Changing technology always brings with it a need for new tools, and today’s new Tire Pressure Monitoring Systems (TPMS) are no exception. In 2000, Congress passed legislation called the TREAD (Transportation Recall Enhancement, Accountability and Documentation) Act in response to tire failures that were causing SUV rollovers. Many of these tire failures were blamed on underinflated tires. Studies have shown that nearly 250,000 accidents a year in the U.S. may be attributed to low tire pressure, and about 75% of all roadside flats are preceded by a slow leak or under inflation.
Low tires also increase rolling resistance and increase fuel consumption. According to the U.S. Department of Transportation, low tires waste 5.4 million gallons of fuel a day! Fuel efficiency is reduced 1% for every 3 psi under-inflation, so keeping tires properly inflated translates to a free tank of gas every year.
The TREAD Act requires vehicle manufacturers to equip all passenger cars and light trucks under 10,000 lbs. with tire pressure monitoring systems starting with 20% of model year 2006 vehicles. The law requires 70% of all vehicles to have it by model year 2007, and every new vehicle to be equipped with TPMS by 2008.
Actually, about 45% of 2006 vehicles have TPMS, and a number of earlier models dating as far back as 1997 (such as Corvettes with run-flat tires) have TPMS as well. So there are more than 20 million vehicles with TPMS on the road today.
Adding TPMS to a vehicle adds yet another warning light. Like all the other onboard electronics, TPMS monitors itself and turns on a warning light if it detects a fault such as loss of signal from one of its tire pressure sensors.
There are two basic types of TPMS, direct and indirect. Direct systems have a pressure sensor in each tire. The sensor may be attached to the inside of the wheel or it may be located at the base of the valve stem. The sensor contains a pressure transducer, battery and transponder that broadcasts a signal periodically. Each sensor is individually coded so the TPMS module can keep track of the signals and identify any tire that is low.
The signals sent by the pressure sensors may be received by the “keyless” entry system on some vehicles, the Powertrain Control Module or the TPMS module. The coded signal is broadcast at 125 kHz, or 315 or 434 MHz (depending on the application), and includes the sensor’s location and pressure reading. On most systems, the signal is broadcast every 30 to 60 seconds when the vehicle is traveling faster than 12 to 15 mph.
If tire pressure drops below a certain threshold (the law says 25% less than the recommended inflation pressure, but some systems will alert the driver if the pressure drops only 15%), the system will warn the driver. On some vehicles, a low tire will turn on a “LOW/FLAT TIRE” warning lamp or U-shaped icon. On others, it may also display that actual pressure reading in psi or kpa.
The main service issue with direct systems is loss of signal from any of the tire pressure sensors. This may happen if the transducer or transponder fails, or the battery goes dead. Most use a long-life lithium battery with a life expectancy of five years or more, but they don’t last forever. Consequently, once these vehicles get some age and miles on them, sensor failures will occur and repairs will be needed. Most OEMs are also recommending the TPMS pressure sensors be replaced when the tires are replaced. This requires a “relearning” procedure so the TPMS module can relearn the position of each sensor (right front, left front, right rear and left rear).
With indirect TPMS systems, there are no pressure sensors in the wheels. Tire pressure is monitored by comparing the relative speeds of the wheels via the ABS wheel speed sensors. Loss of air pressure from a tire causes a decrease in its diameter, which can be detected as a slight change in speed. Indirect systems are not as sensitive as direct systems, but require no additional hardware in the wheels. It’s essentially an enhancement to the existing ABS system. Failures here will be limited to the wheel speed sensors, which will also affect the operation of the ABS/traction control/stability control system.
Though TPMS is primarily an OEM-installed item, aftermarket TPMS kits are available for retrofitting older vehicles. The kits include pressure sensors for each wheel, a TPMS control module/receiver and a dash warning panel to alert the driver if tire pressure is low. Most of these kits cost several hundred dollars, but there are a few low cost kits that sell for under $100 that transmit a pressure reading to a special tire gauge.
TPMS service kits are another item that your shop will need in order to service vehicles with direct TPMS systems. The service kits include all the caps, cores, grommets and nuts that are needed to replace valve stem-mounted pressure sensors in the wheels.
