Q: What Parts Fail Most Often With the Heating and Air Conditioning System?

Blower Motor and the Blower Motor Resistor.

Blower motors and blower motor resistors (also commonly referred to as the blower motor speed control), are easily the most common components to fail in the heating and air condition system. These two components are integral to every system across all manufacturers makes and models.

A blower motor is a rotating electric motor that spins a fan to push air into the passenger cabin. Rotating electric motors have two main failures: the bearings that allow the motor to spin wear out and brushes in the motor wear down creating a bad electrical connection.

The blower motor is situated between the outside world and the interior of your vehicle. It is often subjected to moisture, leaves, sand, dust and other debris the contribute to its failure. Often times a vehicle is parked under a tree or under the overflowing gutters of a house. These two scenarios will lead to premature motor failure. In newer cars, manufacturers have implemented the use of cabin air filters to catch any debris in addition to filtering the air for the passengers.

Electric motor brushes wear down just like the brake pads on your car. As they shorten due to constant rubbing against the motors commutator, the amount of pressure applied is reduced and a poor electrical contact eventually occurs. There is an easy test for this condition. Simply tap on the motor with a moderately heavy hammer while powering the motor. If the motor begins to work, the brushes are worn out. The standard repair for this is to replace the blower motor.

Replacing the blower motor can be simple or complex depending on the make, model and year of the vehicle. Usually it is situated under the passanger side dash, just above where your feet would be. For some vehicles, replacing the blower motor is as simple as removing three or four mounting bolts and an electrical connector, while others require some extensive disassembly of the dash to access it.

Blower Motor Resistor

The blower motor resistor introduces a resistance between the blower motor and the battery. The flow of electricity in a wire is not unlike the flow of water through a pipe or hose. You can illustrate this electrical property with a simple garden hose. Pinch the hose and the amount of water exiting the hose is reduced. This concept is very much like what happens with the blower motor resistor.

A blower resistor will actually have several resistors of varying resistance values. Each resistor represents a different fan speed. The fan switch is used to dictate which resistor the flow of electricity should pass through.

Resistance in an electrical circuit creates heat. When heat is created in this fashion it becomes necessary to dissipate the heat and cool the resistor. If the heat created from the flow of electricity isn’t adequately dissipated, the resistor will eventually burn through in a similar fashion as a fuse. This property of heat creation in a resistor is in fact the main if not sole source of blower resistor failures. Oftentimes the removal of the resistor will reveal a charred piece of plastic confirming the failure.

Blower motor resistors come in a wide variety of shapes and sizes. Each manufacturer utilizes varying electronics technologies to accomplish speed control of the blower motor. For this reason, some manufacturers have higher failure rates than others for these components. Nevertheless, these components are always subject to the same challenges in every vehicle. Their failure is inevitable.

Blower resistors are most often mounted close to the blower motor in a passage that will blow air over them for cooling. Just like the blower motor, the resistor is usually mounted under the passanger side dash above your feet. How difficult a blower resistor is to replace simply depends on the where it has been mounted in the climate control system. Many times they are easily accessable and other times they are buried deep inside the dash.

Blower switch / Fan speed switch.

The failure rates of blower switches vary greatly from manufacturer to manufacturer. A switch can carry the full electrical load of the blower motor or simply be a indication to a computer about the desired fan speed the occupant wishes. On vehicles that carry the full electrical load of the blower motor, there failure rate is significantly higher.

Blower switches are just that, a switch not unlike the ones on your walls in your home. The exception is they have more than two positions. Usually four, sometimes three, and possibly more than four. But four is the most common.

Fan speed switches are commonly part of the HVAC Control Panel. It is a complicated electronics component that requires careful consideration when it is suspected of failing. Much of the time, replacing a fan speed switch requires the replacing of the HVAC Control Panel. Usually this is dependant upon whether or not the HVAC Module is automatically or manually controlled.

Heating and A/C control unit. (AKA, HVAC Controller, HVAC Control Panel, Control Head, Comfort Controls and more)

Everyone is familiar with this part. It’s that part of your vehicle that typically sits below your radio. Without it we would be really hot or really cold. The HVAC controller, as it is usually called, comes in many configurations. It will most likely house the blower motor switch, temperature control switch, mode control for defrosters etc, and any other added feature the manufacturer chooses to include in regards to the comfort of the passengers.

This unit generally comes in two forms, auto and manual control. Auto control HVAC units will house a host of other electronics and will set the temperature for you. Manually controlled units require you to set the temperature slider or dial to your desired temperature. Unlike the auto systems, you will need to make frequent changes in order to find the temperature most comfortable for you.

