TPMS warning light in a Suzuki XL-7
TPMS warning light in a Suzuki XL-7. Click image to enlarge

Story and photos by Jil McIntosh

Oshawa, Ontario – Even though tire pressure monitoring systems have been around for years and have slowly been phased in, the big day came on September 1, 2007. As of that date, the U.S. government mandated that every new passenger vehicle with a GVWR of 10,000 lbs (4,535 kg) or less had to roll off the assembly line with a factory-equipped tire pressure monitoring system (TPMS) if it was to be sold in the United States.

The mandate is the result of the Transportation Recall Enhancement, Accountability and Documentation (TREAD) Act. That act, enacted on November 1, 2000, was a direct result of hearings regarding Ford Explorer rollovers with Firestone tires. Arguably, those rollovers were simply proof that you can’t overload and under-inflate a tire, and then drive like you’re in the Indy 500, but either way, TPMS is here to stay.

TPMS isn’t mandatory in Canada, and so far, there are no plans to make it so. Transport Canada says it is monitoring the effectiveness of TPMS and will not rule out the possibility of mandatory requirement, but also says that it will make no decisions before looking at several issues, including false or no warnings, sensor battery life, long-term system durability, and the extremely low numbers of tire failures cited in fatal collisions.

But automakers are building their vehicles with the systems anyway, and so we can expect to see them on a number of cars sold here, either as standard or optional equipment. While most drivers will never give them another thought unless the warning light comes on, it’s important to understand them: if you don’t, they could end up costing you money.

According to the mandate, TPMS must warn the driver when a tire loses air beyond a pre-set threshold, which for most systems is 25 percent. That number is a source of controversy, with tire industry experts saying it’s too low, and vehicle manufacturers saying they wanted to avoid “nuisance warnings” that a higher threshold may have caused. A group consisting of the Tire Industry Association, the consumer group Public Citizen, and four tire manufacturers brought a lawsuit against the U.S. Department of Transportation to challenge and raise the threshold. The tire groups wanted a threshold set individually to each vehicle’s tire size, dimensions and load-carrying capacity, which would deploy when the tire reached a pressure that could no longer safely sustain the load, but they lost in court.

With the current threshold, on larger tires such as on light trucks, a tire could be low on air by as much as 15 pounds before the TPMS flashes a warning. That’s why, even though your vehicle may have a system, it’s important to check your tires regularly, using a tire pressure gauge.

There are two types of TPMS: indirect and direct. Indirect systems use the vehicle’s wheel sensors to compare the turning circumference of each tire to the other three. Because it’s not as accurate, the threshold on indirect systems is 30 per cent. The system can also trigger false readings if one wheel spins on a slippery surface, and because it compares the tires, it won’t issue a warning if all four lose air equally, such as can happen with temperature changes. For these reasons, it’s mostly fallen out of use.

Direct systems use sensors fitted inside the tire’s air chamber, where they read the pressure directly; these must warn at 25 per cent. Most are attached to the tire’s valve stem, although some systems use “banded” sensors, which are strapped to the inside of the wheel. Depending on the vehicle, there will be a general warning light, an indication of which tire is low, or a computerized readout of the exact pressure in each tire.

Where TPMS will become a headache for many consumers is tire replacement. Because of the sensor’s position in the tire, it’s very easy for an inexperienced technician to break or otherwise damage a sensor. You’ll also have to make sure that your valve stems (and sometimes the caps) are the correct ones for a TPMS system. The sensors use a nickel-plated valve stem, and if the technician inserts the wrong core, the metals will eventually fuse, rendering the sensor useless.

There’s also the issue of “relearning” the system when tires are replaced or rotated, which gets the system recognizing each wheel and reading its sensor accurately. Depending on the vehicle, relearning may be as simple as driving for a few kilometres, to a more complicated system of adding or removing air during a test drive, to requiring a specialized (and often expensive) computerized sensor readout tool.

If you switch to winter or custom wheels, you’ll have one of three choices: move your sensors to the new wheels, if they’ll fit; buy new sensors to fit the wheels; or drive without sensors and get used to the warning light on your dash.

And while the sensors themselves are sealed units, their fittings include lock nuts and seals that should be replaced on a regular basis, such as with every tire change, to ensure that the unit stays watertight.

While Transport Canada has stated that there are no legal requirements for TPMS to be functional in your vehicle, the tire industry is still debating the question of how judges might look at civil lawsuits, should a driver try to blame tire failure on a TPMS unit that wasn’t properly installed or repaired. There’s no question that it’s going to be a grey area for the industry in the near future.

As for your vehicle, the best advice is “protect yourself”. Check your tire pressure regularly, even if your system isn’t flashing a warning. Have your vehicle serviced by a trusted shop, by technicians who have been properly trained in working with TPMS. If valve stems or caps are replaced, make sure the shop has used the correct parts. Don’t use tire sealants or balancing beads unless you’re sure the product is guaranteed to be compatible with TPMS. And if that little light comes on in your dash, don’t ignore it. Every one of your car’s safety systems is only there to mop up after the fact if your tires lose their grip on the road, and the best thing you can do in the name of safety is to ensure they’re doing their job.

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