Photo: Getty ImagesWhen it comes to diagnosing HVAC electrical problems, work smarter, not harder. Don't you love GM's OnStar system? Here's a deal that from thousands of miles away, an advisor working in a nice clean control room can hit a few keys on a computer keyboard and, among other things, run a vehicular diagnostic check. The results of the check will flash up on a computer screen, and armed with that information, the advisor can counsel the motorist on an appropriate course of action. OnStar even offers a service where subscribers can have a remote diagnostic check performed on a monthly basis and receive the results of the examination via e-mail. Some of our peers are of the opinion that this is not necessarily a good thing. But no matter your business-oriented opinion of OnStar, there's no denying that it definitely possesses a high-tech coolness factor.
However, let's revisit the subject of that nice clean control room. If the opportunity presented itself, wouldn't you want to work in that type of environment as well? Conducting a diagnostic operation by pressing a few buttons, then obtaining some type of visual clue that can point you in a more specific diagnostic direction. I'm sure you would agree that working in a clean, comfortable car seat sure beats poking around in a dirty powertrain compartment.
So a vehicle rolls into your shop with a cooling or heating complaint. Your initial assessment reveals that it's positively not in the vehicle owner's head. You verify that indeed, the car's heating/ventilation/air conditioning (HVAC) system is experiencing a malady. After a thorough visual inspection, what should be your next step? Maybe it should be to your head. More precisely, diagnostic help may be available through the HVAC system's control head.
For years, many automatic temperature control (ATC) systems have incorporated on-board diagnostics (OBD). By pushing certain control head buttons in a particular sequence, turning a certain knob a certain way or a combination of both, a trouble code will appear on the control head display or other instrument panel location if a monitored circuit contains a fault.
FORD TALK Top: This illustrates the code retrieval initiation procedure from a 1997 Lincoln Town Car; most Ford vehicles will use this same procedure. Press "Off" and "Floor" simultaneously, then press "Automatic" within two seconds. Codes, if present, will appear on the display shortly afterwards. If no codes are present, most Fords will end the check with "888" or all segments lit (bottom).
(Photos: Pat DeGuiseppi)
General Motors (GM) and Ford ATC systems are probably the ones with which most experienced HVAC techs are familiar. However, many other vehicles are equipped with on-board HVAC diagnostics and some with manual temperature control (MTC) systems.
It gets even better than that. Beyond displaying trouble codes, some of these systems allow you to obtain and observe sensor readings, blend and mode door positions and more. While the information you get by running control head diagnostics may not prevent you from eventually digging out the wiring diagram and multimeter, it can certainly get you off to a good start.
Control head diagnostics While this article can't possibly cover every vehicle that offers control head diagnostics, we can supply you with some general guidelines. Keep in mind that with the exception of most Ford vehicles, self-diagnostic initiation procedures vary greatly, even within the same vehicle manufacturer's family. Keep your service information close at hand and up-to-date. GENERAL MOTORS:It's a good news/bad news thing. First the good news: Just about all pre-1996/pre-OBD II GM ATC systems offer reasonably thorough on-board diagnostics. This dates well back into the '80s and back even further for many Cadillacs, as well as certain other GM cars.Now the bad news: After OBD II hit the scene en masse for the 1996 model-year, you must use a scan tool to extract trouble codes and other diagnostic information from GM ATC systems, with the exception of most Cadillacs and a very few certain other GM cars. In most cases, that scan tool must be GM's very own Tech 2. Unfortunately, most aftermarket shops don't have a Tech 2. Nevertheless, the Tech 2 does some pretty amazing things with many of the newer GM ATC systems, including the display of live sensor data and some excellent bidirectional tests.
As far as GM MTC systems, virtually none offer OBD at this time.
(Photos: Pat DeGuiseppi)FORD/LINCOLN/MERCURY: While no Ford MTC systems that I know of have control head diagnostics, most of their fully automatic ATC systems dating back to the late 1980s do have them. What makes the Ford situation even better is that the same procedure works with the vast majority of the automaker's vehicles. The system will go through its self-checks, and after about 30 seconds, it will display fault codes if any are present.
There are a few variations concerning the initiation of Ford's code extraction process, but usually, it's just that you have to press different buttons and possibly in a different sequence.
DAIMLERCHRYLSER (DCX): You could say DCX is somewhat of a latecomer in the area of control head diagnostics. With a few exceptions, the automaker really didn't start to incorporate these systems until the early '90s. However, once they started, they did some good things.Beyond displaying fault codes, many DCX ATC systems also allow you to run calibration procedures for the mode and temperature door actuators. Some of the automaker's systems also incorporate a feature called a "Cool Down Test": a self-check showing failure results that can be used as a starting point for further diagnostics.
Putting the icing on the Chrysler cake, some of its MTC systems also have OBD.
