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Dorman: How to Install a HVAC Control Module


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    • By Counterman
      Volkswagen HVAC systems have come a long way in the past 20 years. While the basics of heating and cooling may be the same, the controllers and sensors have improved dramatically. No longer does a driver have to worry about dealing with fogged-up windows or bad smells driving behind a semi-truck.
      When a driver sets a temperature in the control head of an HVAC system, what does it mean to the vehicle? Seventy-two degrees could be captured at many different settings depending on the outside temperature, humidity and even the position of the sun. When a passenger then decides that 70 degrees is a better temperature for her zone, things get even more complicated.
      Automatic Temperature Control (ATC) systems require a complex array of internal and external sensors that look at the temperature, humidity and quality of the air inside the vehicle.
      TEMPERATURE SENSORS
      To maintain a preset air temperature, the VW HVAC system will typically have one or more interior air temperature sensors, an ambient (outside) air temperature sensor, and possibly one or two sunload sensors.
      Interior air temperature sensors are usually simple, two-wire thermistors that change resistance with temperature, but some are infrared sensors that detect heat from the vehicle’s occupants. This thermistor-type usually has an aspirator tube that pulls air through the sensor when the blower fan is running. Others use a small electric fan for the same purpose. A plugged aspirator tube or inoperative fan will slow the sensor’s response to temperature changes.
      Most air temperature sensors have a “negative temperature coefficient,” which means they lose resistance as the temperature goes up. A simple way to check this type of sensor is to use a blow dryer to heat the sensor. The resistance should drop as the sensor warms up.
      Ambient air temperature sensors typically have a slow sample rate to even out variations in readings that may be sensed at different vehicle speeds. When the vehicle stops moving, heat can build up quickly around the sensor and could mislead the ATC module into thinking it’s getting hotter outside. So, most ATC modules look at the ambient sensor input only every couple of minutes instead of continuously. On some applications, the ATC module may even ignore input from the ambient sensor when the vehicle is not moving.
      There are other temperatures in the various ducts. Also, most systems will have sensors before and after the heater and evaporator cores. These sensors measure the performance of the system.
      SOLAR LOAD SENSORS
      Many ATC systems also make use of a photodiode solar load sensor on the dash. This sensor allows the ATC system to increase cooling needs when the cabin is being heated by direct sunlight. On vehicles with dual-zone systems, there is often a separate sunload sensor for each side. Sunload sensors receive reference voltage from the ATC module and pass current when the light intensity reaches a certain threshold.
      Some ATC systems have additional temperature sensors located on the evaporator and/or compressor to prevent evaporator icing and to regulate the operation of the compressor. Some vehicles also have duct temperature sensors and heater core temperature sensors to further refine temperature control. These are usually found on the dual-zone ATC systems.
      HUMIDITY SENSORS
      Humidity sensors are capacitance sensors that measure the amount of moisture in the air. The information from the sensor both regulates the volume of air projected onto the windows to reduce misting and manages the humidity levels inside the car to enhance climate comfort. These sensors are typically mounted at the base of the rearview mirror.
      From the data delivered by the humidity and temperature sensor, the HVAC system calculates the dew point temperature of the air. Some systems use an infrared sensor that remotely measures the windshield and side window temperatures, as well.
      The performance of the sensor can degrade over time and cause the sensor to malfunction and give false readings. If this happens, you will see a code stored in the HVAC module.
      AIR-QUALITY SENSOR
      Air-quality sensors can prevent harmful gases and unpleasant odors that can get into the car cabin when the vehicle is sitting in heavy traffic, passing through congested areas or driving through tunnels.
      The sensor signals the fresh air inlet door/ventilation flap to close when undesirable substances are detected. Volkswagen, Audi and other import nameplate luxury car manufacturers are using this sensor. This sensor is typically mounted behind the grill. Just hot and cold? Not anymore!
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    • By Counterman
      Performance Ride Technology (PRT) will be exhibiting its full line of shocks, struts and complete strut assemblies at AAPEX 2022.
      The display will include a new product line and various first-to-market applications, according to the company. Some of the product launches apply to brand-new vehicles such as the 2022 Honda CR-V and 2022 Jeep Cherokee.
      The new product line, PRT air shocks, is an OEM-quality replacement for air-suspension systems, “providing the luxury and comfort of a smooth ride at an affordable price,” the company said. The PRT air shocks and packaging also will be featured in the AAPEX Showcase.
      In addition, the company will be presenting an extensive portfolio of heavy-duty applications. The PRT heavy-duty program offers coverage of more than 4,600 OEM references with 100% gas-charged shocks for trucks, trailers, buses, commercial vehicles, cement mixers, motorhomes and more.
      PRT will be exhibiting in Booth A2261 at AAPEX, which is scheduled for Nov. 1-3 in Las Vegas.
      PRT is a brand of ADD USA group, one of the largest manufacturers of shocks, struts and complete strut assemblies in the world.
      “We are very excited to present the PRT air shocks and all these new applications to our clients,” said Bruno Bello, director of global category and marketing at PRT. “As an OEM supplier, we are continually investing in research and development of new products and technology, which allows us to bring the latest innovations to our clients at AAPEX.”
      As a global presence brand, PRT also will exhibiting at Automechanika Dubai 2022.
      For more information, call 770-238-1611 or visit
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    • By Counterman
      Whenever a shopper finds a product they need on DormanProducts.com, they now instantly can see which nearby stores have that part in stock, using a new real-time inventory tool announced by Dorman Products.
