You’ve heard the old adage, “The more things change, the more they stay the same.” But that’s not necessarily true with today’s air conditioning hardware and design. There’s more to A/C than just cool air and vents, and it’s important you know this if you want your techs to be A/C proficient and your shop to be profitable.

Where do you start? By understanding basic system design differences and by familiarizing yourself and your technicians with the principle features of electronic climate control and new system function and control. With that knowledge, you can better understand the troubleshooting approaches to various systems, some of which are provided in this article.

The Components

Let’s start our discussion of the latest and greatest in A/C hardware and design with a review of the basic required components that comprise a full-blown electronic auto temperature system (EATC). Keep in mind throughout this article that no matter how involved the system or level of complication, the EATC is designed to simply maintain the desired vehicle interior temperature by heating, cooling, re-heating or re-cooling the air distributed, whether it’s a re-circulated blend or sourced from the vehicle’s exterior.

The system is also designed to actively reduce the relative humidity of the air during A/C operation. It functions completely unaided once the driver selects the cabin temperature and rate of climate environmental control. Many of the current systems, however, have provisions for driver, front passenger and even rear passenger override of basic settings. These climate-control “zones” can best be described as localized occupant areas in which temperature, blower speed and directed airflow can be altered to suit the passenger sitting in that spot at the time. In other words, each passenger can pick his most pleasant temperature environment.

What follows is a list of the hardware in the EATC. There are two types of components to know: shared design and isolated.

Shared design refers to differences in two systems: semi-auto-temperature control (SATC) and fully electronic temperature control. SATC mainly involves early systems or low-end options on late-model vehicles. It’s characterized by a slide control panel and often has a cycle A/C clutch. The fully electronic temperature control system uses single- or dual-control sections to operate doors, temperature and A/C compressor flow. A computer monitor and flow control (duty-cycle) is utilized instead of cycle A/C clutch on the compressor. The instrument panel may use dials, but it will also have an indicator that displays the interior temperature setting. System control will work to maintain this setting.

Shared design components include:

1. A/C compressor – This unit may be constant or variable displacement in the nature of airflow delivery.
2. Condenser – This item is located in the front of the vehicle chassis and is designed to take the pressure gas delivered by the compressor and transform it into a high-condensed pressurized liquid. It then routes the liquid through a coil grouping exposed to road draft, ambient air and/or forced cooling fan-delivered airflow. In this manner, it acts as a heat “exchanger” and gives off accumulated heat to the open air, normally in front of the vehicle radiator.
3. Evaporator – This item is the “heat collector” and is located within the vehicle cabin interior. There may be more than one of these per vehicle. Often, multi-zone applications use thermostatic technology, which developed from the early use of multi-evaporator installations common on stretch limos and vans.
4. Receiver/dryer – This is the filter, the moisture removal and sometimes the storehouse for refrigerant items used in most A/C systems.
5. Evaporative controls – This may be an expansion valve, like what’s found on a GM V-5 variable displacement compressor system used in the 2001 Buick LeSabre. Orifice tubes, evaporative pressure regulators, valves and others normally represent the final line of division as the constantly circulating usable refrigerant changes from a liquid to a gas so the compressor can reintroduce the cycle again.
6. Interior blower – This uses fan-forced circulation to route heat-laden cabin interior air across the packed-in fins of the evaporator.
7. Actuators and doors – These are plenum-shaft, gate-like devices that act as directors or deflectors of inside or outside-routed airflow. (Note, in manual or semi-automatic temperature control, these items were always vacuum-controlled.) These days, new EATC systems make use of linear-driven actuators with feed-back switches or potentiometers (also called pots) to return a position or rate of travel signal to a body-control module or climate-control module.
8. Sensors and monitors – These are the temperature-sensitive resistors that create a control stylus. A control stylus refers to sensors such as interior temperature, external temperature, evaporator core temperature and pressure, high and low side pressure differential, and A/C control of compressor (i.e. variable flow control). The control stylus is used by various interior climate-monitoring computers. Input side devices are often thermsistors (thermal resistors), while output side devices are switches or follow-up transducers with a pot attached to give the affected processor a position reading on a door.
9. Ducts and registers – These items are used to send the air to the specified area as programmed or selected by the driver or zone occupant.
10. Control panel – The on/off function, temperature, airflow position and rate selection is chosen by the driver at this fully automatic display panel. It uses touch control or settings dialed in a more conventional fashion. In EATC systems, trouble codes and other diagnostic information can be made available in the control-panel area.

As if this list weren’t enough, you also need to be familiar with the components of the isolated EATC-only function to be able to discern the difference between less complex semi-automatic systems and more complex fully automatic systems.