How often the HVAC unit fails is strictly a factor of the design and build quality of each manufacturer. Some manufacturers have significantly higher failure rates than others. Auto HVAC units can be much more complicated to diagnose as well as being significantly more expensive. For me, the failure rate of this unit is a big factor when I am looking to purchase a vehicle. It’s failure represents a great deal of inconvenience when they fail. If it is expensive to boot, it only adds to your troubles.

Replacing the HVAC Controller involves disassembling the middle part of your dash. Typically this involves the removal of the plastic covers surrounding the HVAC Controller and radio. This can be simple or very involved depending on the design choices made by the manufacturer.

Heater core leak

Your heater core is simply a small radiator. Heater cores and radiators are all about the transferring of heat. In the case of the heater core, it takes some of the heat the motor creates and delivers it to the passenger compartment. This is accomplished by routing hot coolant into the heater core that is mounted deep inside the dash of all vehicles. Behind the heater core is your blower motor. When turned on it blows through the fins of the heater core pushing hot air into the passenger compartment.

They are very simple so there is only two things that can go wrong with them. They can leak or become clogged. Leaks will happen in time for every part that has hot coolant running through it. But the clogging can be avoided by regularly maintaining the coolant in your motor. When the coolant begins to fail, particulates will form and lodge in the small passages inside the heater core. Once these passages become clogged, hot coolant can’t travel through the core and the core will not warm up giving you that wonderful warmth we all desire on those cold winter days.

Replacing a heater core is one of the most challenging repairs a mechanic must perform. In most cases it requires the near or complete removal of the dash. Hidden screws and bolts are numerous as well as wiring harness routing will snake in and out of many passages. There is a high possibility of breaking parts of the dash that are built with plastic. A great deal of care must be taken by the mechanic during this job. Having replaced many a heater core myself, it is truly a sinking feeling when you have completely assembled a dash to find something doesn’t work that previously did.

A/C system leaks.

Your air conditioning system is truly a marvel of modern science. It uses physics, which I won’t get into, to create cold air from thin air, so to speak. The important thing to understand is the A/C system is a closed system that pumps refrigerant through many pipes. This refrigerant is always under pressure. Eventually the seals and or gaskets that allow the system to retain the refrigerant will break down and a leak will occur. As the volume of refrigerant is reduced in the A/C system, so does its capacity to create cold air. A/C systems should only be worked on by qualified technicians with a solid understanding of A/C systems.

Repairing an A/C failure is mostly simple. Diagnosing an A/C system requires the understanding of thermodynamics and pressure. Diagnosis and repair of A/C systems requires special equipment that can be expensive. There are environmental concerns that must be taken into consideration when working on an A/C system.

Diagnosis aside, the repairing of leaks is very common. This requires the removal of the refrigerant with a machine followed by the replacing of sealing o-rings and or hose replacement. All refrigerant is reused or recycled.

Blend door actuators.

The blend door actuator is a small electric motor that opens and closes an actual door. The door can be fully shut or fully closed and every angle in between. When the door is partly half open, as an example, it blends the hot air and cold air entering the cabin evenly. 50% cold air and 50% warm air. The blend door can be used to tailor this blending of air to adjust the temperature in the passenger compartment.

These actuators can be electrically, mechanically or vacuum actuated. Their failure rate greatly depends on their design. Most are fairly foolproof. Others not so much. Replacing an actuator can be simple or very time consuming. It is dependant upon how difficult it is to access the actuator. More complicated systems require partial dash disassembly. Others are as easy as replacing a blower motor and are often mounted in near proximity to the blower motor.

Mode door actuators

Mode door actuator are exactly the same as blend door actuators. The only difference is they direct the air into the various passages in your dash. Defrosters at the windshield, air flow to the feet or in your face.

Clogged evaporator drain

This happens for one reason, a buildup of debris in the drain passage. The drain is nothing more than a small pipe, passage, or hose that drains the water that comes off of the evaporator core. You may have noticed a clear liquid leaking under your car during summer months. This is simply water that had condensed on the evaporator cores exterior. You can observe this physics effect whenever you use a spray can that gets cold if you spray with it too long. Anyone who has used the can of air to remove dust from a computer has seen this effect.

When the evaporator drain clogs, you will experience a dripping from under the dash by your feet or simply wet carpet. Often this condition is confused for a heater core leak. Drains clog from debris such as algae, road debris and small rodents who leave twigs and other items behind.

Most of the time, a mechanic only needs to use a wire or some air pressure from an air compressor to remove the debris from the drain passage.

Should you run into any problems concerning these components, seek the help from a certified mechanic who can diagnose your heating or air conditioning system accurately.