Direct systems monitor the air pressure inside each tire with a wheel-mounted sensor. These systems started appearing around model year 2000 on a growing number of import vehicles including Nissan Quest, 2001 Audi A8 & Quattro, 2002 Infiniti Q45, 2003 Nissan 350Z and Murano SUV, and 2004 Mitsubishi Endeavor to name a few. One way to tell if a vehicle has a direct TPMS is to simply look at the valve stems. If the stems are held in place with nuts at their base, the tires may have pressure sensors inside. The sensor is usually located at the base of the valve stem inside the wheel (though, on some applications, the sensor may be mounted in the drop center inside the wheel and held in place with a steel band).
The sensor contains a pressure transducer, battery and transponder that periodically broadcasts an FM signal. The battery may be replaceable in some sensors, but is usually part of the sealed sensor assembly — which means the sensor has to be replaced if its internal battery fails. The sensor is activated by the motion of the rotating wheel when the vehicle is being driven, and by changes in air pressure inside the tire.
On most applications, each sensor is individually coded and has its own designated location: RF (Right Front), LF (Left Front), RR (Right Rear) and LR (Left Rear). Some systems may also have an additional sensor in the spare tire.
The signals sent by the pressure sensors inside the tires may be received by the “keyless” entry system on some vehicles, the Powertrain Control Module, the anti-theft system 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.
Despite rumors to the contrary, cell phones, radios, radar detectors, garage door openers and remote starters should have no effect whatsoever on the signals broadcast by the TPMS sensors because all operate at different frequencies. So if a TPMS system is not functioning normally, electromagnetic interference is probably not the cause. The fault is usually a bad sensor or, in some cases, a bad receiver, or the system is confused about which sensor is at which wheel location.
WHAT HAPPENS WHEN A TIRE LOSES PRESSURE?
If tire pressure drops below the threshold for warning the driver, the TPMS system is required to turn on some type of warning light. The least sophisticated TPMS systems will simply turn on a “low tire” warning light, or illuminate a U-shaped icon that is supposed to look like a cross-section of a tire. But these systems don’t tell the driver which tire is low or by how much. That means the driver is supposed to realize there may be a problem with the tires, and to stop and check the pressure in each one to see if any are low. The more sophisticated TPMS systems will illuminate a display that tells the driver exactly which tire is low (RF, LF, RR or LR), and may also show the actual pressure reading for each tire.
