Understanding Tire Pressure Monitoring Systems

by Don Elliott on August 24, 2011

Auto tires are an integral part of your car’s safety and operating system. Each year over 40,000 car accidents are directly attributed to under inflated tires. Under inflated tires cause tread separation and tire failure. The U. S. Department of Transportation (DOT) National Highway Traffic Safety Administration (NHTSA) mandated the installation of tire pressure monitoring systems (TPMS) on all light duty vehicles, beginning with the 2008 model year.

Tires take a beating as the contact point between your car and the road. Tires are made of steel, fabric, and rubber. The materials are layered together to perform at a specific tire pressure. When tire pressure drops too low, the bond between the various layers is weakened. This causes the tire to fail and affects the stability of the vehicle. Drivers could potentially lose control of the vehicle.

How does a tire pressure monitoring system work? There are two types of tire pressure monitoring systems, direct and indirect. Both systems operate on the idea that the vehicle dashboard’s electronic warning system will notify the driver  A symbol, like this one, will appear when the tire is underinflated by as much as 25%. If you don’t recognize this as an underinflated tire, you are not alone. Schrader, a tire pressure monitoring system manufacturer, found 46% of people surveyed did not recognize this as a tire pressure warning.

Direct TPMS

Tire pressure monitoring sensors are installed in each tire to notify the driver of low tire pressure. The sensors use battery-powered radio frequency communication with the vehicles on-board computer system. Direct sensors come in two types. Internal sensors are mounted inside the tire. External sensors are mounted on the valve stem systems. Direct tire pressure monitoring systems are generally considered more reliable. They can warn the driver which specific tire is under-inflated.

Indirect TPMS

Indirect tire sensors are installed inside the tire as a band on the wheel. The sensors use the vehicle’s anti-lock braking system to compare the rotational speed of each wheel. A tire that is low on pressure will roll at a slightly different rotational speed than a fully inflated tire. The wheel sensors notify the car’s computer when one or more wheels are rotating at a different speed. Unfortunately, indirect sensors cannot detect which wheel is low. It also won’t report if all four tires are equally low on air pressure.

Tire pressure monitoring systems have proven to be a bit of a nightmare for auto repair shops and tire stores. There are few standards for OEM and aftermarket TPMS products. In addition, there have been a number of false reports of low tire pressure. This may cause some drivers to disregard TPMS warning lights. Automotive repair and tire change technicians need to take care when changing or replacing tires with monitoring systems. Tire sensors are easily damaged and costly to replace. Finally, some TPMS warning lights require a trip to the franchise dealership to have the warning light turned off, for an extra fee, of course.

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