Twenty-First Century A/C Service

Jan. 1, 2020
We zoom down today's roads in carpeted, climate-controlled capsules of concentrated technology. Slicing through the wind, we find ourselves belted into comfortable, heated or cooled seats, surrounded by glass and steel, plastic and rubber, with explo

The bottom line is that while today's units have some interesting hardware, the customer still just wants cold air.

We zoom down today's roads in carpeted, climate-controlled capsules of concentrated technology. Slicing through the wind, we find ourselves belted into comfortable, heated or cooled seats, surrounded by glass and steel, plastic and rubber, with explosively smart airbags to protect us at every quarter.

Our brakes, suspension and steering systems are likely to be as electronically assisted as are our engines, transmissions, climate controls, navigation and entertainment systems, which include perks like on-board Bluetooth networks and built-in iPod connectors.

Some vehicles have wipers that turn themselves on automatically when the windshield gets wet, and your sound system may change its volume setting in response to vehicle speed. Some newer models even have electronic engine mounts.


Climate control systems are changing at least as radically as everything else. In the past, they commonly used 3 to 5 pounds of refrigerant in the past, but present-day capacities have been slashed to the point that many new units have refill capacities between 1 and 2 pounds.

Cabin air filters have become more common since 1995 when they appeared on the Ford Contour. More and more of the air handling doors in a vehicle's dash panel are driven by electronic servos rather than vacuum motors or cables; the Mercedes S-Class has 10 of them.

Even some of the most inexpensive, no-frills vehicles have been equipped with electronic blend door actuators for about a decade. Interestingly, there are almost as many different types of door actuators and systems as there are vehicles, with some manufacturers standardizing to the extreme and other manufacturers moving in the opposite direction.

Many manufacturers are using clutchless compressors with electronically controlled, dynamically adjustable swash plates. These units are equipped with a rotary valve in the rear of the compressor that is electromagnetically controlled to feed refrigerant pressure into a special chamber that causes the angle of the swash plate to change. This shortens and lengthens piston travel to regulate refrigerant flow. As a result, clutch cycling isn't necessary. The pulley does have some replaceable breakaway pins or tabs in case something goes awry. These clutchless compressors are quieter, run cooler and can operate quite efficiently up to 11,000 RPM, according to DENSO Corp.
Scroll-type compressors have fewer moving parts and have been fairly common on some Asian makes for nearly 20 years. They have proved to be quite dependable, but they haven't enjoyed the popularity one would expect.

Many hybrid vehicles – but not all of them – have electric-motor-driven compressors with a high-voltage, orange-sheathed wire harness feeding the motor. These systems generally have more than 200 volts traveling through those leads. On hybrids with high-voltage electric compressors, non-conductive compressor oil (ND-11) is a must.


With automatic climate controls, abbreviations can be confusing. For example, General Motors (GM) calls its fully automatic system "HVAC - Automatic." DaimlerChrysler refers to its as "HVAC - ATC." Ford calls its electronic automatic climate control system "EATC." Hyundai's term is "Fully Automatic Temperature Control" (FATC).

On the Sonata, there are only three electronic door actuators: the Mode Actuator, the Intake (Recirculation) Actuator and the Temperature Actuator. All three of the actuators' position pots are fed 5 volts (orange) and reference ground (pink) from the control head through Asian-style joint connectors, but the motor control and pot signal wires are a proprietary mix of other colors.

The air conditioning (A/C) control head signals the powertrain control module (PCM) with driver requests on two different circuits. The PCM responds by controlling the compressor clutch through the A/C relay, which is mounted in the engine compartment fuse panel.

On vehicles with Hyundai's Dual Zone system, the register temperature is controlled from side to side, but it doesn't include split fan speed control. The compressor on newer units is variable displacement by way of a moveable swash plate, very similar to compressors already in use on other vehicles.

The A/C pressure sensor is installed between the receiver driver and the inflation valve. Sensing coolant pressure, this unit converts pressure into voltage. With this signal, the PCM performs idle control, cooling fan control and A/C compressor control.

The dash-mounted photo sensor is used by the headlights and the A/C On switch. It can be bench-tested with a flashlight because it makes its own voltage.

Vehicle speed is factored in when the ambient temperature display puts its numbers on the panel. Because the reading can be faulty when the car stops moving, the last reading taken while the car was moving is held on the display.

The radiator cooling fan(s) are controlled by the PCM through a pulse width modulator that is located in the forward area of the left front wheel well, near the headlight on that side.

Pulse width modulation (PWM) isn't a new idea, by the way. This type of voltage supply enables a hard-working component such as the engine cooling fan motor to do its job without getting quite so hot, effectively extending the life of the motor and affording more precise control of fan speed. Some electronically savvy technicians might remember the early 1980s GM cars that were equipped with a similar piece of electronic hardware driving the engine fan.

Mounted outside behind the grille along with the ambient temperature sensor is the air quality sensor (AQS). (Toyota calls its a "smog ventilation sensor.") The AQS triggers the control unit to move the air doors in the dash from "fresh" to "recirculate"whenever the sensor detects sulfurous acid gas, carbon dioxide, carbon monoxide, hydrocarbon or allergens.

Interestingly, Hyundai has been using a heater core water temperature sensor since 2004 that affects blower speed much like the old Ford Cold Engine Lockout Switch: If the heater core is below 113°F, then blower operation is regulated automatically in heat or auto modes, preventing most of the icy wind we all hate on those freezing mornings. The blower isn't totally shut down, though. At less than 50°F ambient, if the "Auto" or "Floor" positions are selected and the heater core is at between 113°F and 163°F, the blower won't run above a certain speed, but that speed gradually increases as heater core temperature rises.

The fin thermal sensor is actually an evaporator temperature sensor, and it is easily replaced on newer Hyundai platforms. It prevents the evaporator from icing by triggering compressor shutdown at 37°F, but allows compressor reengagement at 41°F.

In-car temperature sensors also are used because the FATC needs to know the cabin temperature. Some models also have a cabin humidity sensor. The fresh air door will close at 80 percent in-car humidity and reopen at 40 percent.

The blower on FATC-equipped cars is controlled by the control head through an item that is referred to in the Hyundai shop manual as a "MOSFET." So what the heck is a MOSFET? Try Metal Oxide Semiconductor Field Effect Transistor.

It's basically a power transistor that can be checked by connecting a test light to power and touching the test light probe to the transistor base. If the transistor is good, it will deliver a ground to the blower motor.

We were told at the Mobile Air Conditioning Society (MACS) Worldwide convention that the Hyundai FATC system will display two-digit trouble codes when you hold "Auto" and press "Off" a few times. These two-digit trouble codes correspond exactly with the B codes produced by the scan tool. What that means is that the two-digit code isn't any less reliable or less specific than the B-codes, unlike the coarsely defined two-digit engine control codes of that nature on some other makes in bygone years.

The good news is that Hyundai is displacing parts previously supplied by Siemens and Bosch with Delphi-built components.


The refrigerant ports are situated beautifully on the Sonata. Both of them have yellow caps and are poking up right behind the passenger side headlamp. The desiccant bag in the end of the subcooler is replaceable separately.

The bottom line is that while these units have some interesting hardware, the customer still wants cold air, and if you take all makes and models, you're bound to see one of these sooner or later.

That Hyundai 100,000-mile warranty isn't bumper-to-bumper, and it doesn't transfer to the second owner, so get ready for some "Hyundiagnosis" in your A/C bay.

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