With your patience, I am going to deviate for a moment from my normal approach of explaining how to choose and use the right tools to complete a task. My approach this month is a little personal but trust me, it is relevant to auto repair and the systematic approach we should all take when diagnosing and repairing vehicles.
My wife has been fighting a degenerative brain disease for a little over two years. We have been to numerous doctors and have performed countless tests only to be told repeatedly there are more tests necessary. Keep in mind, this is a degenerative disease, meaning the longer things go on without the proper diagnosis and treatment, more and more serious symptoms develop.
Not only have we spent innumerable hours and thousands of dollars going back and forth with the doctors, the frustration of dealing with an ongoing issue takes a mental toll and causes a mistrust of everyone involved in the diagnostic process which, by the way, directly relates to automotive repair.
Finally, we found the right doctor that not only has enough experience with this exact type of condition but has also had success in performing some non-traditional tests to help diagnose her symptoms. Not only did this doctor have the proper test equipment, he also had the correct and current information to validate his suspicions and come up with a treatment plan to manage — but unfortunately not cure — the disease.
I am telling you this to explain a simple fact: whether diagnosing a vehicle or a person, having the most up-to-date and relevant tools combined, with your experience, is essential. Additionally, having access to the most current information and knowing how to use that information will allow you to be more efficient and accurate with your diagnosis.
We will come back to the correlation of a doctor and a technician shortly, but for now on to this month’s topic: air conditioning. A/C systems have been on vehicles for approximately 80 years. In that time, they have gone from an optional extravagance that could be added on at the factory or in the aftermarket to being standard equipment on most cars sold in the U.S.
It is also important to keep in mind that even though refrigerants and many of the controls (inputs and outputs) have changed, the basic premise of how the system works remains very similar to the original design; it removes heat and humidity from within the vehicle.
With many of these changes, most specifically changes to the refrigerants and their operating pressures and capacities, new test and repair equipment is required to work on newer vehicles. When I first started working on A/C systems, the average refrigerant capacity was around 3.5 lbs or 56 oz. One of the major changes in A/C systems has been a reduction in the capacity of the system, mostly due to a combination of changes in refrigerant and to improve fuel economy. Many vehicles built today are using just 16 to 20 oz. in their systems.
The reduction in capacity is significant. When you were working on a system that was as large as 56 oz., 2 to 3 oz. of over- or under-charge or contamination of air was not really a big problem and the system would still function fairly well. With today’s smaller capacities, 2 to 3 oz. of contamination or over- or under-charge can be as much as 10 to 15 percent of the system’s capacity and will have a significant impact on the system’s ability to function correctly.
This fact makes an up-to-date recovery/recycle/recharge (RRR) machine critical to use. The latest model machines will allow you to completely evacuate and remove all the moisture from the system, then precisely measure and install the correct amount of refrigerant and oil. Older machines that do not use current technologies may not be able to measure refrigerant amounts for these smaller systems accurately enough.
A current model refrigerant identifier is also necessary to ensure the tester is capable of testing for all current refrigerants and variants available, and to make sure the vehicle owner has not contaminated the system with any of these refrigerants.
A/C analyzer tools are available that can quickly measure temperatures and/or pressures at various points in order to quickly and accurately diagnose an A/C system. Tools such as the Mastercool A/C System Analyzer use an algorithm to calculate the effectiveness of the system by measuring refrigerant charge, superheat, supercool, and temperature differences within the system, ensuring an accurate diagnosis of the system.
With almost every vehicle you see using some type of electronic A/C controls, a scan tool is mandatory to diagnose the system, especially with vehicles that utilize a CAN bus. With a scan tool, you can monitor each input and output the system uses to regulate temperatures and pressures throughout the vehicle. A scan tool may also be used to activate various actuators such as blend doors within the system and may also be necessary to calibrate these actuators after a repair.
Last, but certainly not least, is current information specific to the vehicle, and updates from your equipment manufacturer. These updates may be relevant if the equipment manufacturer has made changes to their software or hardware to improve their equipment.
