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Another Idiot Light

March 27, 2010
By: Dan Anderson, Farm Journal Columnist

There's another yellow warning light waiting to wink at you from dashboards of 2008 and newer cars, light trucks and SUVs. A federally mandated tire pressure monitoring system (TPMS) warns drivers if any tire's air pressure drops below 75% of the level recommended by the car's manufacturer.

There are two types of TPMS: direct and indirect. Direct TPMS, adopted by the majority of car manufacturers, uses a small battery-powered air pressure sensor/radio transmitter installed

inside each tire. The sensor monitors air pressure in the tire and transmits the information to a central computer. If air pressure in any tire falls below a prescribed limit, a warning light

appears on the control panel.

Indirect TPMS is an alternate method of monitoring tire air pressure that is being used during the transition to mandatory TPMS and by at least one manufacturer to meet the 2008 standard. Indirect TPMS is part of the vehicle's anti-lock brake system and uses wheel-speed sensors at each wheel. When air pressure in a tire decreases, the tire's rolling radius changes and the wheel turns faster. When wheel speed increases beyond calibrated values, a light warns the driver of the low tire.

While TPMS should increase driving safety and improve fuel economy by keeping tires at optimum air pressure, it will also add cost and complicate tire changes, repairs and rotations.

"If you run a tire low and it trips the monitor system, or you rotate tires as part of regular maintenance, you may have to recalibrate the TPMS to get the warning light to stay off,” says Kevin Rohlwing, senior vice president of training for the Tire Industry Association. "That can be a hassle. General Motors alone has 22 different ‘relearn' procedures for its TPMS vehicles.”

Turn off the light. Relearn procedures allow TPMS computers to recalibrate radio signals from the sensor in the reinflated tire. Some relearn procedures are as simple as pushing a button on the dashboard; others may involve driving the vehicle in a prescribed manner, inflating and deflating tires, or other arcane procedures.

Any change to a tire or wheel may require a relearn procedure. If custom tires and wheels are installed, new TPMS sensors will boost the final bill.

"Some of the sensors cost $100 each,” says Larry Guisinger, service supervisor at Karl Chevrolet in Ankeny, Iowa. "Cost depends on the manufacturer of the wheels and the type of TPMS in the vehicle. You can add $400 to the cost of custom wheels and tires, just getting the tire monitoring system up to spec.”

Farmers who repair flat tires on vehicles with TPMS need to use care. Most TPMS sensors are part of special valve stems and easily damaged by abuse.

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FEATURED IN: Farm Journal - Early Spring 2010

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