車子的輪胎氣壓監測系統TPMS，Tire Pressure Monitoring System
My Tire Pressure Warning Light Just Came On. What Do I Do Now?
The culprit could be falling temperatures
By Nick Kurczewski
November 09, 2017
Big changes between high and low temperatures that happen in the fall can trigger an alert on your dashboard—one that may leave some drivers wondering what to do next.
If you see a cutaway tire with an exclamation point, it means your tire pressure monitoring system has gone off. Falling temperatures can trigger the system because tire pressure declines with the thermometer.
“In round numbers, tire pressure changes +/-1 pound per square inch for every corresponding change of +/-10 degrees Fahrenheit,” says Woody Rogers, director of tire information at retailer Tire Rack. “As an example, if you set your pressure today at 60 degrees, and tomorrow morning it is 30 degrees, your tire pressure will be about 3 psi lower.”
But not all systems work the same way or provide the same amount of information. Here’s what you need to know if yours goes off.
What do I do when that warning light comes on?
Pull over as soon as you can and check your tire pressure (some systems give a generic warning, while others give you specifics on pressure). Driving on improperly inflated tires can affect everything from fuel mileage—underinflated tires are less efficient—to how your car rides and steers, particularly in an emergency situation.
And remember that the warning light can come on and then go off.
“Just because the light went out doesn’t mean your tires are properly inflated,” Rogers says.
Of course, regular tire pressure checks always are a good idea, regardless of what your tire pressure monitoring system says.
“You still need to check your tire pressure monthly,” says Gene Petersen, who runs tire testing at Consumer Reports.
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What is a tire pressure monitoring system?
The tire pressure monitoring system does exactly what you’d think—it keeps track of the air pressure in each of your car’s tires. The systems work independently in each wheel, and set off a warning light (the tire/exclamation point icon) on the instrument cluster if the pressure is too low.
The typical trigger happens when the pressure is 25 percent lower than the manufacturer’s recommended cold tire pressure, Petersen says, which can be measured when the tires have been sitting for at least three hours. You can find the recommended pressure level by checking your owner's manual or by reading the sticker on the driver's door jamb.
There are two main flavors of tire monitoring systems: direct systems and indirect systems.
“The vast majority of TPM systems in the U.S. are the direct type, which has a pressure sensor/transmitter mounted to the wheel,” says Rogers. “The sensor/transmitter is most often attached to the end of the valve stem on the end that is out of sight, inside the tire’s air chamber. A small number of direct systems have the sensor/transmitter separate from the valve stem.”
These are attached to the inside of the wheel with a metal strap.
Indirect systems get their information by comparing how the vehicle’s tires are turning in tandem, Rogers says.
“These indirect systems use the wheel speed sensors [part of the antilock brake system] to monitor the revolutions of all four tires as the vehicle moves,” he explains. “The central processor of the indirect system uses advanced algorithms to sense small changes in tire rotation speed created by different inflation pressures among the four wheel positions.”
The tire pressure monitor can alert when tire pressure is significantly below the correct level.
If I get new tires do I need new sensors?
Not necessarily. Tire pressure sensors aren’t permanently affixed to the old tires and they can be reused if they’re still working well.
But, says CR’s Petersen, “typically the battery will last about 10 years and then the sensor needs to be replaced.”
Are TPMS sensors expensive?
The cost depends in part on the work you’re having done, says Tire Rack’s Rogers. If you’re already getting new tires, the sensors will be within easy access and take only a short amount of time and labor to change.
It “becomes a lot more expensive to replace when you aren’t having a tire change done,” says Rogers.
The average price for a direct sensor, according to Rogers, can range from $40 to $70 per sensor, for most common vehicles.
Rogers says that some “niche vehicles” might have sensors that cost hundreds of dollars, but these “very specialized sensors” are extremely uncommon.
Even with tire pressure monitoring systems, it is important to manually check pressures monthly.
Cordless Tire Inflators Prove to Be an Easy Way to Get the Job Done
CR evaluates models from Ryobi, Craftsman, Stanley, and Ideaworks
By Gene Petersen and Chris Jones
September 02, 2017
Keeping your tires properly inflated is key to driving safely, but adding air can be a real chore if you don’t have a compressor near where you live or work. Cordless tire inflators, or cordless air compressors, provide a handy, portable solution. To see whether these products just blow hot air, CR recently bought four to evaluate.
