Auto Service World
Feature   April 1, 2009   by Andrew Ross

Sometimes A/C Diagnostics is all Hot Air

Focus on Airflow in Automatic Temperature Control Systems

When it comes to effecting a/c repairs, most professional shops would agree that diagnostics are at the top of the list. But diagnosing a problem with an automatic temperature control system, not just the refrigerant circuit, may lead to fewer post-repair letdowns.

“Most of the time we don’t think about the effect of air distribution on temperature,” says trainer Glenn Young. “Usually we look at everything in the refrigerant system first, and then we look at airflow. We should be thinking of airflow right up front. I really feel that your diagnostic challenge with aTc is not the refrigeration system; it is airflow control.”

Even experienced a/c techs understand how demoralizing it can be to execute a repair–diagnosed as a low-refrigerant problem, for example–only to have the car blow hot air despite a return of strong cooling system performance.

Looking to airflow problems may be the solution.

Young’s automatic Temperature control Diagnostics presentation at the Mobile air conditioning Society Worldwide convention earlier this year emphasized that controlling airflow is more complex than techs ever encountered with the old vacuum systems or cable-operated mechanical systems.

The number of blend doors has expanded to provide an increasing number of options for passengers, from the dual zone to three-and four-zone systems, and the number of failure modes has expanded in kind.

a self-described “code junkie,” Young warns that because the HVac system is not covered under the same federal regulations as those mandating emissions repair code standardization and component naming, there may or may not be codes set in a failure. In full knowledge of the trouble this can cause, he says, manufacturers often have a “symptoms list” or a “no-code list” that can help immensely with diagnostics.

“This is absolutely crucial to use when we don’t have a code.”

Young says that among the biggest challenges are the sophisticated five-wire actuator systems with logic found on controller area Network (caN) vehicles.

“a lot of systems are not only using voltage to operate the door; they are also using amperage to know what position that door is in.

“The most advanced type of actuator has a logic module built directly into the actuator itself. The logic module has direct control over the door motor. The logic module in the actuator decodes the signal for the control head and positions the door.

“Generally any problems detected will set a fault code, in memory. any problems with the motor movement, or the inability of the circuit strategy to see what it is programmed to do, is going to cause problems.”

What can happen, he says, is that a mechanical failure– stripped gears, for example–can cause a door to get stuck. The system will know that it’s not in the right position and try repeatedly to move it to the correct position, causing a knocking sound.

And then the world ahead, he says, is going to get more complicated.

There is a proliferation of sensors to monitor temperature and humidity, in multiple zones, and pressure switches that hold much more authority over system operation than in the old days. Seemingly unnecessary items such as windshield condensation sensors and, from Lexus, an exhaust gas sensor make things even more complicated.

“This is essentially a five-gas sensor. Now, I know how much a five-gas costs. I don’t know what this costs, but I know it can’t be cheap. When a car is in traffic and the car in front is a polluter, it shuts all the windows and changes everything to recirculate. Who is controlling the HVAC? It certainly isn’t the driver.”

Young says that as systems become more complex, taking a proper diagnostic approach is more important than ever.

It is useful, he adds, to remember that you don’t just have to diagnose the problem today; you might also have to diagnose the failed repair from yesterday.

As computer controls proliferate, a tech cannot assume that the last repair was performed properly, or that the computer was programmed properly for the car in which it was installed (e. g., with or without sunload sensors).

“If you get a hard car to repair, you have to embark on some forensic diagnostics. A decision tree can tell you a lot, but it doesn’t say to check to see what the last idiot did before the car got to you.”

Information in this article was adapted from Glenn Young’s presentation at the Mobile Air Conditioning Society Worldwide Convention. Young is a veteran trainer offering courses through a number of organizations, including Carquest, which sponsored his MACS presentation. Young can be contacted at

Sunload Sensors Explained

Sunlight entering a vehicle can account for up to 60 per cent of the interior heat load that the system must overcome.

Sunload sensors monitor the intensity of sunlight and adjust the HVAC control to improve the comfort level of the driver and passengers on sunny days. Sunload sensors are usually located at the top of the dash and are often mounted on a removable plate, speaker grill, or defroster vent. There is often one on either side of the dash to accommodate differences in sunlight exposure.

Most sunload sensors are the photodiode type, which provides increased resistance as the light intensity increases, so the signal from the sensor drops as the sun shines brighter. The operating range is between zero and five volts. As the sun load increases, the voltage decreases. The signal is sent to the unified meter and A/C amp, which provide instructions to the system to adjust blend doors and fan speed.

Bright sunlight conditions can result in the fan speed increasing, and blend doors opening to improve cooling.

NOTE: Sunload sensors do not respond to fluorescent light, which is read as darkness. When the sunload sensor is checked indoors or in the shade, an open circuit might be indicated. Always check the sunload sensor at a place where sun shines directly on it.

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