Isolated components include:

1. Integrated control panel – This has temperature data and other selective data parameters integrated into function and/or display. This “control head” will usually input data into the programmer based upon operator-selected conditions.
2. Programmer – On certain vehicles, the programmer is a stand-alone device that accepts sensor inputs. It alters flow delivery, A/C clutch function, heater/water valve operation and blower speed.
3. Resistor strip circuit – This converts low-current signals taken from the integrated control panel and converts them into high-current feed signals to the blower motor.
4. Power module – Also called the amplifier/controller, this can be set up to function alone or as part of a programming sequence of the body control A/C output functions. Here, blower speed control at the automatic level is carried out based upon in-car temperature and ambient and sunload sensor inputs.
5. Remote climate-control module – This is the controller that runs interference between the programmer, control head and sometimes the body-control module and/or powertrain-control module. A remote climate-control module can self-test to detect problems in the system-control function.
6. Dedicated protective circuits (such as pressure and temperature switches) – These switches, especially those used to cycle compressors, can establish flow rates for variable displacement compressor circuits. Data is normally sent to the microprocessor to alter flow or to turn the clutch off or on.
7. Powertrain-control module – This vehicle computer controls all major operative functions for driveability, emissions control and fuel mileage aspects. The EATC is often linked through the remote climate-control module, as well as the body-control module, to the powertrain-control module, using it as a database for information with regard to the vehicle.
8. Body-control module – On some vehicle lines, the body-control module serves as the primary control for EATC and is linked by a control area network to the programmer, climate-control module and/or control head.
9. Electronically controlled actuators – These servo controllers are currently designed to operate in a single direction using a linear design that allows for feedback to the controlling source. Individual servos of this nature offer precise control, and diagnostic observations are more readily acquired than with previous bi-directional controlled devices.
10. Humidity sensor – This keeps the cabin from becoming excessively wet or dry. Value may be calculated from a duct-located circuit board combo sensor unit and passed on to a push-button control. Or it may also be sent to the controller, which can function to alter the pulse-width modulated signal used by some systems to control A/C compressor variable displacement.

General Troubleshooting Procedures

Now that you know what you’re looking at, it’s time to figure out how to troubleshoot it.

Automatic temperature control service procedures are often misleading. Why? Because it seems like isolated switches and sensors are everywhere. As a result, you use basic resistance or, in a few cases, voltage tests to verify correct operation. This technique seems to be a carryover from the days of manual and semi-auto temperature systems that had limited, if any, on-board or off-board diagnostics using the climate-control panel or even a dedicated or generic scanner.

Let’s take a closer look at troubleshooting EATC problems on vehicles made by the Big Three car manufacturers:

• General Motors (GM): GM EATC troubleshooting is very dependent upon which version of the microprocessor-controlled electronic touch climate control is used on a vehicle platform. Depending on vehicle model and division, the servo actuators may be a vacuum design or use electro servo motors. Methods of entering diagnostics also vary, but utilizing the climate-control panel is still viable provided you use the exact manual for the vehicle you’re working on. These differences, according to GM technicians, include radical variety in body-control module-related diagnostics on same-car-division lines, dependent upon models of the vehicle used. Seemingly unrelated service standards, such as the availability of factory cellular and multimedia functions, can be a discerning factor that will qualify a vehicle for an alternate mode of on-board diagnostic routines. Local area network nodes can also act to change several diagnostic methods, including climate control, on upscale GM platforms.

• Ford Motor Company: Suppose you’re reading a service code on the climate-control display register that says your 2000 Sable has an open in-car temperature sensor – a code B1251 to be exact. The service manual procedure instructs you to unplug the temperature sensor connector #C233 that looks over pins 1 and 3. Look for loose or bent terminal parts (pin or cavity). If it looks OK, then inspect the remote climate-control module connector #C228, pin 15 and pin 23. Look for a poor fit or other mechanical damage. Next, measure the resistance of the sensor at the component side. By comparing the actual temperature reading in the vehicle with what’s on the chart, followed by the applicable resistance, we can determine if the sensor itself has gone on the blink or if more diagnostics are necessary. The chart can take us to the output voltage test on the remote climate control, and if it’s not within operating prescribed limits, the module will have to go! (If this sounds similar to a procedure used to test engine coolant temperature sensors for defects, you’re right on the money.)

On 2000 and 2001 Ford vehicles, intermittent diagnostic codes can be accessed easily by pushing the OFF and FLOOR buttons simultaneously and then hitting the AUTO button. The test runs for 30 seconds. The climate-control display window will show “888” when the test is complete. Check out of diagnostics and keep the codes by pressing the cooler side of the TEMP button. If you want to deep-six the codes and still get out of diagnostics, press the DEFROST button on the front panel. All remote climate-control module diagnostic trouble codes will clear out.

Remember that Ford systems take more time to troubleshoot since scan-tool-related procedures aren’t as prevalent as on GM or Chrysler.

• DaimlerChrysler: After allowing the DaimlerChrysler vehicle to reach operating temperature, shut off all exterior lights and press the PANEL button. The display should light up, which allows you to enter self-diagnostics. (Note: If the panel didn’t light up, check the fuses and for defective circuitry, including the master BUS power connector.) If all of the protection, power and ground side circuits are OK, replace the ATC computer and continue testing.

When you reach the diagnostic stage, press BI-LEVEL, FLOOR and DEFROST at the same time. If no diagnostic trouble codes are present, the self-test will run for 90 seconds and then terminate by displaying a “75.”

There are four items the computer won’t acknowledge, so a technician must check them himself.

