces air across the heated fins and tube into the passenger compartment. It adds that the core is typically made of aluminium or brass tube, with fins to increase surface area.

Two more jobs sit in the same system. OnAllCylinders says the heater core also plays a large role in defogging and defrosting windows. Wikipedia says the temperature of the air entering the cabin can be controlled by a valve that limits the coolant going through the core, by a damper door that sends part or all of the incoming air around the core, or by a combination of the two. It also says more complicated systems use electromechanical actuators and thermistors to set the valves or doors.

What do the blend door and its actuator do?

MechanicBase says three kinds of doors operate most automotive climate control systems: an inlet door, a mode door and a blend door. It says the inlet door decides whether the air comes from outside or is recirculated, and the mode door decides where the air discharges, such as the vents or the floor. The blend door goes by other names, including the mixture door and the temperature door, and MechanicBase says it is needed to manage the temperature coming from the system.

On the actuator, MechanicBase says most climate control systems use an electric motor to control the position of the blend door, while older vehicles used a mechanical cable. It says power is sent to the actuator when the temperature setting changes, and that the actuator can contain a sensor that keeps watch on the position of the door.

It also says the climate control panel or control module controls the actuator. Wikipedia says that in simple systems the driver moves the valve or door directly, usually with a knob or lever.

MechanicBase says most blend door actuators are found behind the dashboard near the blower fan. It says a system controlled from one place for the whole vehicle should have one actuator, while a dual-zone or tri-zone system should have multiple actuators and blend doors to control each zone. Wikipedia adds that cars with dual climate function may use a heater core split in two, with different amounts of coolant flowing through each side.

What does a failing blend door actuator sound and feel like?

MechanicBase lists strange sounds first. It says a failing actuator can make knocking noises when the climate control is first started, and a clicking sound from the dashboard area. It says the sounds can be faint or loud, repetitive or constant, and sometimes intermittent.

It also says squeaking or droning is possible, and that the noise can get louder as the problem gets worse. Moz Electronics, an electronic-component distributor, calls clicking or tapping behind the dashboard a very common symptom and says worn plastic gears slip or strip.

For temperature, MechanicBase says an actuator problem can show up as a mixture of heat and cold at various times while the controls themselves do not change. It says that once an actuator breaks completely, the wrong temperature can come out of the vents all the time, such as heat coming out with the air conditioning on.

It adds that a driver who has just changed the controls should give the system time to adjust before judging the result. Moz Electronics lists unequal temperature between the driver and passenger sides, saying dual-zone systems may blow hot on one side and cold on the other. It also lists air that is only hot or only cold, which it says is usually caused by a stuck or unresponsive blend door.

On causes, MechanicBase says an actuator can fail simply from wear and tear, that its motor can wear out over time, and that extreme temperatures could make it fail early. It says that if any of the electrical circuits in the heating and air conditioning system fail, the actuator could malfunction, and that a blown fuse can be behind some of the problems.

Wikipedia says blend doors or their control mechanisms can become stuck, possibly due to thermal expansion, and that actuators in automatic climate control can fail. MechanicBase cautions that uneven airflow alone is not proof of an actuator problem, because a failing fan or a blocked intake can cause it too. It says the best way to test an actuator is a diagnostic tool compatible with the vehicle that can send a signal to it.

How do the blower motor and its resistor set the fan speed?

Samarins describes the blower motor as the motor that runs the fan in the heating and air conditioning system. It says the motor sits inside the dash, often on the side opposite the steering wheel, or in the engine compartment on the firewall.

Samarins says a blower motor resistor or a blower motor control module is the part that controls the motor's speed. A plain resistor, it says, is used where the blower has only four or five fixed speeds, and an electronic control module is used with automatic climate control and where blower speed can be adjusted gradually.

Samarins says that in most modern cars the resistor or control module sits inside one of the ducts close to the blower motor, and that this is done so passing air cools it. Its diagram of a typical car shows the resistor bypassed on the highest setting, with the blower powered directly by the fan switch.

The same diagram shows lower settings sending current through one, two or three resistors connected in series. Samarins says that in some cars the blower may still work on High when the resistor is bad. It also notes that adding resistance to an electric circuit reduces the current in the circuit.

