By Jim Kerr

Correct tire pressure is critical for safe vehicle operation. Unfortunately, it’s been estimated that about one out of every four vehicles on the road is running on under-inflated tires. Few drivers check tire pressure on a regular basis, and you can’t tell if a tire has correct pressure simply by looking at it. The U.S. Department of Transport has developed Federal Motor Vehicle Safety Standards for tire pressure monitoring systems, and because most of the vehicles sold in Canada share technology and features with those sold south of the border, Canadians are now seeing these tire pressure monitoring systems on vehicles here.

The U.S. regulations for monitoring tire pressures applies to light duty trucks and passenger vehicles with a gross vehicle weight rating of 4536 kg (10,000 lb.) or less. A warning light on the dash must indicate to the driver there is a low tire whenever a tire pressure drops to 25% below the rated cold tire inflation pressure recommendations for that vehicle. The system must provide this warning within 20 minutes of when a tire pressure drops below the specified pressure. Finally, the Tire Pressure Monitoring System (TPMS) must turn on a warning if the system or one of the tire pressure sensors has a malfunction.

According to the regulations, 20 percent or more of the vehicles produced between October 5, 2005, and September 1, 2006 must have been equipped with a TPMS. For vehicles manufactured between September 1 2006 and September 1, 2007, 70 percent of the vehicles must have a system, and all light duty vehicles built after September 1, 2007 that fall under the regulations must have a system.

Two types of monitoring systems are allowed. Indirect systems use the antilock brake system wheel speed sensors to monitor differences in tire rotational speeds. A tire with low pressure has a smaller rolling radius and will rotate faster. Indirect systems can’t monitor actual tire pressure. They only compare tire rotating speeds.

Direct systems are the most common system installed on new vehicles. These systems have a pressure sensor/transmitter attached to each of the vehicle’s wheels, where they sense actual tire pressure. This information is transmitted by radio frequency to a receiver inside the vehicle, which monitors the system and activates warnings. On many vehicles, it is even possible to read tire pressures on driver information displays so you don’t have to leave your vehicle to check tire pressures.

Tire pressure sensors on direct systems may be strapped to the wheel centre or they may be part of the valve stem assembly. If you see a large metal valve stem that is held in place by a nut, then you probably have a system on your vehicle, but some of the new systems are using a replaceable rubber valve stem that looks very much like a conventional valve stem. Many sensors are powered by a small battery that will last about 10 years on average. Some sensors are powered by an internal crystal that vibrates due to road shock.

All these direct systems work equally well but you will want to know if you have one on your vehicle before you have any tires changed or repaired so you can notify the repair technician. It is easy to damage a sensor when removing a tire, especially if the person is not aware it has a sensor on the wheel.

There are several manufacturers of original equipment and aftermarket tire pressure monitoring systems. Beru systems are used by Audi/Volkswagen, BMW, Land Rover, Mercedes-Benz and Porsche. Schrader make systems for Ford, Chrysler, GM and Nissan/Infiniti. Lexus has used a system by Pacific. Siemans is another provider of monitoring systems. Even though several auto manufacturers may use systems from the same supplier, there are few similarities between manufacturers. Valve stem attachments are usually different. The shape of the sensor differs so it will fit different wheels. The only thing all systems with valve stem sensors have in common is that they use a special nickel-plated valve core. Install a regular brass valve core in one of these sensors and it will corrode and seize in the sensor. Then you will have to replace the complete sensor the next time you want to add air.

Tire pressure monitoring systems seem like a good idea. However, they only warn when a tire is getting dangerously low. These warning systems do not replace regularly checking the tire pressures. There are still some problems with the systems too. Living in a cold climate, there is considerable change in tire pressure between a reasonably warm garage and a frigid winter morning. Many of the vehicles I have driven with the systems have warned me of low pressure because the drop in temperature lowers tire pressure. This happened every morning, even though the tire pressures were adjusted correctly. The systems may not be perfect, but they do direct one to check the tires before tire pressure becomes critically low.