Servicing TPMS systems will require a variety of special tools. For removing valve stem-mounted pressure sensors, various valve core and nut tools will be needed, including 11 and 12 mm sockets. The torque on the nut at the base of the valve stem is critical because too much torque may damage the sensor and too little torque may allow air leaks. For tightening the base nut, plan on using a valve torque tool (essentially a mini-torque wrench that reads 2 to 10 Nm).
For testing sensor transponders and running relearn procedures, you will need a special TPMS test and learn tool and/or scan tool. Dedicated TPMS testers are small hand-held products that communicate electronically with the sensors and TPMS system. The unit typically activates the pressure sensors so they will broadcast a signal. The unit then programs the TPMS module or receiver in the vehicle so it will recognize which sensor is which (RF, LF, RR and LR). The tool can also be used to verify the operation of each individual sensor and to troubleshoot TPMS faults. Many of these tools have a USB port that allows the tool’s software to be updated as needed, and to download information from the tool to a PC for printing or recording.
TPMS test and learn tools are available from a variety of tool suppliers, and typically sell for $500 or less. Most of these tools will work with all types of pressure sensors, but some may be limited to specific models or types of sensors (such as sensors that operate in continuous mode or modulated mode only).
For more advanced diagnostics involving communication problems between the TPMS module in the vehicle and the PCM or keyless entry system, an OEM scan tool or aftermarket professional grade scan tool with the appropriate software will be needed. A scan tool is also needed for most indirect (ABS) TPMS systems, too, for diagnostics and resetting the system.
On vehicles with direct TPMS with pressure sensors attached inside each wheel, the sensor is usually mounted in the drop center of the wheel 180 degrees (on the opposite side) from the valve stem. If you’re changing a tire on one of these applications, avoid dragging the tire bead across the sensor when dismounting or mounting the tire on the wheel as this may damage the sensor or tire bead.
On applications where the TPMS pressure sensor is on the end of the valve stem, the valve stem should be positioned 180 degrees from the bead breaker on the tire machine when dismounting the tire.
An alternative method for changing tires on vehicles with valve stem-mounted sensors is to vent all the air from the tire (by removing the valve core), then remove the nut on the base of the valve stem and allow the sensor to fall inside the tire. The sensor can be retrieved from inside the tire after the bead has been broken loose.
As with normal tire service procedures, new valve stem seals should be installed on the pressure sensor to assure a long-lasting, leak-free installation.
Most direct TPMS systems require a relearn procedure if any of the sensors have been replaced, or if any of the tires have been rotated to a different location on the vehicle. The relearn procedure will vary from one vehicle to another, so always refer to the owner’s manual or service literature for the exact procedure that is required. Generally, one of two methods will be used: a magnetic tool that is placed over each valve stem in a specific sequence, or using a transponder and/or scan tool to reset the system.
For example, on Corvette and Cadillac applications, the relative position of each tire pressure sensor can be relearned either way, by placing a J-41760 magnet over the valve stem, or by using a Tech 2 or equivalent scan tool to run the relearn procedure.
Magnetic Method: With the key on, engine off, press both the lock and unlock buttons on the key fob. A horn chirp within 10 seconds indicates the TPMS receiver is in the programming mode. Place the magnet over the left front valve stem until the horn chirps once. This forces the LF pressure sensor to transmit its code to the main module. Repeat the same procedure for the right front, right rear and left rear sensors in this order.
Note: You have only 60 seconds per wheel to complete this procedure otherwise the system will stop and you’ll have to start over. Verify all four pressure readings are displayed on the driver information center.
Relearn procedure with a scan tool: Choose Tire Pressure Monitor from the menu (Body/Remote Function Actuation/Special Functions/Set Options/Tire Pressure Monitor) and follow the prompts for the relearn procedure. This mode will program a unique code into the TPMS receiver’s memory for each sensor location.
Note: Do not attempt a relearn procedure if there is a similar vehicle with TPMS nearby. Signals from the other vehicle may interfere with the vehicle you are reprogramming.