MEET YOUR MERCEDESThe Recirculation button LED will begin to
flash. When it does, press the left Auto button to display the first code
that might be present. Afterwards, press the right Auto button to look for
the next possible code.This car surprised us with code
"b1234," which is an indication of a problem in the Sun Sensor
circuit. The service information doesn't specify what the 'E' represents,
but my guess is "Error."The "8...82" represents data
being received from the Refrigerant Temperature Sensor. The corresponding
number for that particular sensor is "8", and '82' is the
temperature reading in degrees Fahrenheit. Many other sensor readings are
available.The "12...HI" represents the Left
Footwell Flap position. The corresponding number for that particular flap
is "12," and 'HI' represents the flap's position, which is open
in this case. Data is available for all of the other flaps as well.The initiation procedures for obtaining sensor data and flap positions not only differ from each other, but also differ from the code retrieval procedure. You must carefully follow the instructions in your service information.
(Photos: Pat DeGuiseppi)EUROPEAN IMPORTS:There's a real mixed bag here. With many European vehicles, the only way to retrieve fault codes, as well as observe sensor data and other HVAC system operational information, is through the use of a dedicated scan tool or similar device. However, many other European vehicles have reasonably comprehensive on-board capabilities. HEY NOW HONDA
Top: The initiation of the code extraction process for an MTC-equipped 2003 Honda Civic Si is illustrated. First, turn the blower switch to off, then press and hold the Recirculation and Rear Window Defogger switches. While the switches are depressed, turn the ignition key to the On position, then release both switches. The Recirculation and Rear Window Defogger button LEDs will illuminate (bottom). In about three seconds, the Recirculation switch LED will turn off, and the A/C switch LED will come on. This is the indication that the self-diagnostic test has begun. The test lasts for about 10 seconds, after which the A/C switch LED will go out. At this point, if any fault codes are present, the Recirculation switch LED will blink them out in typical flash code style. Refer to your service information for the interpretive information.
(Photos: Pat DeGuiseppi)JAPANESE IMPORTS: For the most part, it doesn't get much better than this. Since they started installing them, the vast majority of Japanese make ATC systems provide very thorough on-board diagnostics. Many also allow you to access sensor data, information pertaining to blend and mode door positions and more. While many Japanese vehicles also allow the use of a factory scan tool to gather HVAC system data, most do not require one.
A few Japanese make MTC systems also provide OBD, such as Honda and Suzuki.
OTHER ASIAN VEHICLES: Hyundai and Kia ATC systems provide control head diagnostics that very much run along the "capability lines" as the Japanese vehicles.Where do you find the information? There are four main sources of information that contain the procedures you need to extract diagnostic information from control heads.1. Factory service manuals: Like the dearth of post OBD II GM ATC control head diagnostics, this is another good news/bad news situation. The good news is, just as with every other vehicular system, factory books are probably the best place to find what you need. They usually contain the most detailed information, including the very important "Theory of Operation" section. Of course, the bad news is that unless they're specialists dealing with a certain line of vehicles, most shops don't have factory manuals.2. Vehicle manufacturer service information Web sites: In lieu of access to printed manuals, vehicle manufacturer service information Web sites are another source of factory information. That's certainly good news; many shops now have, or if not, can have access to the Internet. Web Resources Many articles have been written about the service information Web sites, including quite a few in this very magazine. The bottom line is that before you hop on the service information Web site bandwagon, there's a lot you should know that is way beyond the scope of this article. The National Automotive Service Task Force's Web site (www.nastf.org) contains most of the information you need. It also contains direct links to all of the automaker service information Web sites that are currently available. I strongly recommend you check it out.But there are also a few disadvantages concerning this alternative. Currently, the only information these Web sites are required to provide is for 1996-and-later vehicles equipped with OBD II systems - essentially, emissions/driveability/computerized engine management material. The specific HVAC information you might be looking for may not yet or ever be available.
3. CD- or Internet-based computerized information systems: Most shops subscribe to these third-party information providers, and the information provided is usually quite comprehensive.
4. "Aftermarket" printed manuals: You can obtain HVAC-specific service manuals, but printed information is scarce. Now, some companies only offer this type of information in electronic format.
The Mobile Air Conditioning Society (MACS) Worldwide still offers its "Specifications Reference Manual." Its main purpose is to provide A/C and cooling system capacity specs, but it includes lots of other information. Among the compressor clutch air gap specs, serpentine belt routing diagrams and other items, you'll also find the procedures for initiating control head diagnostics in the vehicles that provide them.
A closing note With the exception of a few sensor and actuator problems or an occasional bad control head or programmer, ATC and "electrically actuated" MTC systems have proven to be amazingly reliable. Many of the problems that have cropped up are covered in service bulletins, so always be sure to also check them as part of your discovery process.I hope this article has provided you with the necessary information on how OBD can help you get to the bottom of HVAC system problems. The next time you're confronted with one, the solution may be right in your head.
