Q. I’ve read that quite a few cars are now coming with tire-pressure monitoring systems. Can you shed some light on how they work? Is it possible to add this feature to my existing car or truck? — Peter Scalla, Santa Cruz, Calif.
A. A growing number of high-end and performance-oriented vehicles utilize tire-pressure monitoring systems to provide an added element of safety and enhance vehicle performance.
It’s estimated that one out of four vehicles on the road has one or more under-inflated tires, which sacrifices fuel economy and can lead to vehicle instability and tire damage. Proper tire pressure is particularly important for heavily loaded vehicles and those with low-profile tires, as excessive flexing can gradually tear a tire’s internal structure apart.
The federal government is taking this issue seriously and has mandated monitoring systems be phased in on all vehicles weighing less than 10,000 pounds beginning this year. Monitoring systems fall into two categories — those that actually measure the pressure within each tire (direct systems), and those that calculate pressure loss through the anti-lock brakes’ wheel-speed sensors (indirect systems).
Manufacturer-installed direct systems incorporate a sensor/transmitter within the tire/wheel assembly, attached to the back of a metal valve stem (often utilized as an antenna). After-market pressure-monitoring systems usually attach the sensor/transmitter to the drop-center of the wheel using a large band clamp and retain the traditional rubber valve stem. A long-life lithium battery supplies power for the sensor/transmitter, which sends radio signals to wheel-well receivers or directly to the instrument panel as the vehicle is driven.
Indirect systems require no new hardware, as they use the vehicle’s existing ABS (anti-lock brake system) wheel speed sensors to infer tire pressure. A tire low on air has a smaller radius, resulting in a faster-than-its-peers wheel speed signal.
This inexpensive method meets the government’s 25-percent-low warning standard, but falls short on accuracy and can’t identify four simultaneously low tires.
After-market direct monitoring systems are available for about $200 and up, and are quick and simple to install after your tires have been removed from the rims. Accuracy is claimed to be within 1.5 to 2 pounds, and instrument panel display options range from simple LED warning lamps to individual tire pressure and temperature values. (Check www.tirerack.com for SmarTire product information.)
Inexpensive pressure-sensing valve stem caps (such as Accu-Pressure Safety Caps, about $15 at www.gs-7.com) also can be used to guard against under-inflation. Their color-coded indicator changes from green to yellow as the pressure drops four 4 pounds per square inch below specs and becomes red as the pressure falls 10 psi.
Regardless of whether one has a tire-pressure warning system or not, it’s a good idea to check (cold) tire pressure monthly, using an accurate gauge. Be sure to use the specification found on the driver’s door or pillar sticker, rather than the maximum pressure listed on the tire. It’s also important to make your tire-service installer aware of an installed direct system, as the sensors can be damaged during tire removal/replacement, if extra care isn’t exercised.
Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif. E-mail him at under-the-hood@juno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. He cannot make personal replies.
