Tire (Tyre) Pressure Monitoring System (TPMS) - Simple Explanation

Posted by Kannan G on
Tire pressure monitoring system TPMS

The purpose of the tire pressure monitoring system (TPMS) in the vehicle is to notify us the unsafe change in the air pressure in one or more of the vehicle tires that may create unsafe driving conditions. The deviations in the tire pressure or low tire pressure can lead to sudden tire damage which causes the driver to lose control of the vehicle. TPMS continuously monitors the air pressure of the tires of automobiles when in moving. So it provides early warning that allows the driver to rectify the problems before they become severe. Tire pressure monitoring system helps us to keep an eye on the tire pressure and warn us in case of potential tire damage.

Why do we need TPMS?

  • Driving a vehicle with under-inflated tires reduce the lifetime of the tires.
  • Under-inflated tires increase the rolling resistance of wheels which causes a decrease in the fuel economy of the vehicle.
  • Under-inflated tires sometimes increase the braking distance and hence have a potential safety concern. Also, it affects the stability of the vehicle.

Types of TPMS:

  • Indirect TPMS
  • Direct TPMS

Indirect TPMS

In indirect TPMS, the under-inflated tires are identified by calculating the rate of the wheel revolution. An indirect TPMS uses the wheel speed sensors of the vehicle's antilock braking system (ABS) to measure the rate of revolutions on each wheel that can be used by on-board computer systems to compare with each other. If one tire is low on pressure, its change in circumference is enough to roll at a slightly different number of revolutions per mile than the other three tires.

By using the rate of revolution of each wheel, the computer can interpret the relative size of the tires on our vehicle. When a wheel starts spinning faster than expected, the computer calculates and identifies that the tire is under-inflated and notify the driver accordingly.

So, an indirect TPMS does not actually measure tire pressure. It is not electronically processing the same kind of measurement that we see usually with a tire gauge. Instead, an indirect TPMS simply measures how fast our vehicle tires are rotating and sends signals to the control module that will turn on the indicator light accordingly when the rotation of any of the wheels seem irregular.

Advantages of Indirect TPMS:

  • It requires less programming and maintenance over the years than a direct TPMS.
  • Relatively inexpensive compared to a direct TPMS.

Disadvantages of Indirect TPMS:

  • May become inaccurate if you purchase a bigger or smaller tire.
  • May be unreliable when tires are unevenly worn.
  • Indirect systems won't tell the drivers which tire is low on pressure, and will not warn the driver if all four tires are losing pressure at the same rate.
  • They create false warnings sometimes. The false warnings happen when the vehicle is driven around a long curve that causes the outside tires to rotate faster than the inside tires, and when the tires spin on ice or snow-covered roads. In those cases, the false alarms would train the driver to disregard the tire pressure monitoring system's warnings.

Direct TPMS

Different strategies have been developed to implement the direct TPMS. Direct TPMS uses pressure monitoring sensors and a transmitter connected within the valve of each tire that monitors specific pressure levels not just wheel revolution data from the anti-lock brake system. Sensors in a direct TPMS provide the temperature of the tire also.

The sensors and transmitters send accurate pressure, temperature values, etc., to the antennas/receivers present in the vehicle-network in form of telegram or data packets via HF radio signals. A direct tire pressure monitor usually sends all of this data wirelessly. There can be separate antennas for this purpose or antennas of other existing systems, like the Remote Keyless Entry (RKE) system, may be utilized. The direct TPMS sends those data to a centralized control module where it is analyzed, interpreted. If tire pressure is lower than the predetermined value, the data transmitted directly to our vehicle's dashboard where the indicator light flashes instantly.

Advantages of Direct TPMS:

  • It provides actual tire pressure readings from inside the tire.
  • Simply resynchronized after tire rotation or tire replacements.
  • Not prone to inaccuracies because of tire rotations or tire replacements.
  • Batteries inside the sensors usually last for about a decade.

Disadvantages of Direct TPMS:

  • Expensive than an indirect TPMS.
  • Battery rarely serviceable, if the battery is drained, the whole sensor must be changed.
  • Some proprietary systems make installation, service, and replacement confusing for consumers and technicians in auto shops.

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