Keeping Your Cool With Automatic Temp Control

By Dave Hobbs. A climate control system that automatically maintains a set interior climate while conditions outside constantly change might seem complicated. Cool-headed diagnostics involve breaking these systems into manageable parts.

I can still recall the first automatic air conditioning system I attempted to work on as a young and impatient tech in my family’s independent garage way back in the 1970s. It was a GM Comfortron system in an older Cadillac. This system was way over my head at the time. It tested my patience to say the least and I ended up losing my cool (pun unintended). It was no surprise immediately afterward when Dad personally starting taking care of every Comfortron system that came into our shop.

As time went on, a little maturity allowed for more patience to learn the ins and outs of the next-generation system that followed the Comfortron. I didn’t want to give up on the challenge or the flat rate hours, so I had to learn the new systems and develop the needed patience to work on them. Now, some 35 years later, automatic temperature control (ATC) systems are still changing, still challenging and still over our heads at times. All of us, however, can keep our cool and successfully tackle the latest systems if we break them down to easily understandable subsystems.

Mission Possible

Before going further, let’s talk about what any automatic temperature control system’s mission is by thinking about its outputs. ATC systems have to maintain a desired temperature in the vehicle throughout all seasons of operation. First, for cooling in the summer, the system has to control compressor operation. Ultimately, for the sake of engine load management, the power control module (PCM) carries the task of commanding the compressor clutch relay or the solenoid that controls the variable displacement swash plate angle on the brand-new clutchless compressors. On a manual system, the driver presses the a/c “snowflake” button for the control head to request the PCM to enable the compressor. But on an automatic system set to auto, the request is determined by the system, not the driver.

The second mission is to determine how fast the blower should run, followed by a third mission of positioning the blend doors for temperature control and a fourth mission to control the MODE and RECIRC doors for air distribution and air inlet options. All of these missions must happen precisely and automatically—not a simple task.

Hot or Cold?

Unlike residential HVAC systems, mobile ATC systems require extra sensors to detect quick and radical changes in the temperature conditions of the passenger compartment. These sensors can range from a simple thermistor to a high-tech infrared sensor that pulls a temp reading right out of the center of the passenger compartment. IR sensors are gaining in popularity, and are found in select Dodge, Jeep, Chrysler and GM models.

For the systems still using the thermistor-style in-vehicle temp sensor, the trend in recent years has been to improve system accuracy with additional thermistors in the ducts for floor and dash outlets. If the vehicle is equipped with a dual-zone system, this means adding up to four more thermistors to measure in-vehicle temperatures. If you’re comfortable with reading engine coolant temp PIDs with a scan tool or probing an ECT sensor with a meter, you can easily do the same with these thermistors.

Regardless of the mounting location, these negative-temperature-coefficient thermistors work the same way: Resistance increases as the temperature decreases, so an ohmmeter can be a big help in testing as you heat and cool the sensors or just backprobe with the key on and watch the voltage change with temperature. Many vehicles use an evaporator discharge air temp sensor, but its primary purpose is to prevent evaporator freeze.

The main in-vehicle temp sensor has been around quite a while and has almost without exception had an aspirator tube connected to it. This flexible tube runs to the back of the sensor housing, down to a point on the heater/evaporator/blower housing to an area which can develop a “draft” to pull fresh air from within the vehicle’s cabin across the sensor.

Sometimes the aspirator hose falls off or gets pinched behind the dash, in which case the sensor will be measuring the temperature of the dash instead of the surrounding air. Since you can’t see it without major dash disassembly, a trick has always been to take a tiny piece of tissue paper and hold it up to the louvers on the dash in front of where the sensor is mounted behind and turn the blower on HIGH. If the tube is doing its job, the tissue should stay in place over the vent opening when you take your finger away. Some in-dash sensors use a tiny muffin fan to draw air across them, and the motor can be activated for testing with a scan tool.

Another common sense approach to in-vehicle temp sensor testing can be used on the newer infrared sensors. In Chrysler/Dodge/Jeep applications, the sensor is either built into the control head itself or in the instrument panel trim near the control head. Some GM models utilize an IR sensor in the rear HVAC control head. Whatever the case, all you need is something hot (a cup of coffee) or cold (a can of soda from a vending machine), along with either a scan tool that can read HVAC PIDS or a DVOM to measure the blend door command signal. Simply watch the scan tool while reading the temp PID (or blend door command) or backprobe the circuit going to the temp blend door involved for that particular zone of the vehicle. Be aware that if the vehicle has a dual-zone system, the IR sensor is a split sensor for left-side and right-side temp changes. Hold the hot or cold drink up to the little eye of the infrared sensor and watch for a change. No change? There’s your problem. Continue with your diagnostics.