MechanicBase describes the resistor in similar terms: it says the higher the resistance, the slower the fan goes, with less airflow, either cold or hot. It adds that a car with only a blower motor and no resistor would be stuck at one fan speed, and that many newer cars have replaced the traditional resistor with an electronic control module.

What does a failing blower resistor or blower motor look like to a driver?

Samarins says the most common symptom of a failed blower motor resistor is a heater fan that runs only at the highest speed setting and does not work at low speeds. It says that in some cars a failed resistor can stop the fan completely.

MechanicBase lists a motor that stays on one speed, some airflow settings that do not work (for example, lower speeds working and higher ones not, or the reverse), no air from the vents, and fan speed that comes and goes. OnAllCylinders says that if the air is getting warm but the fan speed controls are not working properly, or the fan is not working at all, the blower motor resistor may be bad.

On causes, Samarins says a blower motor resistor most often fails from corrosion or overheating. It says resistance to the motor's rotation can draw excess current that overheats and prematurely damages the resistor, and it gives a fan blade jammed by a foreign object and worn motor bearings as examples. Samarins adds that leaves, twigs and nuts can jam the blade, and that a worn-out blower motor can intermittently make a loud screeching noise.

MechanicBase says the most common reason for a bad resistor is a bad fan motor. It also says the resistor was designed to last the life of the vehicle but can fail early if the heater and air conditioning are used continually or the part is exposed to excessive heat. MechanicBase says that when some fan settings do not work, a defective blower switch can cause the same trouble, and that a proper diagnosis is needed before parts are replaced.

A few owner-level notes come from the sources. MechanicBase says it is not safe to bypass the blower resistor and that doing so can cause a car fire. It says the resistor carries current and can build up heat warm enough to burn fingers.

It also says the only safety issue with a bad resistor is when you need to clean the windshield and the defrosters are not working. Kia's owner's manual for a 2023 model adds that operating the blower with the ENGINE START/STOP button in the ON position could discharge the battery, and that the blower should be operated when the engine is running.

What do the sources say about heater core leaks and clogs?

OnAllCylinders says heater cores are often subject to vibration, heat cycles and potential chemical coolant contamination, and that they can develop leaks. It says some leaks come from a crack at a brazed copper joint, while others are tiny pinhole leaks from pitting and age. In its words, heater core leaks usually show up as the smell of coolant inside the vehicle, soggy and stained footwell carpet, or a chilly interior.

Wikipedia says a leak at a connection to the heater core may first be noticed by smell, because ethylene glycol coolant has a sweet smell. It says the leak may also cause somewhat greasy fogging of the windshield above the heater vent, and that glycol may leak into the car and wet the upholstery or carpeting.

OnAllCylinders lists these signs of a bad heater core: little heat from the vents, a sweet coolant smell in the cabin, defoggers that stop working well, general coolant loss, little airflow when the heat is on, wet floor mats or a damp footwell, and carpet stains that are usually green depending on coolant type. Wikipedia describes a more severe case. It says electrolysis can cause excessive internal corrosion that ruptures the core, and that coolant may then spray into the passenger compartment followed by white smoke, which it calls a hazard to driving visibility.

For clogs, Wikipedia says the heater core is made of small piping, a quarter inch or less, with numerous bends. It says clogging may occur if the coolant system is not flushed or the coolant is not changed regularly, and that restricted coolant flow reduces heating capacity or removes it altogether if the core is completely blocked. OnAllCylinders says heater cores can clog on the inside and the outside.

It says debris pulled in by the blower can lodge in the fins, and that if the vents still move plenty of air but the air is not getting warmer, the core may be clogged internally, with deposits and corrosion keeping hot coolant from reaching the fins. It adds that old, contaminated or improper coolant can gum up the small passages in a heater core over time.

What does one owner's manual say about defrosting and fogged glass?

A 2023 Kia owner's manual says that when the windshield is covered with frost or moisture the front view is blurred, and the frost and moisture should be removed. For maximum defrosting, it says to set the temperature control to the extreme hot position and the fan speed control to the highest speed. If warm air to the floor is wanted while defrosting or defogging, it says to set the mode to the floor-defrost position.

The same manual says to clear all snow and ice from the windshield, rear window, outside mirrors and all side windows before driving. It also says to clear snow and ice from the hood and from the air inlet in the cowl grille to improve heater and defroster efficiency and to reduce the probability of fogging up the inside of the windshield.