HVAC Improper air temperature flow issues

Feb. 4, 2019
Actuator position and temperature values accessed with your scan tool are very useful when diagnosing a system that is not functioning properly.

Tools Used:

·        Vehicle Information sources

·         Scan Tool

·         Thermometer

·         Refrigerant Identifier

·         RRR Machine

·         Various Hand Tools

·         AirSept A/C Blend Door Repair Tool

With all of the electronic systems on today’s vehicles it should come as no surprise that Heating Ventilation and Air Conditioning (HVAC) systems utilize electric motors controlled by a computer module (or modules) to direct and control air to various zones at the desired temperature.

Electrically controlled motors to operate blend doors, or flaps, have been around for quite some time. Some of the newer innovations with these actuators involve the presence of control modules that are able to precisely move the doors to provide comfort to individual zones based on inputs from the driver or passenger. These systems are able to control air flow much better, mainly due to the incorporation of multiple temperature sensors within the system, along with actuators that provide feedback to control modules. Temperature sensors can include sun load sensors, which allow for compensation of extreme interior temperatures, and ambient (outside) air. Many vehicles have multiple sensors in the ducts, as well as other locations within the vehicle.

Temperature sensors and flap position typically report values to the HVAC module in the vehicle. In some cases that will be the Body Control Module (BCM). The module can then send output signals to actuators that move flaps within the system. Changes in actuator position can change the temperature or direction of air flow by moving levers attached to flaps within the air box/ducting system under the dash.

In many cases, the actuator itself is used to provide feedback to the module for the flap position. Actuator position and temperature values accessed with your scan tool are extremely useful when diagnosing a system that is not functioning properly.

For the most part, actuator motors by themselves are fairly reliable, but there are times when other issues occur that cause an actuator to fail. The issues are various, and most of them are what are known as “pattern failures”, which are common issues within a particular vehicle manufacturers product line. Many of the problems can be avoided by performing simple maintenance procedures, which in most cases is just operating the system through all the modes. Occasionally operating the system through each mode allows the doors to move throughout all ranges of their operation and will help prevent binding, which is caused by dust and moisture combining to create a “sludge” which restricts door movement.

A very common issue with many vehicle manufacturers is a “clicking” or “ratcheting” noise coming from the blend doors when an actuator is moving a door. In many cases this occurs at the beginning of summer or winter months. This occurs mainly because the door positions are usually used within a narrow window of operation during warm or cold months when temperatures remain consistent. When the doors stay in a small range and do not receive commands to open or close to their limits, the doors can then stick once a command is given to open or close a door; the actuator gears may strip, or in extreme cases, break the lever that operates the door.

Another fairly common issue is incorrect air temperature either throughout the vehicle, or in one or more zones.

Step 1 – Obtain vehicle information

The initial part of obtaining vehicle information is to both interview your customer to determine what their issue is and operate the system to confirm the problem can be duplicated. After you are able to duplicate the issue, use your information source to see if there are any TSB’s or other service campaigns that match the symptoms of this vehicle. Always verify the applicability of TSB’s before performing the service, especially if the TSB requires programming a module. Other information you should obtain include specifics of how the system operates and what type of sensors are used, along with their operating specifications. Finally, you will need to know the procedure to calibrate the system after service work is performed.

The next part of obtaining information is using your scan tool to observe temperature and blend door position(s). Depending on the vehicle, and the scan tool, the PIDS for temperature and actuator positions can typically be found in the HVAC module, or in the BCM.

Operate the system through each mode and temperature range and note temperature and actuator position changes. In most cases if the problem with the vehicle is due to a “mechanical” problem such as a binding door or an actuator calibration fault, the PIDS will change slightly when different commands are given. If a command to open or close a door is given and the PID does not change, it is likely caused by either an electrical issue not allowing an actuator to function, or by a door flap or actuator that is broken.

Step 2 – Diagnosing the issue

Using data from your scan tool combined with information from a temperature gauge will help to isolate the cause of electrical or computer command issues. Use temperature gauges to verify PIDS from the computer. Note all tests should be performed with the engine running.

Some items to look for include:

·         Is the system voltage within specifications? With the engine running, battery (system) voltage must be within specifications in order to supply enough power to operate all electric and electronic components.

·         Is the ambient air temperature PID consistent with the outside air temperature? If the ambient temperature PID does not match the actual air temperature, the HVAC system cannot operate correctly.

·         Does the desired air temperature from each zone match the outlet air temperature? If the desired temperature does not match the actual temperature, the cause is likely due to an air delivery (actuator or door) problem.