Greenhouse Effect

Solar sensors in the top of the dash sample levels of sunlight to combat the change on passenger heat load. When the sunlight increases, a photodiode inside the simple two-wire sensor causes the reference voltage supplied by the ATC system control module to be pulled low.

A real quick test without breaking out your scan tool or meter is to completely shade the sensor for a few minutes with the a/c set to a temperature close to ambient while the engine is running. Now move the vehicle into in the sun and remove the covering from the sensor. Blower speed should pick up a bit and the blend door(s) should move a bit toward cold. More in-depth testing might include alternating between shielding the sensor from light and shining an incandescent light on it while either watching the voltage change (key on) or watching an ohmmeter go from OL while the sensor is hidden from light to some kind of resistance (anything but OL) when it’s exposed to sunlight.

Sensor supply voltage varies by manufacturer, and even among a manufacturer’s model lines. Toyota ATC systems, for example, send 5 volts to the sensor on the Prius line and battery voltage to the sensor on Corollas. GM sticks with 5 volts across all model lines. By the way, I’ve yet to connect my scan tool to any Prius and not see a DTC B1421, which is a fault for the solar sensor. Nothing is wrong; just driving it into the bay under the shop lights (I presume) sets the code. I’ve not found any TSBs for this, so who knows? They’re not broke so I don’t fix them!

Dual-zone systems can use either two sunload sensors mounted on the driver and passenger sides of the top dash pad or a single sensor with two elements in one assembly angled to handle both sides. These two-in-one sensors must be mounted in the correct orientation to allow them to determine which side of the vehicle has most of the sun beating in on it, so make sure you don’t mount them in the wrong position.

OATs Not Just for Breakfast

Vehicle ATC systems must also figure out whether the driver wants to cool down to 70° or warm up to 70°. The sensor that does this is called an outside air temp (OAT) sensor, or an ambient air temp (AAT) sensor. Most, like the in-vehicle temp sensors, are the familiar 5- or 12-volt analog two-wire sensors utilizing a negative-temperature-coefficient thermistor. Like the main in-vehicle temp sensor that needs a sample of fresh air moving over it, the OAT sensor is mounted in the front of the vehicle, usually behind the grille. No problem there if you’re moving down the road getting fresh air samples. It’s when the vehicle is stopped for an extended time idling when the sensor can be tricked. Most manufacturers have corrected the issue of engine heat soak by tricking the sensor into thinking the ambient air is much hotter than it actually is by filtering the logic in the control module into ignoring the air temperature reading unless VSS is sensed.

Again, testing this sensor is exactly like testing the in-vehicle temp sensors: Watch the scan tool PID or backprobe with an ohmmeter or voltmeter. It’s when observing the sensor with a scan tool or watching it on the DIC that you have to remember about the software filtering of the sensor’s output. If you cool the sensor down with some circuit chiller (ignition on), you’ll see an immediate update on most models. But when you heat the sensor up with a heat gun, don’t expect to see a change in the PID/DIC on most vehicles without a road test or at least lifting the vehicle and getting the drive wheels to spin at 30 mph or so.

One notable exception to this filtered OAT sensor is the sensor on 2001-05 GM full-size trucks with the Duramax engine. For whatever reason, this sensor doesn’t get its update delayed based on VSS; therefore, you’ll see a jacked-up temp reading on the DIC when the engine sits and idles while the sensor gets soaked with engine heat. A TSB tells you to remount the sensor to a radiator support screw near the left headlight. The wiring will be too long now, so be sure to secure it with a tie strap or two.

Actuator Motors:Pistols or Pianos?

Controlling modes, choosing RECIRC or FRESH AIR and blending the temperatures are jobs for either electric motors or vacuum diaphragm actuators. The doors are usually simple flat metal or plastic panels on a metal shaft that pivots when force is applied. They can become warped or the pivot shaft can become rusty due to condensation from the evaporator. Foam strips used for sealing can dislodge and cause the door to bind. Even with gear reduction to increase torque (and reduce speed), the best electric motors won’t turn a stuck or binding HVAC door.

I’ve seen cases where the plastic gears inside the motors cracked from applying force to change the position of a door that wouldn’t move. One common scenario when this happens is the customer comes in with a temp complaint along with some kind of indication of an electrical problem displayed on the ATC system control head. Whether the head controls the system (smart head) or is simply a display with buttons and knobs, there’s usually some kind of on-board diagnostic feedback in the form of an icon or LED on the display flashing to indicate a problem. On some models a BXXXX DTC may set.

