The fatal rollover accidents involving Ford Explorers and Firestone tires in the 1990s were blamed on low tire inflation pressures and less-than-best tire design. While the Ford/Firestone story filled the headlines, the problems with tire manufacturing and low tire pressures went much deeper.

In 2000, the U.S. Congress passed the Transportation Recall Enhancement, Accountability and Documentation (TREAD) Act to address the growing concerns regarding several major recalls of defective and improperly inflated tires that created dangerous and unsafe driving conditions. A major portion of the TREAD Act addressed the dangers of low tire pressure, which can cause a number of serious problems: poor vehicle stability, poor braking ability and reduced steering control.

Realizing that consumers do not check their tire pressures adequately and that many other climate, road hazard and driving conditions cause tire inflation failures, the legislation mandates tire and vehicle manufacturers to help improve overall safety.

The TREAD Act requires vehicle manufacturers to equip all passenger and light truck vehicles with tire pressure monitor systems (TPMS) by the 2008 model year. These systems are required to alert the driver if tire pressure falls to 25 percent under the recommended pressure.

How the TREAD Act Affects You

Will you know what to do when the TPMS light pops on?

It's an accepted fact that consumers just don't pay proper attention to the inflation level and condition of their tires. During National Car Care Month a couple of years ago, more than 400 check lanes were set up nationwide to conduct free vehicle inspections for consumers. The findings regarding tires were shocking: 21 percent of the vehicles were found to have improperly inflated tires; 16 percent had worn tread and were in need of replacement. Another 11 percent of vehicles had damaged tires and were in poor condition. In short, nearly half of the vehicles inspected were driving on dangerous tires!

Tire pressure monitoring systems are an integral part of automotive service. Simply stated: If you rotate, repair, replace or even inflate tires as part of your business, you must be thoroughly knowledgeable of how they operate and be ready to service them. Don't wait until you need to service a vehicle with TPMS to start educating yourself and your technicians. You can't send vehicles with TPMS to the dealers and expect your customers to return if you want to maintain and grow your customer base. Prepare now to service TPMS on your customers' vehicles.

The subject of servicing TPMS can be confusing because of the wide variety of types, sensors and service procedures in the industry. Every automaker has developed its own approach to TPMS using a combination of its own corporate technology and the use of existing hardware available from suppliers. Additionally, they had to adapt these systems to the wheels and tires used on their vehicles. For these reasons, the sooner and more fully you educate yourself and your technicians, the quicker your shop will be up to speed.

Two Basic Types of TPMS

There are two basic types of TPMS in use today: indirect and direct.

Indirect TPMS

The indirect systems appeared first because it is simpler to engineer into the vehicles. Indirect TPMS uses software algorithms in the ABS control module to determine when one wheel is rotating faster than the others. This is normally due to low tire pressure causing a tire to have a smaller diameter, therefore turning faster. These systems function by learning the wheel rotation rate and sending a warning when the rate increases.

Band-type direct TPMS sensors are fastened to the bvalley of the wheel.

While this type of TPMS is relatively simple for manufacturers to build into a vehicle - because it doesn't require additional components - it does have drawbacks. Indirect systems can't usually detect a problem when all four tires are equally low, which is quite common. Some indirect systems can't even determine when both tires on a single axle are low. To determine which tire is underinflated, all four must be checked. After inflating all tires to the proper level, the system must be reset - usually by pushing a button on the instrument panel - to again relearn the rotation rate of the tires. An additional problem with indirect systems is that if the system is reset when any or all of the tires are low, the system recognizes the reset level as normal.

The 1986 Porsche 959 was the first passenger vehicle on U.S. roads to use such a system, but the 1990 Chevrolet Corvette was the first mainstream vehicle to incorporate what GM called "a Low Tire Pressure Warning Indicator," as a $325 option. A couple of years later Cadillac adopted the system, but it didn't spread to other GM vehicles until a few years later, when the technology improved. Common vehicles with ABS-based indirect TPMS include some 1997-2003 GM vehicles and the 2001-03 Ford Windstar. Some European vehicles of the era have sophisticated indirect TPMS that can detect slighter deflation more accurately, but this is found mostly on German luxury cars.

Direct TPMS

Direct TPMS use sensors in the valve stem body of each tire to monitor the inflation pressure and a transmitter relays that real-time information to a control module located within the vehicle. The sensor/transmitter unit lies on the interior end of the valve stem, inside the tire.

Each sensor has a built-in battery to provide power to transmit the signal, which also identifies its location. The control module checks the signals from each wheel and alerts the driver by illuminating a warning light if the pressure in any wheel drops below the safe level.

