Tire Pressure Monitoring Systems

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

Dear Tracy,
My friend just purchased a new car and one of its features is an internal system that monitors tire pressure. Can you tell me how this system works?
-Lucy G.
Dear Lucy,
A tire pressure monitoring system, also referred to as a TPMS, is an electronic system that continuously monitors the air pressure of all four tires. The TPMS alerts the driver when tire pressure falls below a preset limit by illuminating a warning light on the dashboard.
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. Earlier model vehicles can be retro-fitted with a tire pressure monitoring system installed by a qualified service professional.
Tire pressure monitoring systems come in two designs –  direct and indirect. A direct monitoring system places a pressure sensor on each tire, usually on the valve stem or band mounted. The sensors used in direct systems are powered by separate lithium batteries, which ultimately need replacement. This means that direct tire pressure monitoring systems need to be serviced regularly and should be part of scheduled maintenance.
The technology used in indirect tire pressure monitoring systems is based on the calculation of factors, including tire size. The diameter of a tire is smaller when it is not properly inflated, and when one tire is smaller than the other three, it will have to spin faster to keep up. 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.  This calculation is used to determine if one is spinning significantly faster than the others, and therefore underinflated. While the indirect system does not require servicing, the design does have some issues, such as the problem that if all four tires are underinflated, the system may not detect an abnormality.
All tire pressure monitoring systems installed on 2008 vehicles and later are required to detect and warn the driver when the system is not functioning properly through a malfunction indicator. For some systems, a malfunction is indicated by a flashing of the low tire pressure warning light for a period 60 to 90 seconds with the warning lamp remaining illuminated following the flash sequence. The flash and illumination sequence will repeat at each subsequent vehicle start-up until the problem is addressed. If your vehicle has a TPMS, be sure that you are familiar with the malfunction warning for your specific system.