Dear Tracy,

I have heard that some new cars come equipped with an internal system that monitors tire pressure. Can you tell me if this type of system can be put in any car, and how the system works?

-Katherine R.

Dear Katherine,

Tire Pressure Monitoring System (TPMS) is an electronic system installed within a vehicle to continuously monitor the air pressure of all four tires. The TPMS alerts the driver, typically through a warning light on the dashboard, when tire pressure falls below a preset limit.

TPMS Indicator Light, image courtesy of www.safecar.gov

There are two types of TPMS –direct and indirect. A direct TPMS is designed with a pressure sensor on each tire, usually on the valve stem or band mounted. The sensors employed by direct systems operate by separate lithium batteries, which eventually die. This means that the TPMS needs to be serviced and should be part of regularly scheduled maintenance.

Indirect TPMS technology is based on the calculation of factors, beginning with the fact that a tire’s over-all diameter is smaller when it is not properly inflated. A wheel that is smaller than the other three will have to spin faster to keep up. Wheel speed sensors applied at each wheel position 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. While the indirect system does not require servicing, the design does have some issues, including the snag that if all four tires are underinflated, the system may not detect a problem.

For vehicles that do not have TPMS installed, portable systems are available and can be installed by a qualified service professional. In the future, this will not be an issue because as of 2008, the National Highway Traffic Safety Administration has required that all passenger cars, light trucks and vans (Gross weight less than 10,000 pounds) be equipped with a TPMS.