Case study
Now, we will discuss a vehicle that has had an ongoing A/C issue for about a year. This vehicle is a 2010 GMC Sierra Pickup with an automatic front and rear climate system. The used vehicle was purchased about six months ago by one of our customers, so they had no real information about any prior issues with the HVAC system. The issue they brought it in for was that the A/C would work fine for about 20 to 30 minutes, then the air coming through the vents would change flow to the floor and the air would get warm.
This sounds like a simple issue with many potential causes such as an overcharge, blocked airflow to the evaporator, or a cooling fan issue, just to name a few. We began our diagnostic testing by confirming the problem. Once the A/C had been running for about 20 minutes, the inside air became warm and the air diverted from coming out the dash vents to coming out at the floor.
Once we were able to duplicate the issue, we connected the vehicle to our refrigerant identifier which indicated there was a small amount of air contaminating the system, but it did have 134a in the system with no other contaminants. We the connected our RRR machine and performed a service on the system. Once the service was done, we drove the vehicle and it once again had the same issue.
We returned to the shop where we observed the A/C compressor to make sure it was functioning. We also connected the RRR machine again to recheck pressures. The compressor was on, but it was not cycling. Pressures were slightly higher than before, but within normal operating specifications. We then connected our Mastercool analyzer which indicated the system may have a restriction in the evaporator, which is what we suspected.
We then checked the vehicle information on Mitchell 1 to see if there were any relevant service bulletins or TSB’s; there were none.
Before we removed the evaporator, we decided to research this particular issue a little more since neither we nor the owner had any of the vehicle’s history. First of all, we noticed the A/C control panel looked like it had been replaced as the normal fading of the temperature indicator was not present. Since we suspected this had been replaced, we decided we should use our scan tool to calibrate the system, which resets all of the door actuators to their normal positions. Once we did this, we drove the vehicle again. To our surprise the system seemed to function normally.
It seemed as though someone had replaced the controller but had not taken the time to recalibrate the system, which caused the blend doors to malfunction and block air from coming across the evaporator, allowing the evaporator to freeze and block air flow.
We drove the vehicle on a longer road test to make sure that it continued to work well. It did until we lowered the temperature a few degrees, then the system started blowing warm air again.
We returned to the shop where we connected the scan tool to observe the blend door actuator positions. Each door operated correctly and when commanded, opening and closing fully. All of the pressure and temperature sensors were within normal operating ranges.
This is the point where we got a little creative, much like the doctor that diagnosed my wife. If you have an issue on a vehicle that has a production date near the beginning or end of a model year, take a look at the system in the year earlier or later to determine if there are any variations that may be affecting the vehicle. In the case of this GMC, the production date was August of 2010, making it a 2010 model year. But there was a discrepancy in the available sensor information between a 2010 and a 2011. Starting in 2011, GMC used a temperature sensor in the evaporator to monitor the temperature of the core itself, which then allowed the computer to command the A/C compressor on and off in order to maintain the proper pressures and temperatures within the system.
When we scanned the vehicle using the VIN correct 2010 model year, the sensor read 0 degrees. When we connected the scan tool but changed the VIN to read as a 2011, the sensor data showed a negative number, meaning the sensor was unplugged. Now, keep in mind we had no previous repair records on this vehicle, so we had no way of knowing what issues had been addressed before, or if any modules had been reprogrammed.
Since we found an open circuit in the evaporator sensor, we found and re-connected the harness. Once the harness was connected, we scanned the vehicle and found normal readings as shown in the image. An extensive road test was performed while setting the climate controls at various temperatures, with no additional problems.
This A/C system was able to cause an issue by having a sensor disconnected, but it did not set a fault code. What we determined was due to the merging of two model year technologies. The sensors were installed in the 2010 GMC, but the logic that would cause a fault code to set was not present in the BCM which controls the HVAC system.
In the 2011 model year, GMC switched over to an HVAC module to control the system rather than the BCM. Since this system was a blend of different technologies, it did not follow the normal diagnostic test procedures which should have easily shown where the fault was. But, just like the doctor that finally thought outside of the box to diagnose my wife, moving away from pre-conceived notions we had, we were able to diagnose this a/c problem by thinking outside the box.
