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Automotivr Air Conditioning Theory, ..............Procedures.......................And Diagnosis.

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Climate Control is very hard to Understand, Read this over and over

Automatic Temperature Control Is Twice The Problem

Another change in A/C technology that is already having a big impact on the aftermarket is automatic temperature control (ATC). Designed to make life easier for the vehicleís occupants, the complexity of many of these systems is making life anything but easy for technicians who have to fix them. Automatic temperature control systems have been around for many years, but with each passing model year the complexity of new systems has continued to grow at an exponential rate. The climate control system is now so intertwined with the power train control module, body control module and other system modules that itís difficult to figure out what might be causing an A/C cooling problem.

In the good olí days, you just turned or flipped a switch to turn on the A/C. The switch sent voltage to a relay that energized the clutch on the compressor. Fan speed was selected manually with a knob while temperature control was controlled manually by cables or vacuum motors that operated the blend, recirc and duct doors. Now you donít even have to turn the A/C on. It automatically comes on as soon as the engine is started. You just reselect a temperature setting and the ATC system does the rest. It provides heat when the vehicle is cold, and cool air when the vehicle is hot. It selects the blower speed as well as where the air exits the ducts. Whatís more, we now have "dual zone" A/C systems that provide separate temperature settings for the driver and front seat passenger, as well as front/rear systems that have separate controls for the rear passengers in minivans and SUVs.

Some of these ATC systems have "smart" control heads that contain the decision-making electronics that regulate system operation. Others have a "dumb" control head that just passes the driver settings on to a second module (typically the body control module) that actually carries out the commands. Both kinds of heads can be very expensive to replace (up to $2,000 or more on some vehicles!). Dumb heads typically have four to eight wire connectors while smart heads usually have connectors with 20 or more wires.

On many vehicles, the power train control module has the final say as to whether or not the A/C will provide any cooling. The PCM controls the relay that energizes the A/C compressor. It will turn on the A/C compressor when it receives a request from the ATC module or body control module - but not under certain operating conditions. The PCM may not turn the A/C compressor on if other sensors tell it the engine is overheating, the throttle is wide open or idle speed is dropping because of demands being placed on the engine by other accessories. In addition to the control head, ATC systems also use a variety of sensors to monitor temperatures as well as whatís happening in various parts of the system. The two basic types of sensors are thermistors that change resistance to monitor the ambient, interior, outlet, evaporator and refrigerant temperatures, and feedback sensors that monitor the position of air door control motors. Some newer vehicles now use infrared sensors that read the surface temperature of the vehicleís occupants in the front or rear seats, and many others have "sun load" sensors that allow the ATC system to compensate for the extra warmth created by direct sunlight.


Some systems now use compressor rpm sensors to detect belt slippage that could indicate a compressor is on the verge of failing. Others use pressure sensors to monitor the refrigerant. Most (but not all) of the newer ATC systems also have moved away from vacuum motor actuated blend and duct doors to electric motor controlled doors. There are three basic types: three-wire "smart" motors that keep the ATC module informed of their relative position, five-wire smart motors that do likewise and five-wire dumb motors that require a separate position sensor. On some systems, the ATC control module counts the pulses of the motor commutator to determine its relative position. This may require a special self-calibration procedure if any of these components are replaced so the ATC module can relearn the correct position of the motors and doors.


Troubleshooting problems in these new ATC systems requires a scan tool on most vehicles, though some can provide diagnostic trouble codes through the control head by pushing various combinations of buttons simultaneously or in a certain sequence. Either way, the technician needs a thorough understanding of how the system is designed to operate along with a wiring diagram, diagnostic charts and test procedures needed to repair the system. The problem is, many aftermarket scan tools donít have as much of the ATC info or capabilities as the new car dealer scan tools. Some vehicle manufacturers (notably Mercedes-Benz) also have been very slow to release their latest service information to the aftermarket. Hopefully, this situation will change as a result of legislation that requires the vehicle manufacturers to release OBD II-related service information.



