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TPMS Sensor 101: What is It & How it Works?

Dec 2nd 2020

TPMS Sensor 101: What is It & How it Works?

Why are tire pressure monitoring systems so relevant in modern cars? Do they have a function or are they just an added expense to vehicle owners? Are the TPMS units located in the tires or on the wheel?

We will try to answer these and some additional questions in today's blog post.

What is a TPMS Sensor?

A tire pressure monitoring system (TPMS) is a system in your car that alerts the driver when the air pressure drops inside the tire. This is an electronic system that connects the sensor (placed on the rim) with the monitor (on the vehicle's dashboard) in order to show whether the tires are properly inflated or not.

When air pressure loss occurs in the tire, the TPMS will notify the driver with a flat tire warning light. This means the tire is not at its proper inflation for some reason - which may be due to tire or valve damage, but either way, the tire needs to be reinflated and repaired.

These sensors ensure the tires do not reach dangerous levels of deflation, which can further damage the tire, the rim, and even the TMPS unit if one is not careful. For this reason, they have become a mandatory part of vehicle manufacturing in the US since 2008.

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.

How Does a TPMS Work?

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.

The two types are:

  • Direct TPMS
  • Indirect TPMS

Direct TPMS 

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. When the air pressure in a tire drops below 25% from the recommended inflation level, the sensor notifies the computer system of the vehicle and the low-pressure warning light on the dashboard turns on.

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.

There are two types of direct TPMS systems:

  • High Line System
  • Low Line System

Direct TPMS - High Line System 

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.

Direct TPMS - Low Line System

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.

Indirect TPMS

Indirect TMPS systems work with the vehicle's Antilock Braking System. The ABS monitors the wheel speed and it helps out the indirect TPMS system as well. 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.

The Indirect TMPS system does not need additional components to be mounted on the wheels as it works with the ABS monitor. As a result, when the system needs to be replaced, additional labor costs and parts will not need to be paid for. 

Direct TPMS vs Indirect TPMS

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. However, when all four tires are deflated to a certain degree and function in that manner, the indirect TPMS will not notify the driver. This is still classified as low inflation pressure and it will cause an issue in the long run, but the indirect TPMS system will not notice it.

Under-inflated tires result in a shortened service life, a longer braking distance, a ruined fuel economy, and weaker traction. Therefore, direct TMPS sensors are more reliable for the average driver and they will result in fewer headaches during the vehicle's performance. 

Benefits of Tire Pressure Monitoring System

The benefits of a tire pressure monitoring system (TPMS) are the exact opposite of what under-inflated tires cause. When the tire pressure is monitored in the tires, it gives the driver control over the vehicle and ensures that it can perform in the correct manner. A tire pressure monitoring system will allow the driver to notice incorrectly inflated tires, and to prevent any possible problems when the low pressure light comes on.

Proper tire pressure inflations optimize the tire's surface contact throughout its performance. In this manner, they optimize the tire's traction, ensure a shortened braking distance, improve the fuel economy and ensure a slower and even wear along the tread area.

Therefore, when the TMPS system receives regular maintenance and performs in the proper manner, it will ensure the vehicle's optimal performance.

When to Replace the Tire Pressure Monitoring System

When the tire pressure monitoring system (TPMS) needs to be replaced depends on its type. The main reason for the TPMS' replacement is damage, corrosion, and run-down batteries.

The location of the tire pressure sensors on the wheels makes them face a higher possibility of damage during the vehicle's everyday performance. This damage can be caused by a flat tire, potholes, or accidents, which do not necessarily just harm the sensors but can create bigger issues as well.

Corrosion is usually the result of the road salt used in winter weather conditions. If the tires do not use snap-in, rubber valve stem TPMS sensors, the system faces a higher chance of corrosion and excessive rusting. Road salt can easily harm the rest of the vehicle as well. For more information, check out our blog post about the topic.

TMPS sensors, like most electronic devices, run on battery life. While they utilize a battery-saving operating method, their lifespan is still limited. Generally, sensors can run for 5-10 years, depending on what type you decide to get. If you notice that the sensors are nearing the end of their battery life, it will be more cost-efficient to change them when replacing the tires - it will save you on the labor cost!

Still, regular checkups and maintenance of the tire pressure monitoring system are recommended for safety. It will allow the tire pressure sensors and the low pressure light to function in the necessary manner, without having to result in emergency fixes when the system breaks down, due to any of the before mentioned reasons.

Unfortunately, the TMPS sensor of tires cannot be repaired. When a TMPS unit goes bad for any reason, either the entire system or the bad unit will need replacing. 

Frequently Asked Questions (FAQ):

Are TPMS sensors required by law?

Yes, the United Stated Congress legislated the TREAD Act, which requires all light motor vehicles to be equipped with a proper TPMS sensor. As of 2008, all new passenger vehicles must have a TPMS sensor installed. For trucks and vans, a TMPS sensor is not mandatory.

Can you drive with a broken TPMS sensor?

The vehicle can be driven when the TPMS sensor is broken, but it is recommended to have it fixed as soon as possible. If the TMPS sensor is not doing its job correctly, you will not know when the tires lose air pressure. This can lead to damage to the tire, the rim, and the vehicle if you are not careful.

How much does it cost to replace TPMS sensor?

To replace a TPMS sensor, will typically cost you between $220 and $240. The price to replace the parts needed for the TMPs is around $170 and you will need to add the labor cost, which is usually between $50 to $70 depending on where you take your vehicle.

What does it mean when TPMS light comes on?

When the tire pressure monitoring light turns on, it means the tire or tires have lost air pressure. In such instances, the tire pressure runs lower than what is recommended and it needs to be fixed to ensure the vehicle's performing security. 

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