Vehicle affected: 2011 GMC Sierra Crew Cab 4X4
Issue: Intermittent poor A/C performance
Tools used:
- Vehicle information
- Scan tool
- RRR machine
- DMM with temperature probe
- Infrared thermometer
- Thermal imager
Modern automotive HVAC systems have many similarities with previous versions because they work on relatively the same principals. Even though some of the components, refrigerants and delivery methods have changed, the main purpose of HVAC is to maintain occupant comfort and safety by adjusting temperatures and keeping the windows clear by controlling humidity. Additionally, many newer vehicles have added individual comfort zone controls along with heated and cooled seats.
Even though systems are similar, due to some changes in operation, such as variable displacement compressors, and with the proliferation of individual comfort controls, it is now necessary to change your approach to diagnosing issues with these systems.
Tried and true methods of relying on pressure testing of the air conditioning system are no longer the starting point, nor are they conclusive tests to determine root causes of problems. Many systems now use temperature sensors, in addition to pressure sensors, as additional inputs dictating when and how the compressor and air distribution systems operate. With that in mind, temperature testing and using the right tools to perform tests are essential to being both efficient and correct when diagnosing the system.
Step 1: Gather vehicle information
Before beginning any active testing, you should use information sources such as Alldata to check for any related TSBs and to obtain both operating theory and specifications for the vehicle you are working on. It may not be a bad idea to check the underhood label to see if the label matches the system specifications indicated in the information source. If there have been modifications, such as a retrofit or a programming change, there may be an underhood label describing the change.
As with any other vehicle repair you will need to confirm the issue utilizing information gained from the customer as to when the issue occurs. In the case of HVAC, this information is very important because the system can operate fine under some conditions and have problems with small changes to ambient (outside) air temperature or driving conditions. Our customer indicated the A/C would work fine for a time then stop blowing cool air.
There were no relevant service campaigns for the air conditioning system issue. We were able to duplicate the customers’ concern during a road test.
At this time, we checked the operating parameters, paying close attention to necessary pressure and temperature requirements for normal operation. You should remember depending on where you live in the country, you may not be able to test air conditioning throughout the year because of low ambient air temperatures. This may be overcome by testing the system in a heated shop. In warm months, it will be necessary to supply air flow across the condenser by placing a large fan in front of the vehicle.
Step 2: Perform scan tool tests
Armed with system operational specifications and parameters, you can use your scan tool to first check for any related fault codes. Many late-model vehicles use the CAN bus to transmit information and commands for controlling HVAC functions. Because of this, it is necessary to scan all the modules for individual codes and “U” communication error codes. If any U-codes are present, it will be necessary to diagnose and repair those before going any further.
A code check on this vehicle showed no current, history or U-codes in any module.
Once a code check is complete, use your scan tool to monitor system PIDs, paying close attention to temperatures while the system is working properly and if it stops functioning. Even though you will probably see some HVAC PIDs in the PCM, it may be necessary to look in the body control modules of the HVAC system to obtain complete data.
When the A/C system was working correctly, the scan tool indicated normal readings, comparing the PID data to the information obtained from Alldata. After normal operation for approximately 20 minutes, the compressor turned off. According to scan tool data, the compressor was commanded off by the PCM, but the A/C controller was still sending the request. Further inspection of PIDs found the outside air temp sensor was reading a filtered temperature of 40 degrees F, and an unfiltered temperature of 220 degrees F (see Fig. 1).
First of all, the outside temperature on that day was 80 degrees F. Second, an unfiltered temperature of over 200 degrees F is an implausible number and needed more diagnosis. According to our research, this system turns off the compressor under 45 degrees F, so the compressor being commanded “off” due to the ambient temperature that was being reported seemed normal. We suspected a faulty sensor, but wanted to confirm there were no wiring issues prior to replacing the sensor. Following the wire diagram in Alldata, we traced the wires, inspected the connections and used a DMM to measure resistance in the circuit.
We found no issues within the electrical circuit and replaced the sensor, which restored compressor operation.
Step 3: Confirm the repair
You may think that is a complete repair, but to avoid any potential comeback we have the policy of servicing the system and doing a performance check on all A/C repairs.