Handheld cordless tire inflators compressors run on rechargeable batteries, as cordless drills do, making them easy to use without having to drag around a power cord and a long hose. They typically come with a connection to inflate car tires and a needle adapter to inflate basketballs, footballs, and the like. Most also include a special adapter for blowing up inflatables (like pool toys), and some have adapters for inflating high-pressure bicycle tires.
In our evaluation, we checked out how well they inflate automobile tires only. All four did a good job of topping off a tire that needs just a bit more pressure. These compressors can inflate to more than 100 psi at low volume. Inflating a completely flat car tire seems feasible, but based on our experience, continuous operation could overheat these devices. You should consider buying a large compressor with a tank reservoir if you often need to inflate completely flat tires.
Handheld cordless tire inflators compressors have a trigger-like switch to add air, and two of the evaluated units (the Craftsman C3 Digital Inflator and Stanley AIRit 120) have an automatic feature to inflate to a specific setting.
The Ryobi One+ ($78.97 with battery and charger) gets good marks for its relatively quick inflation time. It proved able to add 5 psi to our car tire in about 40 seconds. The Craftsman C3 ($110.98 with battery and charger) takes a bit longer, at 90 seconds. Both the Stanley AIRit 120 ($49.98) and Ideaworks ($43.03) took 2 minutes or longer to add 5 psi, but they are the cheapest units (with battery included).
The best deal is the Ryobi One+ at $19.97, but that's only if you already own other Ryobi cordless tools and already own a compatible battery and charger.
These gadgets are good to have around to keep your tires properly inflated and are also helpful in topping off higher-pressure temporary spare tires.
Below are highlights from our experiences with portable tire inflators used at the CR Auto Test Center, presented in order of the tire team's preference with the price paid and the retailer we bought it from.
The Ryobi One+ had the most muscle with the fastest inflation performance. It’s a bit pricey when you add in the cost of the battery and charger, but the battery can be used with dozens of other Ryobi cordless power tools. We liked the convenient air chuck, the piece that clamps onto the tire's valve stem. The dial gauge is legible and accurate to our test gauge, but it is not as easy to read as the digital gauges on other units.
Cost: $19.97 for the inflator. $59 for the 18-volt lithium battery and charger, sold separately.
Retailer: Home Depot
Shop Ryobi One+ Tire Inflator on Amazon
Craftsman C3 Digital Inflator
The Craftsman Digital Inflator has an easy-to-read digital pressure gauge that we found to be accurate when compared with our test gauge. The tool can be set to inflate to a specific pressure setting, although we found the actual tire pressure to be about 2 psi lower than the pressure we asked for. The handheld trigger makes it easy to inflate and fine-tune the pressure. The Craftsman compressor air chuck conveniently clamps to a tire’s valve stem. The pricey battery and charger can be used with several other Craftsman cordless tools.
Cost: $40.99 for the inflator. $69.99 for the 19.2-volt lithium battery and charger, sold separately.
Shop Craftsman C3 Digital Inflator on Amazon
Stanley AIRit 120
The Stanley handheld tire inflator works well. The battery pack isn’t removable as it is on the other units, but the unit comes with two chargers; that lets you recharge the battery from either a home outlet or from a car’s power outlet. We like the illuminated pressure gauge, and the light that can light up the tire's valve stem. The digital gauge showed pressures that were in line with what our test gauge found. The screw-on air chuck connection is a bit more tedious to use than the clamp-on type found on other units. Similar to the Craftsman unit, there is an automatic feature to set pressure and it shuts off just shy of that setting, but it's easy to control the amount of air you want using the handheld trigger switch.
Cost: $49.98 for the handheld compressor, battery, and charger.
Ideaworks Cordless Inflator
This handheld compressor can add air to a tire, but the tiny dial gauge is almost illegible and is inaccurate. If using this tool, have your own separate air gauge handy. The screw-on valve chuck is less convenient than the clamp-on type, and the short hose forces you to stay close to the tire.