t Figs. 4 and 5: temperature reading of the left vent (left) and the right vent (bottom), using an analog thermometer. B. Hoyland Tool Briefing Circle 154 on card, or enter at “e-inquiry” on VehicleServicePros.com www.VehicleServicePros.com FEBRUARY 2017 PROFESSIONAL TOOL & EQUIPMENT NEWS 51 VEHICLES AFFECTED: 2011 GMC Sierra Crew Cab 4X4 ISSUE: Intermittent poor A/C performance 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. 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. 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. 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. 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. 1). • DMM with temperature probe – used for “contact” testing 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 p Fig. 1: Techs can use an infrared thermometer equipped with a laser pointer, such as the Mastercool Infrared Thermometer with Laser (No. 52224-A) to ensure proper temperature measurements of specific components, including the compressor, condenser and evaporator. p Fig. 6: Using a digital thermometer, check the compressor temperature compared to engine component temperature. B. Hoyland
Professional_Tool_And_Equipment_News_February_2017
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