Trouble with Tire Pressure Monitoring Systems.

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The most idiotic of all "idiot lights" in your car may be the Tire-Pressure Monitoring System (TPMS). The problem isn't with the TPMS itself. Rather, it's that the TPMS light doesn't illuminate until your tires are severely or, possibly, dangerously underinflated. Imagine a burglar alarm that doesn't go off until the bad guys have already carried out the TiVo.

Lights On, Anybody Home?

If the TPMS light is not illuminated, many drivers assume their tires are fully or, at least, adequately inflated. Often, they are wrong. Many who used to regularly check tire pressure will stop doing so, incorrectly assuming TPMS now takes care of that chore. And many will continue driving long after the TPMS light glows: Idiots often ignore idiot lights. Even the shape of the TPMS warning symbol—an exclamation point inside a slice of a tire—isn't too bright because many don't recognize its meaning.

The reason the federally mandated pressure-monitoring devices are arguably worse than worthless is that government regulations do not require the systems to provide a warning until a tire is 25 percent underinflated. When underinflated, even brand-new tires can't overcome deep water. Also, underinflated tires don't provide enough grip for emergency accident-avoiding maneuvers on damp or even dry pavement.

Underinflation Dangers

But here's something that's even more important: When underinflated 24 percent—the TPMS warning light is not glowing—a tire's internal components may be overheating and deteriorating. The Rubber Manufacturer's Association says that tires on some vehicles are barely able to carry a full load even if tire pressure is set exactly to the vehicle manufacturer's recommendation. The AAA says that tires are "well below the pressure required for safe driving" when underinflated by 25 percent. (For new cars, the correct tire pressure is on a placard on the driver's doorjamb. In older cars, it can be many places including the lids of the glove box, trunk or center console.)

Tire-pressure monitoring systems were mandated by the TREAD Act passed by the U.S. Congress in 2000. TMPS have been on all new cars since the fall of 2007 and were on some cars well before that. A National Highway Traffic Safety Administration (NHTSA) spokesman said TPMS regulations were meant to warn only of "imminent tire failure." Several tire makers and a consumer-watchdog group sued the Federal government to raise the TPMS threshold, but lost.

It's air pressure that allows a tire to carry a load.
It's air pressure that allows a tire to carry a load. To picture this, lay a new, full, unopened two-liter plastic soft drink bottle on its side, hold onto something solid and step up onto the bottle. Then, empty the bottle and stand on it again.

An underinflated tire won't immediately burst if you fill your minivan with high school football players or your pickup with gravel. Instead, the tires will flex too much and overheat, which will begin to break down the internal components, as well as chemical bonds between components. The damage is permanent and adds up over time. Under-inflate or overload a tire often enough and it will come apart, sometimes long after the harmful incidents. It's kind of like when your fat uncle sat on the antique chair at Thanksgiving: The damage it suffered will cause it to collapse when your frail grandmother uses it at Christmas. Damaged tires are most likely to fail at high speed on a hot day with a heavy load aboard: Summer vacation is prime time for tire failure.

Types of TPMS

There are two types of TPMS: Indirect and direct. Indirect systems don't measure tire pressure but rather uses the anti-lock braking system sensor to calculate how far one tire rolls in comparison to the others. Direct systems have sensors on the wheel that relay actual pressure to the car's computer. Indirect TPMS have a large margin of error, while top-quality direct systems can be very precise. It's unknown if any vehicle manufacturers set their direct systems to trigger significantly higher than the required threshold. At least one major manufacturer that uses direct TPMS still employs the 25 percent threshold.

Those who live in cold areas and have the luxury of a heated garage are often befuddled by TPMS. They responsibly set their tire pressure to the vehicle manufacturer-recommended pressure inside a 65-degree garage before driving onto frigid roads. Within a few miles, the TPMS illuminates. False warning? No, the extreme cold has reduced the tire pressure. Tire pressure changes about one psi with every 10-degree Fahrenheit change in outside temperature. Tires set to 32 psi inside a 65-degree garage will drop to about 27 psi if it's 10 degrees outside. Likewise, those who set tire pressure on a hot autumn day that's followed by a cold snap will get a TPMS warning. The tire requires its pressure to be correct for the outside temperature.

Other things that cause TPMS issues include the fact that it's not unusual for a tire to lose almost one psi a month. Also, tire gauges aren't always accurate. We once purchased the most expensive tire gauge in the auto parts store and it read two psi optimistic: The tires were really at 30 psi when the gauge said 32 psi. Another thing that causes TPMS irritation is the calibration process, which differs among systems and manufacturers and can cause false warnings if performed incorrectly.

One thing TPMS does well is alert the driver of an actively leaking tire. If the TPMS illuminates, believe it: Slow down smoothly, find a safe place to stop and check your pressures. The bottom line: It's still up to you to check and set your tires pressures once a month. Don't rely on the TPMS, but don't ignore it either.

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