Although there are many different types of A/C systems, each falls into one of two categories, depending on how the system is controlled: manual and automatic. Manual air conditioning systems have an on-off switch, a temperature control knob or slide switch and a knob or switch for adjusting fan speed. The driver turns on the A/C when he or she wants cool air and selects a temperature setting and blower speed. If the air gets too cold, the driver can turn down the blower speed or change the position of the temperature setting. If the air isn't cold enough, the driver can crank it all the way up.


The temperature slide switch is connected with cables or vacuum hoses to airflow control doors inside the HVAC unit under the instrument panel. Changing the temperature setting opens or closes the doors either to increase or decrease airflow through the A/C evaporator. It's a relatively simple control system that doesn't require a lot of complicated electronics.

Automatic temperature control systems, by comparison, can be very complicated (and troublesome to troubleshoot). The basic idea behind these system is to control both passenger heating and cooling with a single temperature setting. The system then automatically chooses heating or cooling and blower speeds to maintain a fixed temperature that has been preset by the driver and/or passenger. The "dual zone" automatic temperature control systems that are found in many newer vehicles allow occupants of each front seat to choose their own comfort setting. So if one person is always hot and the other is always cold, both people can be comfortable.



Of course, this is easier said than done. Maintaining a relatively constant temperature within a moving vehicle isn't as easy as it sounds because the temperatures inside and outside the vehicle are constantly changing.


The ambient (outside) temperature affects how much heating or cooling is required to heat or cool the incoming air to the desired temperature. As the air temperature inside the vehicle gradually changes, the climate control system has to compensate by altering the blower speed and temperature settings. It's a constant balancing act that requires a number of inputs and controls. This includes an ambient air temperature sensor, one or more in-vehicle air temperature sensors, a "sun load" sensor to compensate for heat entering the vehicle through the glass, one or more electronic control modules and vacuum or electronic controls for the various HVAC airflow control doors.


Most of the newer automatic climate control systems use small electric motors (actuators) to operate the airflow doors in the HVAC unit. There are five-wire, three-wire and two-wire motors, all of which operate differently and must be replaced with the same type of motor. The five-wire motors have a feedback circuit to keep the control module informed about their position. The three-wire "smart" motors often have their own microchip to control and self-calibrate their positions. The two-wire motors are simple, reversible 12-volt motors that push the airflow doors one way or the other. The controller keeps tabs on their position by running the motors full open and full closed, then counting the revolutions of the motor armature to figure their exact position. Needless to say, these are complex, sophisticated temperature control systems.


Troubleshooting today's automatic climate control systems has become a daunting task that requires specialized training and tools. If a cooling problem isn't in the refrigeration circuit (bad compressor, plugged orifice tube, low refrigerant, etc.), it's in the automatic temperature control system - which may require the use of a scan tool, DVOM or other diagnostic tools to pinpoint the cause. More complicated systems have self-diagnostic capabilities and can generate fault codes that indicate the nature of the problem. But in most cases, the technician still has to check out various components by measuring voltages, resistance, looking for opens or shorts in the wiring, etc. Once the problem has been diagnosed, the defective part can be replaced. What kind of parts are we talking about? Switches, sensors, relays, resistors, vacuum valves, vacuum motors, electric motors, blower motors, wiring harness repair kits or even controllers. Depending on the application, some of these climate control parts may only be available from a new car dealer. But many are available through aftermarket A/C parts suppliers. On some vehicles, the climate control module is a separate component. On others it may be part of the body control computer that regulates other functions such as an electronic suspension, air bags or other onboard electronics. The most expensive ones to replace are those where the controller is part of the HVAC control head assembly in the instrument panel. Some of these units can cost $1,200 to $2,400 or more depending on the application. These are typically dealer parts that have to be sourced from the vehicle manufacturer. Because of the high cost of most controllers, it's important to make sure the system has been correctly diagnosed. Many controllers are returned because they didn't solve the problem, or the controller really needs to be replaced and the problem isn't due to something else like a sensor, switch, relay or motor.

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