Diagnosing Computer Controlled HVAC Systems

Computer Controlled HVAC Pre OBD II

Vehicles built prior to the advent of OBD II typically used electronic control modules only to operate their automatic temperature control systems. On these models, the control module was typically contained in the HVAC control head in the same unit housing the driver’s system control switches. Warning indicators may or may not be included on the HVAC information display or instrument cluster. Access to any faults in these early electronic systems was generally performed by pressing control panel switches in a specified sequence, with a code numbers being displayed on the panel directly or by the flashing of an indicator light. Check the OEM service information for the specifics of the vehicle you are working on.

Once the trouble code number has been recovered, look up the definition of the code in your service information system. Most of these early systems set trouble codes for electrical problems in actuators or switches, and were diagnosed with simple voltage and resistance tests using a volt­ ohmmeter. Often, the use of an, automotive lab scope is an appropriate tool, allowing the technician to observe the function of an electrical component or circuit over time and helping him/ her spot an intermittent loss like ones often found in components using a potentiometer as a variable voltage device.

NOTE: Understanding how  any system functions is fundamental to effectively diagnosing them when they fail. Spend the time to review all the information you can before you pick up a tool or replace a component. Besides the service in­ formation, be sure to look for any Technical Service Bulletins (TSBs) that the manufacturer  may have issued. There are also several after­ market informational sources you should include while researching the potential cause of any vehicle performance issue.

Computer Controlled HVAC Post OBD II

Failures in the air conditioning system can impact vehicle emissions.  For  this  reason, most late model vehicles  rely on the engine management computer (more commonly referred to as the “ECM” or “PCM”) to decide when the air conditioning compressor will turn on. Computer control of the compressor and other HVAC components is not limited to those models with automatic temperature control either. That makes it more important than ever for the technician to understand modern computer diagnostic strategies.

HVAC Diagnostic Trouble Codes

Diagnostic Trouble Codes (DTC’s) are the computer reports of a failure within the system(s) it monitors. If it is a fault under the control of the ECM, it will turn on the Malfunction Indicator Lamp (MIL, also called a “Check Engine” light). However, if it is in a HVAC system module, there may be no visible warning that there is a problem. DTC’s  are separated into four different types; “P” (powertrain), “B” (body), “C” (chassis), and “U” (communications). In addition,  each  type  may  have a generic SAE definition or a more unique manufacturer’s definition. Regardless, never rely on the simple definition that may be displayed on your scan tool. Always look up the full definition, and the requirements needed for that fault to be recorded, in your service information system. The operation of most modern HVAC systems relies on more than one module doing its job, and if it can’t talk to the others on its network, DTC’s will set and system function will suffer. Always diagnose and repair these “U” codes before moving on to any others stored in the system.

NOTE: Many electronic module code failures require that a mod­ules software be re-calibrated, a process called “re-flashing’. It is im­portant that you check the OEM TSB’s for this information before wasting valuable diagnostic time.

HVAC DTC Diagnostic Procedure

The first step in any diagnostic procedure is to verify the customer’s complaint. As well as following the basic system test procedures outlined earlier, make a habit of connecting an appropriate scan tool and checking all the vehicle modules for DTC’s. Often, a fault in one may be the cause of a symptom in another. Record any DTC’s found.

Next, look up the individual definitions to see if any have shared requirements for failure detection. Understand fully how the specific system you are working on functions and what is needed by the module in question to log the failure. Be sure to check for any related TSBs as noted above for updated software and/or components. Many scan tools offer what’s called “bi-directional” control functions that allow the technician to command the operation of components like the compressor clutch, air blend doors, etc. Using these commands while monitoring the scan tool data stream can often assist you in locating a fault. Each Parameter Identifier (PID) on the scan tool screen has a meaning and use in troubleshooting. Many manufacturers supply diagnostic flow charts, or “trouble trees”, for their system codes. They can be a great aid in diagnosing HVAC DTCs but don’t follow them blindly. Make sure you know why you are performing a specific test, perform them all, and in the order listed by the manufacturer, to get the most success from them.

There are also several sources of aftermarket information to assist you. With today’s power of the internet, the answer may be only a few clicks away! Not all internet information is created equal, so be sure you verify the accuracy of what you find there. You can also access the factory information sites, albeit for a small fee, and have the same information as the dealer has. Once you have gathered the necessary information, it’s time to grab the appropriate tool and isolate the problem. In addition to a capable scan tool, you’ll need a good volt-ohmmeter to perform any electrical tests outlined in the flow chart. Many technicians utilize digital storage oscilloscopes to perform performance tests of HVAC components like blend door actuators and potentiometers. Any circuit you would prefer to monitor over time and while its working is a good candidate for the use of this tool. The last, and most important, step is to verify your repair after performance test before you return the vehicle to your customer.

This post was written by: Martin Hand


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Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.

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