On April 8, 2005 NHTSA issued a Final Rule (Docket No. NHTSA 2005-20586) that adopts a new Federal Motor Vehicle Safety Standard No.138 "Tire Pressure Monitoring Systems (TPMS)". Previously, the Final Rule issued by NHTSA on this same subject was vacated by the US Court of Appeals because it ruled that it did not follow the TREAD Act requirements of requiring that a TPMS system be capable of detecting when any combination of tires, up to all four tires is significantly under-inflated. That previous Final Rule would have also allowed a one-tire under-inflation detection capability.
Consistent with the Court's ruling, FMVSS 138 will now require new passenger cars, multi-purpose passenger vehicles, trucks, and buses with a gross vehicle weight rating (GVWR) of 4,536 kg (10,000 pounds) or less, except those with dual wheels on an axle, to be equipped with a TPMS to alert the driver when one or more of the vehicle's tires, up to a total of all four tires, is significantly under-inflated. Specifically, the TPMS must warn the driver when the pressure in one or more of the vehicle's tires is 25 percent or more below the vehicle manufacturer's recommended cold inflation pressure, or a minimum level of pressure specified in the standard, whichever pressure is higher.
If any tire drops below the standard's activation threshold, the TPMS is required to provide the low tire pressure warning by illuminating a yellow telltale within 20 minutes of additional travel within a speed range of 50-100 km/hr. This telltale must remain illuminated (and re-illuminate upon subsequent vehicle start-ups) until the under-inflation condition has been corrected. NHTSA states that they have determined that the specified under-inflation threshold and the detection time will allow the TPMS to provide a timely warning that permits the driver to take corrective action before adverse consequences ensue. Thus, NHTSA believes that the low inflation pressure detection requirement of the standard both fulfills the mandate of the TREAD Act and meets the need for motor vehicle safety.
The final rule provides that compliance testing will be conducted on a specific test course, namely the Southern Loop of the Treadwear Course in and around San Angelo, Texas. NHTSA states that this approach offers several advantages. First, testing can be conducted in a timely fashion without the need to design or build a new test track. Further NHTSA states that this course has already been used for several years by both NHTSA and the tire industry for uniform tire quality grading (UTQG) purposes. NHTSA believes that the specified test course provides an objective test that is representative of a variety of roadways and real world conditions.
These TPMS systems will be allowed to be certified with the original equipment tires, but NHTSA has also required that a malfunction indicator also be required to warn drivers in case replacement tires would not be compatible with the TPMS system. If incompatibility exists such that low tire pressure warnings cannot be given this indicator will show a telltale or combined TPMS/malfunction indicator telltale warning the driver of this condition as well as other system faults.
This Final Rule also allows both carry-forward and carry-backward credits. The phase-in percentages are 20% for model year 2006, 70% for model year 2007, and 100% for model year 2008 vehicles. The malfunction indicators are not required until the beginning of the 2008 model year.
For vehicles that are manufactured in two or more stages (and for those vehicles that are altered, after having previously been certified in accordance with Part 567) are not subject to the same compliance timing requirements. Instead, vehicles that are manufactured in two or more stages or that are altered must comply with this standard beginning on September 1, 2008, with no phase-in requirements, although they can certify TPMS systems on their vehicles prior to that date.