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Automotive climate control systems are convenient and dependable. With dual-zone climate control we can even select our individual temperature preference. We need several components to make this happen. When things go wrong, it takes an understanding of the system to find the problem.

Mechanical climate control

Mechanical control panel using cables to control the system

Early vehicles use mechanical controls for temperature and mode adjustment. Sliding cables connect control levers or knobs to the necessary components. To switch from vent to defrost, we move a lever. This lever moves a cable, which causes the mode door to move into position.

cable operated mode air flow system

Other designs control multiple doors with a single cable and an ingenious crank mechanism. Mechanical systems can give many functions available the later digital system. With mechanical systems, they use analog switches to control fan speed. These temperature control devices are very robust and still used on a few vehicles.

A drawback to a mechanical system is, it cannot automatically adjust to changes in ambient temperature. If the passenger compartment gets too cool, we must slide the lever toward warm. This simple design causes few problems but also limits the features available with digital climate control.

Digital climate control

Today, most temperature control systems use digital circuits and electronic actuators. The actuator contains a small motor, gears and a feedback circuit to report the current position of the output shaft. Actuators use 12 volt direct current motors and pulse-width modulated stepper-motors. Before testing, we must consult service-data to decide the type of system being used. Connecting direct current to a stepper motor may ruin it.

Temperature control actuator

Actuators contain no serviceable parts and we must replace them as an assembly. The best way to test such actuators is with a scan-tool, designed for bidirectional communication. Such a tool can read the pulses sent to the actuator and the position it reports. When the commanded position does not equal the actual, an error code is set. This is the most common failure with actuators.

General Motors truck and SUV problems

The Silverado, Sierra, Tahoe, Yukon and similar vehicles often have problems with hot air on one side when the air conditioner is supposed to be on. Most commonly, a failed actuator is the problem. Occasionally, resetting the actuators may help. We list the GM reset procedure here for convenience.

GM HVAC system re-calibration

  1. Place the ignition switch to the OFF position.
  2. Remove the fuse labeled HVAC/ECAS a minimum of 10 seconds.
  3. Re-install the HVAC/ECAS fuse.
  4. Start the vehicle but do not touch the controls.
  5. Wait 40 seconds to see whether the control module will self-calibrate.
  6. If the system still does not work, repair will be needed.

How actuators control vehicle mode and temperature

A plastic case under the dash contains the air conditioner evaporator-core and the heater core. Air flows through the case and a series of doors that the actuators open and close to control this flow. The position of the doors determines where air flows and the temperature. We refer to the different air flow positions as modes.

Air flow and temperature control system

Aggie, the AGCO Automotive logo The mode actuator controls the position from which air will blow (e.g., vent, floor or defrost)

Aggie, the AGCO Automotive logo The blend actuator controls the blending of warm and cold air. Dual zone system may have two or more blend actuators.

Aggie, the AGCO Automotive logo The fresh/circulate actuator controls inside and outside air used in the system. Sometimes also called fresh air re-circulate.

Aggie, the AGCO Automotive logo A Heater control actuator can control heater flow. On some systems this is controlled by the mode actuator.

Aggie, the AGCO Automotive logo Some vehicles also have individual right and left temperature actuators. This allows individual temperature selection from side to side.

Fresh air and recirculated flow

The blower motor moves air through the system. Air may be drawn from the outside or we can recirculate the air that is in the vehicle. This is necessary for maximum cooling and greatly reduces the load on the air-conditioning system. When cooling, dehumidifying it and removing heat makes air far more comfortable. We must continually condition air drawn from the outside. Placing the input to recirculate reduces this load.

Other times the outside air may be comfortable or we may wish to replace the air in the vehicle. The fresh air mode will accomplish this.

Temperature control

Air is most comfortable when we lower the humidity. The water we see dripping under the vehicle, when the air conditioner is on, is humidity being removed. This also cools the air, perhaps more than desired. Temperature actuators blend the cooled air with warm air to achieve the temperature we want. They do this by allowing some air to flow through the heater core, after flowing through the evaporator core.

Dual zone systems

The dual-zone system adds more actuators and doors. Here we divide the air flow before flowing through the evaporator and heater cores. This allows the left side of the vehicle to operate at a different volume and temperature than the right side. After conditioning, the mode actuators further control flow. The mode-position decides if air blows from the vent, at the floor or at the windshield.

How dual-zone HVAC controls individual temperature

Climate control in modern vehicles is a quite complex system.  The final drive-gear of the actuator attaches to one of a series of in-dash doors.  It also contains a sensor that monitors the position of the door.  This sensor reports the door position to the control unit.   This information is sent to the control panel by one of the internal networks.  The control panel then varies the position of the various doors to control temperature and air flow.

Each door can operate independently of the others to achieve almost any effect desired. Unfortunately, with such a complex system, a lot can fail. The actuators sometimes fail and the doors can stick and break.

When the system fails to operate, a diagnostic trouble code or DTC will be set in the body control module. This code will help the technician know the area in which the problem exists. Unfortunately, a DTC does not say what is wrong. Testing the components around the failure is necessary.

Broken air flow doors

When the plastic doors that direct air in the dash break, we must replace them.  Repair can be involved.  Access can be quite a problem, often the dash must be removed to repair the situation.

Access to broken Ford blend door

Ford has been particularly prone to this problem.  The plastic blend-door shaft tends to fracture.  This can keep the door from sealing off the heater core.  The effect is the air conditioner does not get cool or the heater may blow when not desired.

A broken Ford blend door 

Even when this is properly diagnosed, the problem is bad enough.  The dash must be removed and the blend door replaced.  Sometimes, the problem is mis-diagnosed.  The air conditioner does not get cold, so more refrigerant is added to the system before diagnosing the actual problem.  This does not help the lack of cooling and instead over-charges the system and destroys the compressor.  The vehicle owner now has two very expensive problems, instead of one.

Some of the symptoms of climate control problems  

   Heater and/or air conditioner does not work

The air conditioner does not cool as well as before

The heater does not blow as hot as before

Heater or air conditioner is erratic

Clicking noises from under the dash

The system does not respond to commands

With climate control, proper diagnosis and repair are the keys to lower cost.  When the air conditioner or heater stop working or work erratically, bring your vehicle to the professionals at AGCO.  We understand temperature control systems and can properly diagnose and repair them.  Quality diagnosis and repair, it cost a lot less.

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