Tire Monitoring System: Description and Operation

LOW TIRE PRESSURE WARNING SYSTEM (LTPWS)





The LTPWS consists of a sensor transmitting module and a receiver/control module. One sensor is mounted on each wheel of the vehicle except the spare wheel. The receiver/control module is located in the instrument panel behind the Personalized Automotive Security System (PASS Key) module.

The LTPWS performs as follows: When the ignition switch is turned to the "RUN" position, the receiver/control module will audit itself (lamps, wiring, connections, microprocessor, lamp drivers, etc.) by illuminating the two associated lamps in the information center telltale for two (2) seconds. The lamps are; (A) "LOW TIRE PRESSURE" and (B) "SERVICE LTPWS." If the two lamps fail to illuminate, or if only one illuminates, or if both lamps remain "ON" after two seconds, then the receiver/control module has failed its Power-On test.

A pressure switch within the sensor is activated when the tire pressure (in one or more tires) is below 172 kPa (25 psi). An electronic code (signal) representing a low tire pressure condition is then transmitted by the transmitter portion of the sensor. No signal will be sent until the vehicle has moved sufficiently far enough for a "power generation cycle" to be completed. The power cycle is completed when the vehicle has been driven approximately 30 seconds over 40 km/h (25 mph). This power cycle results from the wireless, batteryless, and patented method of generating electricity (Piezo-Electric) from the vibrations produced when the tire is rolling. When the power cycle is complete, the transmitter within the sensor is engaged for approximately 128 milliseconds, during which pulse codes are to be sent to the dash-mounted receiver/control module. As long as the tire pressure remains below the alarm set point, a repeat low tire pressure signal will occur every power cycle. Only one successful communication signal is necessary to activate the "LOW TIRE PRESSURE" lamp.

The receiver portion of the receiver/control module is constantly "listening" for this electronically-coded low tire pressure signal. On successful reception and decode, the module will illuminate the "LOW TIRE PRESSURE" lamp, located in the telltale cluster. This lamp will remain "ON." That is, no changes take place in the display when tires are inflated while leaving the vehicle running. When the ignition is turned "OFF," and back "ON," the receiver is completely reset requiring another low tire pressure signal from a sensor to illuminate the "LOW TIRE PRESSURE" lamp again.

The receiver unit is also "listening" for diagnostic signals. At least one from each tire must be received during the time period called a "Diagnostic Cycle." At the end of this diagnostic cycle, the receiver unit checks that all sensors have reported. If they have all reported in (vehicle moving - all sensors functioning), then no action is taken and another system diagnostic cycle begins. But if one, two or three wheel sensors have not reported (signaled), then the "SERVICE LTPWS" telltale lamp is illuminated. The lamp will go out, on either of two conditions: (1) Ignition switched "OFF," (2) All sensors signal again.