·         If there are inlet and outlet air temperature PIDS, do they match actual temperatures? If they do not match, either an air delivery issue, or a temperature sensor can be the cause.

·         Is the outlet temperature consistent on the left and right sides of the vehicle? Inconsistent temperatures from side to side may be caused by air delivery, but far more likely is caused by low refrigerant. A low refrigerant charge can cause cold outlet temperatures on one side of the dash close to the evaporator and a higher outlet temperature as air moves further from the evaporator.

Prior to using your RRR machine to verify refrigerant charge, connect a refrigerant identifier to assure there is no contaminated refrigerant in the vehicle. An identifier will also let you know if there is excess air in the system that can affect proper operation of the A/C system

Connect your RRR machine that is compatible with the refrigerant in the vehicle. Check system pressures, and if necessary, recover and recharge the system.

Step 3 – Repair the vehicle

Some of the most common actuator or blend door repairs involve accessing components attached to the “air box” under the dash. Follow the instructions for removal and replacement found in your vehicle repair information data base.

In some cases, it will be necessary to remove air bag components in order to access actuators. If so, follow the repair instructions closely when disconnecting or connecting air bag system components. On a side note, I personally do not like to use “battery memory savers” when I am working on air bag systems. Even though it may take a few minutes to record and restore radio station and other “memory” features, along with another few moments to reset adaptive strategies in the computer, I believe it is worth the time instead of inadvertently activating an air bag.

There are quite a few tools that can be used to make under-dash work easier. One of my favorites is using one of my Milwaukee electric drivers with their Right-Angle Attachment. This attachment allows the user to use any ¼ drive bit and access tight areas with the speed of a power driver, saving both time and frustration.

Other good tools for tight areas are the “Stick” series of ratchets from EZ Red, and the Flex Head Ratcheting Combination Wrenches from Gear Wrench. All of these tools have been designed with high tooth count ratchet mechanisms that provide the technician the ability to access and remove/install fasteners in areas that typically do not have substantial room to move a wrench.

Some door actuator issues can be taken care of by performing a calibration. Follow the vehicle specific instructions from your repair source. Some vehicles will require the use of a scan tool while others can be accomplished by just removing the A/C system fuse for a few moments and allowing the system to self-calibrate. If a scan tool is required, it may be necessary to access the commands for calibrating the system in the “Special Functions” menu of your scan tool. If an actuator or door is replaced, it will also be necessary to calibrate the system following vehicle specific procedures.

If a door or flap is stuck, continued operation can sometimes cause the shaft of the blend door to crack. For GM, Ford, and some Jeep vehicles AirSept has developed a tool to repair the shaft in about two hours, rather than in some cases ten hours or more to remove the dash or air box to replace the entire door flap. After replacement, this will also require you to calibrate the system.

All in all, HVAC actuator and door issues are relatively easy to resolve with the right tools. They can also provide a good ROI on your tool investment because most repairs are accomplished with tools that have other uses in the shop.

Related Products:

CarSmart Inspector  CPS Products

Arctic Commander 4000 Automatic R-1234yf and Hybrid RRR Machine Mastercool

1700C Multi-Refrigerant Machine  Robinair

ArcticPRO ACX1215  Mahle

Legend Series Refrigerant Analyzer  Neutronics

About the Author

Barry Hoyland

Barry Hoyland has been in the independent aftermarket for more than 45 years as a technician, technician instructor, shop owner, and shop management consultant. He owned and operated a successful Southern California automotive repair center that offers complete auto care and specialized in emission and diagnostic services for over 28 years. Hoyland also owned a company that modified vehicles to perform as emergency response units and mobile command centers, incorporating high-end electronic components into today’s vehicles. Hoyland has experience with all size and types of vehicles including traditional gas, hybrid electric, alternative fuel, and heavy duty diesel trucks.

Hoyland has provided consulting services for many automotive shops, fleets, and government agencies in order to improve their operational efficiencies.

In addition, he has worked with many NHRA drag racing teams as a crew chief on supercharged alcohol and nitro-methane fueled cars and currently serves as a crew chief on a Top Alcohol Funny Car, a Nostalgia Funny Car, and a Nostalgia Alcohol Dragster

Hoyland holds certifications in ASE: A1, A6, A8, and L1, MACS 609, maintains a California Advanced Emission license, and a CDL with endorsements for double and triple trailers, tankers, and HazMat.

When he is not helping to run a shop in the Pacific Northwest, Hoyland travels across the U.S. as an instructor of technical and shop management courses, many of which he has developed. 

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