User's Manual

Introduction

vehicles must comply. The total population of vehicles with TPM

systems will grow from approximately 4 million units in 2003 to close

to 50 million units by the end of the decade.

The NHTSA, which is responsible for establishing and enforcing

vehicle safety standards, published a mandate requiring that the

TPM system must be capable of monitoring all four tires and alerting

the driver if a tire is under inflated by 25% or more of the OEM’s

recommended tire pressure. Testing showed that proper tire inflation

significantly improves vehicle handling safety, decreases tread wear,

improves fuel economy, and decreases stopping distances.

Underinflated tires are dangerous because they build up internal

heat that can damage the tire structure, potentially resulting in fatal

blowouts.

Direct Versus Indirect Systems

The NHTSA’s mandate on TPM systems allows two system

architectures. The first, called the “direct” system, uses a direct RF

link from each wheel to a central receiving control unit, and the

second, called the "indirect" system, works in conjunction with the

vehicle's antilock-braking (ABS) system.

 Direct Systems. Wireless sensor in or on each wheel that

measures internal tire air pressure and temperature

separately and communicates the readings by RF signals

to a receiver/controller control unit at regular intervals. The

sensors have an integral low frequency transmitter and

are typically built into or mounted on the valve stem. The

end of the valve stem serves as the sensor’s antenna.

Most direct systems share the receiver control unit with

the vehicle’s keyless entry system.

 Indirect Systems. Monitors rotation of tires via the

vehicle’s antilock braking system (ABS) to calculate

whether they are properly inflated. Does not measure tire

pressure or temperature. The system measures the wheel

speed of all four tires and calculates a “rolling

circumference” for each one. In the event of a tire

pressure loss, the rolling circumference of that tire is

reduced, producing a change in the tire’s rotational speed.

The NHTSA found that direct TPM systems are preferred

because they are better able to detect under- or over-

inflation and are quicker to provide alerts. Virtually all

vehicle manufacturers today have moved to direct

systems.