1. With the power on, all of the display symbols and indicators must light up.
2. The blower motor must operate at high speed.
3. Air should flow through panel outlets.
4. The air temperature should change from relatively hot to cold.

Any failures or trouble codes should be handled in accordance with manufacturer service manual flow charts. If any hard faults are detected, record the panel-flashed code and then press PANEL to proceed through the remainder of the test.

Don’t forget that in many vehicle systems, electronic modules banter back and forth with each other. This may provide your scan tool diagnostics with several levels of diagnostic information. For example, two or three different system diagnostic trouble codes read as a problem indicator but with a different parameter indicator data stream (PID) reference. The powertrain-control module, the climate-control module, the programmer and/or control head may all indicate the same problem but perceive each in their own respective ways. Often the body-control module can originate diagnostic codes that will help verify problems the other controllers have already nailed down.

Cooler Heads Prevail

The EATC is a system that bodyshop service people will continue to deal with daily because it often experiences shock damage or is ruined in a crash. And the fact that the driver, passenger and back seat occupants all have separate controls will be a necessary “check through” test for body shop techs after a repair has been made and before the customer regains custody of the vehicle.

I’ve presented a general overview here to give you the basics, but you should still seek out in-depth training on the various systems from sources like I-CAR to ensure that you can correctly service and repair the EATC, no matter what the level of complication.

The changes we’re seeing in automatic climate control are to be expected. And your customers rely on you to stay on top of such technological advances so your facility can make observations and run various tests to determine if a system is functioning normally. This doesn’t mean you won’t have to go out-of-house for the final repairs. It does, however, offer you a chance to produce an inspection process that will identify general problem areas and provide the opportunity for a complete repair.

Cool Technology on the Mercedes C-320 The C-320’s state-of-the-art automatic temperature-control system can be diagnostically challenging for the unprepared tech.

Today’s automatic temperature systems are state-of-the-art designs – and pretty cool to check out.

It doesn’t take a genius to look at Mercedes technology over the last five years and place it at the top of the heap. The same can be said for the top-notch automatic temperature-control (ATC) system on the 2001 C-320. The C-320 ATC uses an LCD display/back-lit switches, 10 positioning motors, a multi-function sensor, independent zone-specific air-direction control, illuminated dash vent controls, a sun position sensor and interior filters designed to use activated charcoal, which filters out dust, pollen and fumes.

Slamming the doors on a purebred luxury sedan can be a headache because many of these cars are so airtight that they resist the sudden compression of trapped interior air. Not so on the C-320. Its climate-control system overcomes this ticklish problem by setting the fresh/recirculated air flap at 50 percent when the ignition is switched off. This allows cabin air to escape when closing vehicle doors. But if this feature were to be ignored during a collision restoration, a customer who’s acclimated to the correct function may interpret the door-closing problem as a poor-fitting door or improper adjustment.

On the C-320, the blower motor will always be switched off during engine cranking – despite where the EATC settings might be or if the system is switched to one of the seven manual settings. After an accident, collision shops will need to get a handle on how ducts, nozzles and registers are laid out in the C-320. (See accompanying diagram.)

The old duo valves used in previous Mercedes vehicles aren’t found on the C-320. Instead, blend air flaps have taken their place. This excellent system uses linear travel actuators to control temperature and direct airflow in a very precise manner.

Refrigerant circuit changes on this vehicle include a new design receiver dryer with a serviceable element, a low side service fitting easily accessible from under the hood and a refrigerant pressure/temperature sensor located under the left section and slightly in front of the condenser. (Note: Never attempt to remove the internal hex nut with a charged system to replace the element. The system must be discharged first.)

What else can you find on the C-320?

• Interior sensing is well-rounded with an evaporator sensor, dual in-car temperature sensors that offer a calculated sum reading for temperature setting and a sun sensor located in the cowl just ahead of the windscreen. The ambient sensor is found in the lower passenger-side front area adjacent to the signal-light chamber.
• A pulse width modulated compressor allows for the correct positioning of the compressor stroke control and is used to control flow of refrigerant during all phases of operation. (Look, Mom! No clutch!)
• A new multi-function sensor tied in with the interior sensor group measures dew points and allows for humidity corrections. Plus it can sense smog and close outside vent doors when it gets a sniff full of the nasty stuff.

But the C-320 can offer a few diagnostic challenges if you don’t know how to attack a problem. Basic on-board system diagnostics can be called up using a Mercedes-only dedicated scanner. Generic OBD 2 codes won’t help you with the climate-control problems you may recognize, so the dedicated scanner is still the best way to handle actuator and/or airflow-related problems.

Shops working on these new vehicles should be able to relate well to the European method of diagnostics, and a trip to a Mercedes dealership for the most recent technical service bulletin information is always a good place to start.

Diagram courtesy of Mercedes-Benz Corp.

Writer Bob Leone, a retired shop owner and contributing editor to BodyShop Business, is ASE three-way Master Certified and is a licensed secondary and post-secondary automotive instructor in the vocational school system in Missouri. He is also a former NAPA A.S.E. Technician of the Year.