On the subject of blend and mode doors, not every vehicle uses the conventional pivoting blend door. Some late-model vehicles use a scroll-type door made up of a fabric that works like a window shade. On an even more unconventional note, the Lincoln LS and Jaguar S-Type Dual ATC (DATC) systems don’t even use a blend door. The heater core is split in two and two pulse-width-modulated (PWM) solenoids control the flow of hot coolant. When there’s 0% duty cycle, there’s 100% coolant flow, and vice versa, so you can imagine the complaint when the solenoid has an open or short—full heat.

As for the motors that run these doors, they come in a wide variety of shapes and sizes. Some look like conventional zinc-plated round electric DC motors attached to a plastic assembly with gears and a lever, while others are completely encased in plastic. Some motor actuator designs are shaped like a grand piano, while others are shaped like a pistol. What’s more important than appearance is orientation and circuitry. Knowing this will help you understand how they work, and that will help you diagnose problems that don’t fall into the quick-and-simple DTC/symptom flowcharts.

Be careful not to install the wrong motor in a particular position. Some GM models, for example, have motors that look identical to each other for the driver and passenger blend doors but have different part numbers due to their orientation. Get them mixed up and you get the door going in the wrong direction . . . along with a temp complaint. A scan tool that has the ability to command door positions will move it and you’ll get the cold or hot air you’re looking for, but a DTC will most likely be set due to commanded and actual door positions being way off.

Door actuator motors on ATC systems usually have the ability to give position feedback to the ATC system controller. This is done in a variety of ways, depending on manufacturer and model year. I will lump them into four types. The most recent type to come on the scene is the two-wire pulse-count motor. This design utilizes the same process we use when watching the waveform of a fuel pump to calculate its rpm. The ATC system controller counts the pulses as the motor turns and small spikes are created. If it wants the door to go to a particular position, it simply sends the correct polarity to activate the motor and move the door in the desired direction, then quits sending power when the proper number of pulses are seen on the power feed circuit.

A more common actuator motor that’s been around a long time is the five-wire so-called dumb motor. They’re considered dumb motors because they don’t have a processor in them. They’re sent power and ground in the correct polarity by the controller to move the motor in the desired direction and have a position feedback sensor (think TPS-style potentiometer) built in.

Next are five-wire smart motors with feedback. Here, the ATC system controller sends power and ground to a processor inside the motor pack, along with a signal wire for control and the remaining wires used for the position sensor. These motors can receive a set voltage, such as 5 volts to command it to move toward cold, 0 volts to move it toward hot and 2.5 volts to tell it to stop.

A variation of this last type has three wires (power, ground and control signal) but no feedback. These are used in two-position applications such as MODE and RECIRC doors. Vacuum actuators are often used here instead of motors and are controlled with electric solenoids either built into the ATC controller (a.k.a. programmer) or in a remote solenoid pack and are subject the usual woes of vacuum leak and supply problems.

Here would be another use for your smoke machine: It’s a sealed vacuum system, so simply connect up the pressure and watch your flow gauge while changing MODE/RECIRC requests. If the ball floats, watch for smoke under the hood and dash.

Speaking of actuator diagnostics, almost all electric motors will give you some kind of clue when they get lost regarding the ATC system’s function of knowing where the motor is. That’s why it’s so important to perform an initialization/calibration procedure, which is spelled out in the service manual. Some are as simple as connecting the battery and waiting a few minutes, others are a little more complicated and require you to press some buttons on the HVAC control head, while still others require a scan tool to perform the reset/calibration.

Never connect a motor to power or ground while it’s out of the vehicle. If the motor wasn’t overextended and lost before, it will be now. You can, however, use a scan tool to command the motor in question to one position or another to watch it physically and observe the feedback on the scan tool if your scan tool has that ability. You might see the position readout in volts, in a percentage, in a number of pulses or in a number called counts, which will be between 0 and 256. The main thing to remember is that the HVAC actuator PID commanded and actual positions should be close to each other. If they aren’t, perform an initialization/calibration (if applicable) before further diagnosis.

The intelligence in any ATC system can be found in an electronic control module referred to by several different names, depending on the manufacturer. A few that come to mind are programmer, amplifier, Heater Control Module (HCM, in GM lingo) and EATC (Electronic Automatic Temperature Control, in Ford lingo). The main module for intelligence is incorporated into the control head on some vehicles, while the control heads on other vehicles are nothing more than displays with buttons sending analog or digital serial data bus messages to the actual smart module in the system.

An easy way to determine if the control head is the brains of the system or simply a display head is to look at the connector drawing in the service manual. If you see only a handful of circuits with functions like serial data, dimming and the usual power, grounds and ignition, you have a “dumb” head on your hands. If the control head contains pins for the actuators, in-car temp, OAT and sunload sensors, you’re looking at a “smart” head that runs the show. Knowing if the control head is the smarts of the ATC system is important in diagnostics. For example, if the system doesn’t maintain a set temperature correctly, a “dumb” head has little to do with that function and is the least likely component to blame.

