Indirect System: Uses the ABS wheel speed sensors to monitor the rolling radius of the wheel and tire assemblies. Rather than directly monitoring tire pressure, the ABS measures the rotational speed of each tire. If one tire starts to spin faster than the others, the ABS determines that a tire has become smaller and has likely lost air pressure. This is done by measuring tire rotations within a given distance. A tire that has a reduced rotation that meets a predetermined percentage is considered to be low on air pressure.

Direct Systems : This system monitors the air pressure in the tires with a wheel mounted sensor. The direct type of TPMS can detect actual pressure levels and transmit the information to the driver more quickly. Tires are monitored individually by sensors attached to a special tire valve, or strapped to the drop center of each wheel. Each sensor monitors and transmits tire pressure and temperature every few minutes. The temperature is monitored to compensate for cold and warm pressure variations. In order to reduce battery consumption, the sensors revert to a “sleep” mode when the vehicle is parked or not moving. During this “sleep mode,” sensors may only activate and transmit signals about every 45 minutes.

To avoid damaging sensors when mounting and demounting tires, it is beneficial to know where the sensors are located. Sensor assemblies are commonly attached to the valve stem, with the valve stem acting as an antenna to assist with transmitting RF signals. They may also be strapped to the drop center of the wheel. If this is the case, the sensor assembly should be located in-line with the valve stem valve stem acting as an antenna to assist with transmitting RF signals. They may also be strapped to the drop center of the wheel. If this is the case, the sensor assembly should be located in-line with the valve stem.

To avoid damaging sensors when demounting tires, do not break the bead within 45 of either side of sensor assembly locations. Be careful not to damage sensors when mounting tires as well. Sensors are not repairable and require replacement if damaged or when their

battery is completely drained. It is recommended that the sensor be carefully “dropped into the tire” prior to demounting to avoid damage. When reinstalling the sensor, it is suggested that the locknut, gasket and valve stem be replaced and torqued to the proper specification. It is also necessary to use the proper valve cap as it is also an integral part of the system. It may be necessary to reprogram the TPMS when the tires are rotated, when a new sensor is installed, or after the vehicle loses power. This usually requires a scan tool andmay involve the use of a special magnet. Refer to the vehicle service manual to

properly reprogram the TPMS.