      To use the new functionality, visitors to
      link hidden, please login to view can search for a product using either vehicle year, make and model; keywords; VIN; OE number; or visual-hardware drilldown. When they find what they need, they can scroll down on that product description’s page to the updated “Where to Buy” section. There they can view on a map what stores near them have that part available, as well as get directions and click to call. “We are always looking for ways to make it easier for repair shops and DIYers to finish a job. With this new capability, it’s now incredibly easy to find our full range of products,” said Steve Gisondi, Dorman’s vice president of marketing. “You can quickly search our tens of thousands of parts and components on a computer or smartphone, and find where they’re available near you, without the hassle of having to call around to multiple locations.”
      This industry-first capability is made possible through a strategic relationship between Dorman and Epicor, a global leader of industry-specific enterprise software. By leveraging Epicor channel connectivity among leading parts-distribution businesses across North America, the Dorman website now connects directly with store-inventory systems at a variety of retailers and distributors to let users know if parts are in stock at specific locations, according to the company.
      “Dorman is one of the automotive aftermarket’s most innovative suppliers, so it’s no surprise they are on the leading edge of solutions that bring increased access and convenience to the industry’s end-users,” said Tim Hardin, senior vice president and general manager of Automotive at Epicor. “We are very proud to have played a role in enabling this important new solution for Dorman and the many parts distribution businesses that offer Dorman’s products.”
      Dorman parts and components are stocked by hundreds of retailers and distributors throughout North America. Real-time inventory is available now on DormanProducts.com for most of Dorman’s largest retail partners, and new partners and locations are continuously being added.
      For more information,
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    • By Counterman
      I get “flop sweats” when I panic. One of my worst cases was when working the front counter at a tire dealer. We had just inspected a Ford Taurus with a broken passenger-side spring and about 75,000 miles on the clock. The customer dropped off the car in the morning complaining of a knocking noise and notchy steering.
      I was on the phone with the customer at 9:30, getting approval on the recommended work. The estimate I wrote was for four struts, two springs and front upper strut mounts. I spent extra time finding parts, looking up labor times and calculating the out-the-door price.
      Then the customer asked, “Is there a less expensive option?” That’s when the flop sweats started. It got worse after he asked, “Can I get away with just replacing the fronts?”
      I quickly pulled out my calculator and rewrote the estimate on the fly for just the front struts, springs and upper mounts. After giving him the price, he asked if it would be half that price if he did only one side. My manager could see the sweat on my face and tapped me on the shoulder. “Tell the customer you will call him back in 10 minutes with an accurate estimate because you need to talk to the technician.”
      He told me that the only way the shop would do the job was to do it the right way. He explained that if we did just the front struts, it would make the vehicle unstable. Replacing only one strut would make the car unsafe and impossible to align. He said the customer would be unsatisfied if we did not do both front and rear struts. He might complain about a steering pull tomorrow, uneven tire wear in a month or that he could not control the vehicle.
      “If the customer does not buy something from us today, we will not go out of business. But, if they buy something unsafe, it could put us out of business.”
      Looking back, I was more concerned about the customer walking out of the shop without buying anything than doing the job the right way to make the vehicle functional and safe. I was looking for approval for my ego and not approval for a sound repair.
      All or Nothing
      Shocks, struts and springs do not operate independently. The condition of one corner of the suspension affects all the corners of the vehicle when driving straight or cornering. At low speeds, it might not be as noticeable, but it still happens. At higher speeds and cornering forces, the inability to control the movement of the suspension and body in one corner can change the contact patch for all the corners. During a panic situation, an unbalanced suspension with worn ride control can alter the contact patch to the point where the vehicle oversteers, understeers or can’t stop.
      The stability-control system measures the effectiveness of its correction in real time. How effective the correction is depends on the condition of the contact patch of the tires. What influences the health of this contact patch is the tire’s condition (construction, traction and even inflation) and the condition of the chassis components like the struts.
      The computer does not assign a value to the condition of the contact patch and there are no parameters (PIDs) in the programming for most systems. The corrections and the effectiveness are measured by the sensors in a high-speed feedback loop.
      A stability-control system will never set a malfunction light if the condition of the contact patch crosses a set threshold. But, as the condition of the chassis and tires diminishes, the corrections become less effective and more actions will be needed to bring a vehicle under control.
      These stability-control sensors are essentially blind to the condition of the state of the ride control and suspension. The software and sensors can’t diagnose a strut that has lost its gas charge or if a spring is weak. It just sees the results as data coming from the sensors. To the algorithms in the software, it could be a patch of ice or an over-loaded car. But, the reality is that it could be a worn out strut and a spring with a missing coil.
      Basically, if the suspension is not up to snuff, the correction by the system will take longer. This results in longer stopping distances or, in some cases, the vehicle ending up on its roof.
      For example, the most basic stability and ABS correction is a panic stop in a straight line. When the driver mashes the brake pedal, weight transfers to the front tires, loads the front suspension and causes the springs to compress. How much the springs compress is influenced directly by the condition of the shocks or struts. When the springs compress, the contact patch of the tires changes due to the weight and geometry of the suspension.
      The rear suspension is even more interesting. When the weight loads the front tires, the rear spring expands, which makes the contact patch of the rear tire smaller. If the shock or strut is unable to control the rebound of the spring, additional weight is transferred to the front, which nosedives even more.
      Applied to the front counter at your shop, it means that you should never sell just one strut, shock or spring. Also, you should be very careful selling just front or rear ride-control units. This approach might get the customer back on the road, but not in a safe vehicle.
      So, what did the customer do with his Taurus? After calming down and wiping the sweat from my brow, I explained why we recommended replacing all four struts. He finally understood and decided to replace all four struts and both front springs.
      The moral of the story: Stand by your recommendations. You’re not doing your customers any favors by recommending a less expensive part or repair.
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      Suspensia is offering RockAuto customers an exclusive 10% off instant rebate on select Control Arms.
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