Blowin’ In the Wind

Blower motors on ATC systems are controlled by a remote power module that’s controlled and monitored by the ATC control/programmer. Because the blower motor draws a large amount of current, there’s a big heat sink on these modules located in the evaporator housing for cooling. The input to control blower speed comes from the ATC system controller via a variable pulse-width-modulated signal. The blower module sends an output on either the power side or the ground side of the circuit. This output can be either an analog voltage to control speed or a high-frequency PWM power or ground feed.

Some blower motors have the blower power module built into them. The biggest enemy of a blower’s power module is the current draw of the blower motor itself. Always check the current draw both at start-up and during continuous operation. You might replace the module, only to find it fails again in a few days or weeks. Some techs have turned to installing an inline fuse between the blower power module and the blower motor in case excessive blower motor current is indeed wiping out the power module. In this scenario, the fuse will blow before the power module fails.

It’s usually the power transistor shorting to B+ internally in these modules. Power modules that fail typically keep the blower motor turned on HIGH at all times to comply with Federal Motor Vehicle Safety Standards (FMVSS), so if there is a problem, at least the blower will run on HIGH. FMVSS also require the air delivery to be on the windshield in event of an air distribution problem, so at least the windshield will be clear of fog or ice to prevent an accident.

Testing the power module begins with a simple determination of a good supply of power and ground to the module. When diagnosing a blower operation problem, always load the output to the blower with a load substitute, such as a headlamp, and perform a voltage drop test. Then move on to the blower power module PWM input from the ATC system controller and measure duty cycle with a DVOM or DSO while changing speed requests on the head.

More Diagnostics

You can break down diagnostics of ATC systems to the basics. The very first thing, of course, is to verify the customer’s complaint. The customer often doesn’t know how the system is supposed to work. Some are not aware that their system will not work in automatic mode if it’s set to the most extreme temperature (hot or cold), nor may they be familiar with the relationship between the front and rear heads. An inoperative rear head may only be a rear head locked out by the front head. A review of the owner’s manual is sometimes the only tool needed to fix the problem!

Next order of business is to see if the compressor can engage on a properly charged and functional a/c system. Some dual-zone systems have delivered uneven side-to-side cooling temperatures from a lack of cold liquid refrigerant due to an undercharged a/c system. Don’t forget other obvious things like a functional cooling system supplying plenty of hot coolant to the heater core.

Next in line is to look for DTCs in any module that may be involved in the ATC system. Not only would the control head be a natural place to look for DTCs, but also the ATC programmer, the PCM and possibly even the body control module (BCM). Some vehicles use the BCM to be the translator of the discrete (off or on) input for a/c request from the control head. The BCM then puts that compressor request input into a serial data bus message to pass along to the PCM for compressor control.

Next, don’t “reinvent the wheel”; always look for TSBs and software updates. I emphasize both here because many software updates are not included in TSBs. Also, some OEM information doesn’t make it into aftermarket service info. Often, bulletins that are preliminary in nature can be found only via the factory online manual. Keep this in mind, as you may be doing some research at various OEM websites. All OEM sites can be found via www.nastf.org, and all but a handful allow for inexpensive short-term subscriptions.

Complaints of improper temperature or air distribution are due either to inputs (sensors), outputs (actuators and motors), the control unit itself or software. Many HVAC complaints can be addressed with a software update. Whenever possible, check the current software level in the ATC system and compare to the latest factory calibration. Some OEMs allow for programming of ATC systems with a J2534 device. The “smelly evaporator” complaint with some GM models (mid-2000s midsize SUVs, for example) can be repaired by disinfecting the evaporator core and flashing the ATC system controller with a new calibration that enables a blower module afterblow feature. This dries out the evaporator after the vehicle is shut off, to prevent future occurrences of the nasty odor.

Finally, familiarize yourself with any built-in diagnostics if your scan tool isn’t up to communicating with a particular ATC system. Pressing OFF and WARM­ER have been GM mainstays for many years to look at DTCs and input/output info. Dodge/Chryslers often allow for a “cool-down test” when the A/C and POWER buttons are pressed at the same time. This test will run the system at full cold and check the evap temp sensor to determine proper system performance.

Regarding Dodge/Chrysler minivans, another button press is helpful to enter into actuator motor calibration, which should be done whenever a motor is replaced. Press POWER and RECIRC for at least 5 seconds and wait 2 minutes for the motor calibrations to take place.

Most manufacturers have some kind of self-test you can initiate to enable minor diagnostics in lieu of a scan tool that can communicate with vehicle’s ATC system. Whichever the case, you can be confident that a little knowledge obtained from breaking these complex systems down into manageable subsystems will help you keep your cool when diagnosing ATC systems this summer!