EagleTribune.com, North Andover, MA


August 19, 2012

Bumpy road may set off tire light

After I drive my 2005 Scion TC coupe 15 minutes the tire pressure light comes on, on the dashboard. I have checked and rechecked the tire pressure in all four tires and they are up to spec. Is this dangerous and should I spend the money to have the tire pressure gauges replaced?

The TPMS (Tire Pressure Monitoring System) originated in the mid 1980s on high-end European cars and has been a required element of U.S. light motor vehicles built since September 2007. Many manufacturers voluntarily implemented either direct or indirect systems prior to the required date. Direct TPMS employs pressure sensors within or on each wheel that transmits wireless information to a central receiver. This is the best method, but sensor or sensor battery replacement is a burden and care must be taken not to damage the sensors when installing new tires.

Indirect TPMS takes the cheapo route by using the ABS wheel speed sensors already in place to infer tire pressure. An underinflated tire’s smaller diameter results in a higher rotational speed than its siblings, resulting in an illuminated warning lamp. Indirect systems can be fooled by a variety of situations such as differing tire sizes, tread patterns or tread wear, slippery or bumpy roads, trailer towing, unusual speeds and temperature, aggressive cornering, vehicle loading, and a few other less likely scenarios.

Your Scion employs indirect TPMS. In follow-up communication you mentioned the lamp glows steady (blinking indicates a detected system malfunction), two tires required renewal due to tire injury, and you frequently travel over a bumpy road section. You are also familiar with the reset button, which reinitializes the system after tire replacement or an under-inflation warning.

Since you have not enc

ountered a blinking TPMS lamp or an illuminated ABS warning lamp, it appears the Scion’s hardware (wheel speed sensors and anti-skid controller) are functioning normally, and the system may be reacting to a condition mentioned above. In the fine print within the re-initialization procedure I noted that it takes over an hour of driving for the procedure to complete. I wonder if you may have missed this and the system has not successfully shaken off a previous issue such as the tire replacement or a bumpy road episode.

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