The push for tire pressure monitoring systems, required on all new cars and light trucks since 2008, can be traced to the Ford Firestone tire controversy in 2000. That's when the failures of the Explorer's Firestone tires first came to the public's attention, prompted by reports of tire tread separations leading to rollovers that caused injuries and deaths.
Low tire pressure was deemed to be a major culprit. This can be exceptionally dangerous if the vehicle is driven at high speeds, with heavy loads or on hot summertime pavement. Each of these conditions can produce tire temperatures high enough to prompt catastrophic failures. Hearings highlighting the problem prompted Congress to require tire pressure monitors in all vehicles with a gross vehicle weight rating of 10,000 pounds or less, except those with dual wheels on an axle.
Obviously, if you have one of the cars with a tire-by-tire readout, your job of maintaining proper tire pressures is easy. If you have a car with a single tire cross section warning light, however, you'll end up checking all four tires to see which ones have strayed from the manufacturer's recommended tire pressures.
Now, some more bad news: You still have to check tires manually in a car with a single low pressure warning light. Since the triggering point is a loss of about 25 percent of the mandated air pressure, it's possible for tire inflation levels to drop into truly dangerous territory without triggering a warning in some cases. The only way to prevent this is to check the pressures manually at least once a month.
Here are some other problems:
* Tire rotation will often require recalibrating the warning system so that it knows which sensor is in which position. Otherwise, the warning will send you to the wrong wheel in search of a low tire pressure.
* Mounting new tires on the rim can also result in problems. Some tire pressure monitors are attached to the valve stem; others are mounted on the center of the rim with a band. Either sensor can be destroyed by improper tire mounting procedures.
* False alarms are possible, too. Technicians have told me that a car with low tire pressures using the same transmitter system as your car might trigger your low pressure warning light while both of you are stopped in traffic next to each other. If your warning light illuminates, then goes out when traffic begins to move, that could be the problem.
* People who change tire sizes, especially those folks who use large diameter, low-profile tires in a car that was not intended to be outfitted with these wheels, can also get erroneous warnings or no warning at all when tire pressures drop.
Finally, people with older cars that use indirect monitors may never be warned of low tire pressures. These systems use the anti-lock brake hardware to keep track of tire rotation. As a tire loses air, it rotates faster than the other tires that have maintained their proper air pressure. Unfortunately, if all four tires lose air pressure in tandem there will be no rotational abnormalities to detect and no warning, even if all four tires are nearly flat.
Jim MacPherson is the host of "The Car Doctor" show airing Sundays at noon on WTIC-AM. Paula MacPherson is his wife and new-car review partner. Send comments, questions, suggestions in care of Special Publications, Hartford Courant, 285 Broad St., Hartford, CT 06115, or email firstname.lastname@example.org.
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