Tire Monitoring System: Description and Operation

General Description

The LTPWS consists of four sensor transmitting modules and a receiver/control module with self- diagnostic capabilities. One sensor is mounted on each wheel of the vehicle except the spare tire. The receiver/control module is located in the instrument panel.

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 driver information center for two (2) seconds. The lamps are; (A) "LOW/FLAT TIRE" and (B) "SERVICE LTPWS." If the tw0 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-up 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 he 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 and battery - less 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 he 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/FLAT TIRE" 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/FLAT TIRE" lamp, located in the driver information center. A low tire Diagnostic Trouble Code (DTC) depicting which tire or tires are low will then be set in the receiver/control modules diagnostic memory. The "LOW/FLAT TIRE" lamp will remain "ON" and a DTC stored in memory even if the tires are reinflated, until the module is reset. Only when the ignition is turned "OFF" and back "ON" will the "LOW/FLAT TIRE" lamp turn "OFF" and stay "OFF." When the ignition is turned "OFF" and back "ON" and good tire pressure signals are received will the low tire DTC stored in the receiver/control module memory be erased. The receiver is now completely reset requiring another low tire pressure signal from a sensor to illuminate the "LOW/FLAT TIRE" lamp again and set a low tire DTC.

The receiver 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" lamp is illuminated and a wheel sensor DTC is then stored in the receiver/control module diagnostic memory.

The lamp will go out, on either of two conditions: (1) Ignition switched "OFF," (2) All sensors signal again, however, a wheel sensor DTC will still be stored in the receiver/control module diagnostic memory until it is manually cleared.