We start by using our RRR machine to completely evacuate, vacuum and recharge the system with the correct amount of oil and refrigerant. We also have the policy of replacing service port valve seals to assure no leaks will occur after service.
We have put great faith in not only observing pressures but performing temperature tests. Temperature tests can confirm correct operation as well as assist in diagnosis if there is an issue.
There are many different tools to measure temperatures in an HVAC system, and some are more accurate than others for some testing.
Here is a short list of measurement tools and what they should be used for:
- Analog or digital thermometer – used for measuring vent outlet temperatures and ambient air in front of vehicle.
- Digital infrared thermometer – used to measure specific components such as the compressor, condenser, refrigerant lines, accumulator and evaporator (see Fig. 2).
- DMM with temperature probe – used for “contact” testing of components when measuring between two points on the same component is necessary (see Fig. 3).
- Thermal imager – a relatively new technology that is making its many uses known in the automotive repair industry. This tool has many uses in HVAC, such as obtaining temperature of components within the dash without removing dash panels to get at the component (see Fig. 4).
There are some rule of thumb temperature tests you can perform to determine if the system is functioning normally, or use to determine what some of the likely causes of failure are.
Starting with an analog or digital thermometer, check and record the ambient air temperature one to two feet in front of the radiator. Then insert a thermometer in a dash vent on both sides of the vehicle (see Figs. 5 and 6). With the air conditioner on high, record both temperatures. Remember to place a fan in front of the vehicle to allow sufficient air flow across the condenser and radiator.
The results of these tests will determine if the system is cooling sufficiently and if it is delivering cooled air equally side to side. The outlet vent temperature on most vehicles will be 30 degrees F or more lower than ambient temperature on a well-functioning system. It should also be noted because ambient air temperatures can affect how well refrigerant condenses; you cannot expect outlet temperature to be at 58 degrees F on a 110 degrees F day because you were able to reach that temperature on a 90 degrees F day.
It goes without saying a quick and safe way to obtain temperatures of underhood components with a running engine is an infrared thermometer. Using an infrared thermometer equipped with a laser pointer ensures you are measuring the components you desire (see Fig. 7). We used the Mastercool 5224 Infrared Thermometer to test the compressor, condenser and evaporator. Measuring the compressor in the center of the housing away from hoses and lines should be within 20 degrees F of other engine components. Higher temperatures may indicate a low oil charge or problems within the compressor.
Using the infrared thermometer, test the condenser to check if there is a sufficient temperature drop between the inlet and outlet temperatures. There should be between a 20 degrees F and 50 degrees F drop in temperature. If not, higher than 50 degrees F readings can indicate a low charge or other issues, while under a 20 degrees F drop can indicate insufficient air flow.
Due to the radiator being so close to the condenser, it may be difficult to differentiate radiator temperature from the condenser. In this case, use a DMM with a temperature probe on the condenser lines (see Fig. 8). This method of “contact” testing may be more accurate as it will read temperatures of only the component(s) you are testing.
Use a DMM with the temperature probe to test the inlet and outlet evaporator lines. They should read within +/- 5 degrees F of each other. If the temperature variance is higher or lower it may be due to a low refrigerant charge, orifice tube issues or too much oil; all of which can cause problems later.
Using a thermal imager, such as the Snap-On EETH300, in the shop has proved to be increasingly useful for vehicle diagnosis. Because it may be a relatively new tool to you, it is a good idea to start building a library of “known good” and faulty components. Over time as you utilize this tool, you will be able to have a visual display of temperature variances and be able to obtain them much faster than before.
This tool has advantages such as having the ability to “see through” other components like the dash to visualize warmer components and record their temperature (see Figs. 9 and 10). With HVAC issues, this can help diagnose faulty evaporators, heater cores and blend doors, among other components.
Thermal imagers are also quickly becoming the “go-to” tool for testing heated and cooled seats, along with heated windows and mirrors (see Figs. 11 and 12). Without a thermal imaging tool, it is difficult to see if these systems are working correctly, if at all.
To sum it up, accept the fact that technology changes. With those changes, it is necessary to utilize new tools and access the abundance of information in order to keep up with technology and to diagnose and repair vehicles efficiently and effectively.
It is important to thoroughly review all aspects of the repair. Having the proper tools in your arsenal helps to ensure the repair has been diagnosed and fixed correctly.
