What is a TPMS (Tire Pressure Monitoring System) and how does it work?

A Tire Pressure Monitoring System (TPMS) is designed to monitor the air pressure in a vehicle’s tires and to report potential inflation problems to the driver, usually through a yellow indicator light on the dashboard.  These systems have been in use for over a decade, but they have only recently become a mandatory safety system.  The Transportation Recall Enhanced Accountability and Documentation (TREAD) Act requires that all vehicles sold in the U.S. after September 1, 2007, come equipped with a TPMS. (See “History” below for more details, including exclusions to this requirement).

Figure 1

The warning indicator light will be yellow and may be one of these two accepted symbols:

There are 2 types of systems in use today: direct and indirect systems.  The indirect TPMS is less complex and less expensive than the direct system, but the direct TPMS is quickly becoming the more popular system among vehicle manufacturers due to the inability of indirect TPMS to satisfy all of the requirements set by government safety guidelines in the TREAD Act.

Direct TPMS

Direct TPMS operates using a sensor mounted inside of each wheel on a vehicle, either valve-stem mounted (Figure 4) or band-mounted (Figure 5).  These sensors take direct air pressure measurements from within the tire and communicate this information back to the vehicle’s computer system using radio frequency.  If a tire’s pressure falls below a pre-determined threshold (typically 25% below the manufacturer’s recommended cold inflation pressure), a yellow warning indicator light will appear on the dashboard to alert the driver to an inflation problem.

Figure 3

Valve-mounted Sensor      Band-mounted Sensor

Figure 4                             Figure 5

According to the Tire Industry Association (TIA), routine maintenance of valve-stem-mounted sensors should be performed every time a tire is repaired or replaced.  This maintenance involves disassembling the sensor and replacing a few key components (shown below in Figures 6 and 7) to ensure a proper air-tight seal and to clean away any corrosion that may have built up between services.  For vehicles equipped with band-mounted TPMS sensors, routine maintenance is typically not required, though some systems require reprogramming when a tire rotation is performed.  Both valve-stem- and band-mounted sensors are powered by a small lithium battery which is encased in rubber and is non-serviceable.  Once the battery comes to the end of its life, the entire sensor must be replaced and the new sensor ID must be programmed into the vehicle’s computer system.  The typical lifespan of a battery can range from 4-10 years, depending largely on how many miles are driven.  TPMS sensors are produced by several different manufacturers and may use different radio frequencies, so care must be taken to select the appropriate sensor for the specific vehicle being serviced.

Four key components of a valve-stem mounted sensor should be replaced at every tire service:

Figure 6

1. Sensor-to-wheel rubber grommet seal
2. Valve stem nut (fastens the sensor to the wheel)
3. Nickel-plated valve core (located inside the valve stem)
4. Self-sealing plastic or metal valve stem cap

Figure 7

Indirect TPMS

The indirect method of identifying an underinflated tire is based on the fact that a tire’s overall diameter (total height) is reduced when the tire loses air.  When one tire is “smaller,” or underinflated, it must spin faster to keep up with the “larger,” or properly inflated, tires.  Indirect TPMS utilizes wheel speed sensors located at each wheel position to identify an underinflated tire by comparing the rotational speed of each wheel with the average speed of all four wheels to determine if one is spinning significantly faster than the others.  If a problem is detected, a yellow warning indicator light will illuminate on the dashboard.  (See Figure 1)

2005 Tire Industry Association        Figure 2

Typically, routine maintenance of the indirect TPMS is not necessary.  However, as with any system, introduction of new components such as different sized wheels and/or tires may cause undesired results.  The threshold for a warning to be displayed may be increased or decreased, causing the system to become either hyper-sensitive or not sensitive enough when it comes to alerting the driver to a potential inflation problem.  If different sized wheels and/or tires are installed on the vehicle, it may be necessary to recalibrate the system. (Please contact a Sullivan Tire near you for exact vehicle specifications.)

Currently, there is one significant flaw with the indirect TPMS which poses a threat to the use of this system in the future.  According to the latest ruling by the National Highway Traffic Safety Administration (NHTSA), a TPMS must warn the driver when the pressure in one or more tires falls 25% or more below the vehicle manufacturer’s recommended cold inflation pressure or a minimum inflation pressure specified in the Federal Motor Vehicle Safety Standard, whichever is higher.  Since the design limitations of current indirect TPMS technology prevent the system from detecting when all four tires are equally underinflated, vehicles with these systems are no longer compliant with TPMS requirements.  If indirect TPMS technology improves to address this issue in the future, these systems may again be seen on new vehicles.