A/C service advancements

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A/C service advancements

Weber, president of Virginia-based Write Stuff Communications Inc., is an award-winning freelance automotive and technical writer with over three decades of journalism experience. He is an ASE-certified Master Automobile Technician, and has worked on automobiles, trucks and small engines. He is a member of the Society of Automotive Engineers (SAE) and numerous other automotive trade associations. He has worked as an auto mechanic, a shop manager and a regional manager for an automotive service franchise operation.

The advent of computer controls, and especially with the 2000 introduction of CAN (controller area network), has changed much of the way we service automobiles. The heating/ventilation/air conditioning (HVAC) system is no exception.

When is the last time you pulled out your manifold gauges and hooked them up to the refrigerant lines’ fittings? It is so much quicker, easier and accurate to plug your scan tool into the diagnostic link connector (DLC) and get the pressure readings digitally. Of course, some systems also use the self-diagnostics of the HVAC control head in the dash by pushing various buttons or a similar method.

The cool thing about scan tool diagnosis is bidirectional control allowing the technician to check the operation of such things as the compressor clutch relay, blend door position and such. The scan tool can also report powertrain related diagnostic trouble codes (DTCs), those with the P prefix as well as body control module codes that have the B prefix. In some cases, network codes having the U prefix are also available.

What has not changed is how we service the air conditioning system components at the mechanical level. Compressors, condensers, evaporators and blowers still require wrenches in the skilled hands of agile technicians. Granted, accessing some of the components has become a hassle over the years though.

What has changed is system control and management. Electronics and their myriad sensor inputs are the realm of the skilled minds who must diagnose them. Here, we shall review many of those components, their function and their testing.



Systems in review

Automatic temperature controls are fast becoming the norm versus switches, levers and knobs. The body control module (BCM) as well as the engine control module (ECM) need to know stuff such as engine rpm, coolant temperature, ambient (outside air) temperature, cabin temperature and relative humidity, and so on. Decisions based on these inputs are used to control the HVAC.

Let’s take a look at how the General Motors automatic system on a 2010 Chevy Malibu works. Let’s start with the air distribution system then examine the temperature control system.

The air distribution system has five major areas:

1. HVAC control components.

2. Air speed.

3. Air distribution.

4. Recirculation operation.

5. Automatic operation.

The control module is a component on the CAN network and is the interface between the driver and the system’s air temperature and flow. Although it has a keep alive memory, that memory can be lost if the battery is disconnected. The control module controls the mode actuator for the distribution and blend doors by sending either zero volts or 12 volts to the actuators and reverses the voltage polarity to move the other direction. As the actuators operate, a signal ranging from zero to five volts is sent to the module to report position.

A control processor controls the blower motor speed by means of a pulse width modulation (PWM) signal. The blower motor gets a constant 12 volts and the ground is modulated to change the speed.

The driver selects the speed which is processed by the control module. The higher the selected speed, the longer the circuit is connected to ground. The lower the selected speed, the less time the circuit is modulated to ground.

After the engine is shut off and the controller goes into the sleep mode, the BCM may send a wake-up signal to the module to switch the blower to high for two to three minutes. This afterblow feature dries out the evaporator and case to prevent microbial growth that causes the legendary locker room odor when the A/C is operated the next day.

Air distribution is selected by the driver to the desired outlet for defrost, bi-level, dash vents, floor. Again, the control processor relays the wishes to command the doors.

When the driver chooses automatic operation, the control module will maintain the desired temperature by cycling the A/C compressor clutch, the blower motor, the temperature actuator and the mode actuator.

Now that we have covered the air distribution, let’s take a look at those sensors and components.

The air temperature control system can be broken down into four areas:

1. Radio-Driver Information Center (DIC).

2. Automatic operation.

3. The heating and air conditioning


4. The A/C cycle.


The radio display shows the outside air temperature. It is on the low speed bus of the CAN network and as such does not refresh instantaneously. However, when using a scan tool, there is no interface between the sensor and the reading so the temperature on your scan tool may not match that on the radio, but that does not mean there is a problem.

As with the air distribution, the control module is the pivotal player on the team but in this case it supports not only the afterblow feature, but the purge, personalization and actuator calibration functions.

 The temperature control actuator is an electric motor that, like a window motor, can operate in two directions. The control module reverses the ground polarity to change motor direction via a five-volt reference and two control circuits that vary between zero and five volts. So, the connector has a total of five wires.

There are four air temperature sensors in this GM system:

1. Ambient.

2. Cabin.

3. Sunload sensor.

4. Pressure sensor.

Each is a simple two-wire negative temperature coefficient thermistor, very similar to a typical coolant temperature sensor. The control module converts the analog voltage that ranges from zero to five volts into a digital signal that goes from zero to 255 counts.

You will find the cabin temperature sensor inside the control module (it can’t be replaced separately). Its signal goes to the body control module (BCM). The HVAC control module then gets commands to the engine control module (ECM) to engage the compressor clutch and HVAC control module to move actuator door positions to obtain the proper air temperature.

The sunload sensor is a photo diode that detects the brightness (from dark to bright) of the light entering the car. Its information is received by the BCM which then sends commands to the control module to cycle the compressor clutch and actuators.

An A/C refrigerant pressure sensor is a familiar three-wire transducer with a five-volt reference that sends a signal of nearly zero to five volts to the BCM. When the refrigerant pressure is low, the signal is nearly zero.

When the pressure is either too high or too low, the signal is ignored and the compressor stops running.

The ECM will disengage the compressor clutch at wide-open-throttle, if the A/C pressure exceeds 435 psi, if the A/C pressure drops below 36 psi, if the engine coolant temperature exceeds 248 degrees Fahrenheit, if the engine exceeds 5,000 rpm, during transmission shifts, if there is an excessive load on the engine (from hard launches and hill climbing), or if the idle is poor.

Working in harmony like the two squads on a football team, the automatic HVAC system keeps its fans (occupants) happy.

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