In 2000, a new Federal law, the TREAD Act went into effect, mandating among other things that by 2007 all new cars of 10,000 pounds or less must be equipped with Tire Pressure Monitoring System, or TPMS. This was a partial response by Congress to the Ford Explorer/Firestone Tire rollover accidents. One of the major controversies in that affair was Firestone's claim that Ford kept the tire pressures on the Explorer artificially low in order to better stabilize the top-heavy SUV. The low tire pressures, Firestone claimed, made the tires more likely to delaminate and blow out. It was eventually determined by Congress that since low tire pressures were in fact an issue in the accidents, all drivers should have better warning of when their tires were becoming dangerously low. The mandate requires all TPMS systems to activate a warning light on the dashboard when tire pressures differ from rated pressures by 25% or more.
It is entirely clear that TPMS systems save lives, not to mention tires. Tire safety advocates will tell you that most people do not check their tire pressures enough, if at all. (Tire safety advocates are incredibly boring at parties.) Running on underinflated tires for a substantial period of time is a recipe for disaster. If the tire becomes low enough, the sidewalls collapse and the inner liner of the tire begins to rub against itself, scrubbing rubber off the liner until the tire blows out. Having this happen at highway speed can potentially kill the driver and others.
TPMS systems existed in some quantity even before the new law; the first TPMS system was installed on the 1986 Porsche 959, and many high-end vehicles have had some form of TPMS installed, especially those that also sported the new runflat tires. BMW, Porsche, Corvette and Mercedes all used TPMS systems extensively throughout the 1990's and 2000's.
There are two types of TPMS systems; direct and indirect.
Direct systems consist of a set of pressure monitors with radio transmitters installed inside the tires and an electronic control unit (ECU) in the dashboard that receives the transmissions. The monitors detect when a tire has lost 25% of it's correct pressure, and send a signal to the ECU, which lights up the low pressure light on the instrument panel. Direct systems constitute the vast majority of TPMS in use today.
Indirect systems work by monitoring the rotational speeds of the tires through sensors in the axle or ABS systems. If a tire is losing pressure, its rotational speed becomes different from the tire on the other side of the axle, and the sensors send an alert to the ECU. Older indirect systems are very rarely used anymore, as they do not meet US DOT standards under the TREAD Act, and suffer from a major flaw anyway. An impact or event that affects both tires on one axle can lead to both tires deflating at roughly the same rate. Since the system works by comparing the two tires' rotation, this kind of tandem deflation can fool the ECU, preventing an alert for seconds or even minutes - critical time in a deflation event. However, a newer type of indirect TPMS system that reportedly solves the tandem deflation issue and meets all relevant standards is now being produced by the Swedish company Nira Dynamics and is rapidly becoming standard on many Audi and VW vehicles.
Most direct TPMS monitors are part of an assembly that includes the valve stem, although some carmakers still use an older style where the monitor is fastened to a metal band around the inside barrel of the wheel. When the valve stem type is installed, the monitor sits under the valve stem hole against the inner barrel of the wheel. Most are bare-bones systems that only transmit low pressure warnings. Some also transmit the pressure data so it can be displayed on the instrument panel. Some even transmit tire temperatures.
There are also a number of aftermarket systems available. In general, most aftermarket systems are direct systems which will monitor pressures and even temperatures in real time. Almost all will include some form of multifunction display which can be placed on the dashboard or installed somewhere in the instrument or radio panels to display the tire information being sent by the monitors.
Tire techs tend to dislike TPMS systems because they are the ones who ultimately get to deal with issues like their fragility, the large variety of different sensors, and the equally large variety of reset procedures. However, most tire techs also recognize the very real advantages of TPMS systems to the driver, including fewer tire-related accidents, better gas mileage and longer tire life. Not to mention longer driver life!