With few exceptions, this was the only pressure-checking tool ordinary consumers had at their disposal. One of the outcomes of this legislation is that most vehicles sold in the United States since include a tire pressure monitoring system of some kind.
Not every TPMS works the same way. An indirect TPMS typically relies on wheel speed sensors that the anti-lock brake system uses. These sensors measure the rate of revolution each wheel is making and can be used by on-board computer systems to compare with each other and to other vehicle operation data such as speed. Based on the rate of revolution of each wheel, the computer can interpret the relative size of the tires on your vehicle.
When a wheel starts spinning faster than expected, the computer calculates that the tire is underinflated and alert the driver accordingly. Instead, an indirect tire pressure monitor simply measures how fast your tires are rotating and sends signals to the computer that will actuate the indicator light when something in the rotation seems amiss. Tire-pressure sensors are usually attached to the valve-stem assembly of each tire and are powered by batteries.
In the event that the sensor detects that air pressure in a tire has dropped to a dangerously low level—25 percent below the recommended air pressure, according to federal government regulations—a warning message or light will display in the instrument cluster. Inflation pressure recommendations for the tires that originally come on a car can usually be found on a label on the driver's doorsill.
TPMS sensors are powered by batteries designed to last several years, but they do eventually lose their charge. Because the sensors cannot easily be removed, when their battery dies the entire sensor must be replaced.
Replacement tire sensors vary in price depending on your vehicle and can be found from many sources, from Amazon to Advance Auto Parts to Tire Rack —or at your local tire store. You can continue to drive indefinitely with a failed TPMS sensor in one or more of the wheels, but then the system will never be able to warn you if you've picked up a puncture in those tires and are on the verge of a blowout.
There is a second type of tire-pressure monitoring system, which works entirely differently. It does not employ tire-pressure sensors in the tires, relying instead on the anti-lock-braking system's wheel-speed sensors to determine if any particular tire's rotational speed is out of sync with the others. This indicates that the suspect tire's circumference has changed and that it could have lost air pressure.
To ensure that your vehicle's TPMS system's pressure sensors remain operational for as long as possible, always reaffix the valve-stem cap after checking air pressure or inflating the tires. This helps prevent valve-stem corrosion, particularly where salt is used to clear the roads in the winter.
If the TPMS warning light does illuminate—the light looks like a flat tire as viewed from behind the car—check your instrument cluster display and then use a tire-pressure gauge to check all four tires to determine which ones, if any, are low.
Fill them to the manufacturer's recommendation as soon as possible, remembering that, if you've driven several miles to get to an air hose, your tires will have warmed up and you should inflate them evenly. For example, if when you get to the gas station one tire is at 20 psi and the other three are at 34, bring the low tire up to the other three, as long they are at or above the vehicle manufacturer's recommended pressure. However, if the TPMS warning comes on again after a short period of time—hours or days—you most likely have a leak in one of your tires.
Get to a tire store and have it repaired without delay. Driving on a badly underinflated tire can destroy it; worse yet, it can cause you to lose control of your car and have an accident.
Finally, for drivers who keep a set of winter tires mounted on a second set of wheels for use in the snowy months, we recommend having compatible TPMS sensors installed on the second set of tires, too. That way, you're covered no matter the season or which tires you have on at the time. After all, we wouldn't want you to feel, ahem, deflated by getting a flat. For this reason, they have become a mandatory part of vehicle manufacturing in the US since Tires that are not inflated correctly will have accelerated tread wear, shortening their service life.
The lower inflation widens their footprint, compromising their driving safety and causing more pressure build-up during their performance - which will ruin their fuel economy. Additionally, the wider footprint will ruin the tires' wet weather performance, while also increasing the braking distance. In other words, it will ruin the vehicle's performance and driving safety.
There are two different types of tire pressure monitoring systems available on the market. Generally speaking, their purpose is the same, but the way they operate is completely different. Direct TPMS systems are the most common. This type uses sensors mounted onto the rims, inside the tire, to measure the inflation of each tire separately. Direct TPMS systems consist of 5 main parts: the 4 sensors mounted onto the wheel of each tire usually near the valve stems and the indicator in the vehicle's computer system.
Therefore, when it is time for the vehicle's maintenance, replacing a direct TMPS system will be a bit more costly. High line TPMS systems are fitted with low-frequency transmitters near the wheels , which are used by the vehicle to force the transmission between the sensor and the computer system.
These types of sensors are not turned on and do not transmit constantly. Instead, the vehicle will regularly ask for information regarding the tire pressure levels from the sensors when the ignition is turned on and repeatedly during the drive. The high line system activates the transmitters one after the other, which will help later determine which sensor has sent the low tire pressure warning. The sensors can be located based on their unique ID, which will show the position of the sensor.
This system type offers the advantage of not draining the vehicle's battery. Low line TPMS systems use the units mounted on the wheels to transmit the tire's air pressure levels on fixed or random intervals. However, as the sensors of the tires are not connected, they may transmit simultaneously. This can lead to the collision of the pressure messages, and measures must be taken for the vehicle to be able to correctly receive them. To combat the collision of the messages, one TMPS system can send the same message multiple times.
Some low line systems make sure the TMPS units transmit more often and send more frequent transmissions when sudden change or high temperatures are detected. In this manner, the sensors make sure the vehicle receives the transmission. Most vehicles are equipped with this type of direct TMPS as it is less costly.
When the tire pressure drops in the tire, it will roll at a varied wheel speed, compared to the other tires. The computer system notices this and makes the low-pressure light come on. As a result, when the system needs to be replaced, additional labor costs and parts will not need to be paid for. The main difference between the direct and indirect tire pressure monitoring system types is the manner in which they measure whether the tires are correctly inflated or not.
But which TPMS type is the better one? For everyday vehicles' and customer needs, the direct TPMS system seems to work better. This is due to the nitpicky control of indirect TPMS systems, which need to be recalibrated each time the pressure in the tires is changed or when the tires are replaced.
As a result, such systems give too much control to the vehicle's owner, which is probably not a good idea with critical safety functions. Furthermore, the biggest issue with indirect TPMS systems is that it only turns on the low tire pressure light when one tire has different pressure levels from the rest.
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