Another kind of direct TPMS sensor is the band-type. These are mounted to the valley of the wheel and secured in place with steel bands. They can be found on some 2006-2007 Ford SUVs and minivans: the Ford Freestar, Explorer and Escape, as well as the Mercury Mountaineer, Monterey and Mariner. For 2008, Ford is returning to the valve-type sensors, so you can expect many of those vehicles to be retrofitted. Band sensors can also be found in many aftermarket TPMS conversion kits.

The only real drawback to the direct type of systems is that the transmitter can be adversely affected by steel wheels, which can block the signal. This problem has been addressed and should not be an issue for 2008 models.

Direct systems began to appear on some luxury vehicles as early as 2001, but most domestic manufacturers began placing them on some 2003 models. Of the two systems, the direct method has proven far more reliable.

Direct TPMS Sensors

Stem-mounted TPMS sensors are most common type.

There are several types of direct TPMS sensors in use. Some are frequency-based transmitters, while others are magnetically actuated by the centrifugal force of the wheel rolling. Because the majority of TPMS sensors on the road are the stem-mounted type, that form will be used as an example.

When the vehicle begins to move and the wheels start rotating, a centrifugal switch inside the sensor activates the pressure measurement and signal-sending functions. At about 20 mph, the sensor begins taking pressure measurements every 30 seconds and transmits the results once each minute to the control module. This electronic "handshake" keeps the wheels in touch with the control module. Each wheel sensor has a unique ID code so the module can recognize it. When the vehicle is parked and has not moved for 15 minutes, the sensors stop transmitting a signal to prolong the life of the internal sensor battery.

If the tire pressure drops below the minimum setting, the module will then illuminate the warning light. The warning light will not usually indicate which tire is low so all tires need to be checked with an accurate tire pressure gauge. Even though the culprit tire may be visibly evident, all other tires must be checked. Some luxury vehicles, however, incorporate a sophisticated driver information center (DIC) on the instrument panel. These DICs may indicate which tire is low.

After inflating the tires to the correct pressure, the vehicle must be driven to 20 mph or more. The warning light will then go off after a few minutes if the pressure is now correct. Some vehicles require the system to be reset, either by pushing a button or using a reset tool. The operation may vary per brand or model, so be sure to check the correct service information for the vehicle you are servicing.

Some luxury brands - especially those with run-flat tires, such as Corvette and Cadillac - have highly sophisticated TPMS that may require special diagnostic tools to reset the system. On many of those systems, the sensors are dedicated to a particular wheel and may not be changed to another location without triggering a diagnostic trouble code.

Registering Wheel Sensor ID Code

Most vehicles equipped with TPMS will have a sticker near the tire placard.

After any service that moves the wheel from its original position on the vehicle, the use of a TPMS ID code registering device is required to re-establish communication between the wheel sensor and the TPMS control module. Such a device triggers the sensor to transmit an ID code signal to the TPMS control module so it can recognize the sensor and identify its location. This does not apply if you are simply removing the wheel for tire service, brake work or any other under car procedure. As long as you put the wheel back in its original position, the TPMS warning light will not illuminate.

On most systems, the TPMS warning light illuminates and stays lit to inform the driver of a simple low tire pressure condition. If it flashes, there is a problem that must be diagnosed and corrected.

Servicing TPMS
The Correct PSI

Vehicles equipped with TPMS usually have a placard on the driver's door pillar indicating the proper tire inflation level and on the same tag or another nearby, indicates the vehicle is equipped with TPMS. With some vehicles, the information stickers may appear in the glove compartment or information is merely listed in the owner's manual.

The Correct Service Information

The first thing you want to remember is that every manufacturer's TPMS is engineered to that company's electronic control systems and therefore are different. And, slight differences may occur from model to model and year to year.

During the TPMS phase-in under the TREAD Act, the automakers moved quickly to test, install and perfect their systems to achieve the best results. Already, many technical service bulletins have been issued by automakers to correct or alter service information.

For the latest, most accurate service information, always refer to your primary service information providers, the manufacturers' service manuals or technical Web sites as listed on the National Automotive Service Task Force at www.nastf.org.

Next issue: In part two of "Keeping an Eye on PSI: Tire Pressure Monitoring Systems," the correct tools for servicing TPMS will be discussed, including tool sources as well as servicing tips.

Tom Nash, AAM, a veteran automotive industry writer, is a freelancer whose firm, Blue Car Media, is in Sterling Heights, Mich. He is a 2005 graduate of the Automotive Management Institute, from which he received his Accredited Automotive Manager designation. His e-mail address is tom.nash@hotmail.com.

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