Tyre Pressure Monitoring System (TPMS)

The purpose of the tyre pressure monitoring system (TPMS) in your vehicle is to warn you that at least one or more tyres are significantly under-inflated, possibly creating unsafe driving conditions. The TPMS low tyre pressure indicator is a yellow symbol that illuminates on the dashboard instrument panel in the shape of a tyre cross-section (that resembles a horseshoe) with an exclamation point.

TPMS Low Tyre Pressure Indicator

That indicator light in your vehicle has a history. It’s a history rooted in years of uncertainty about proper tyre pressure and many serious car accidents that might have been avoided had drivers known their air pressure was low. Even now, it’s estimated that a substantial number of vehicles hit the road each day with underinflated tyres. However, proper tyre maintenance with the aid of a TPMS can and does help prevent many serious accidents.

Before this indicator light became commonplace, knowing whether your air pressure had reached unsafe levels meant getting out, crouching down, and using a tyre gauge. With few exceptions, this was the only pressure-checking tool ordinary consumers had at their disposal.

Then, in response to a surge in accidents due to underinflated tyres, the US government passed the Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act. One of the outcomes of this legislation is that most vehicles sold in the United States since 2007 include a tyre pressure monitoring system of some kind.

Not every TPMS works the same way. The illumination of the low tyre pressure indicator represents the final step in the process of either an indirect TPMS or a direct TPMS.

Indirect TPMS: What is Indirect TPMS & How Does it Work?

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 tyres on your vehicle. When a wheel starts spinning faster than expected, the computer calculates that the tyre is underinflated and alert the driver accordingly.

So, an indirect tyre pressure monitoring system doesn’t actually measure tyre pressure. It’s not electronically processing the same kind of measurement you might see with a tyre gauge. Instead, an indirect tyre pressure monitor simply measures how fast your tyres are rotating and sends signals to the computer that will actuate the indicator light when something in the rotation seems amiss.

Advantages of Indirect TPMS

  • Relatively inexpensive compared to a direct TPMS
  • Requires less programming/maintenance over the years than a direct TPMS
  • Less overall installation maintenance than its direct counterpart

Disadvantages of Indirect TPMS

  • May become inaccurate if you purchase a bigger or smaller tyre
  • May be unreliable when tyres are unevenly worn
  • Must be reset after properly inflating every tyre
  • Must be reset after routine tyre rotation

Direct TPMS: What is Direct TPMS & How Does it Work?

Direct TPMS uses pressure monitoring sensors within each tyre that monitor specific pressure levels – not just wheel revolution data from the anti-lock brake system.

Sensors in a direct TPMS may even provide tyre temperature readings. The direct tyre pressure monitoring system sends all of this data to a centralised control module where it’s analysed, interpreted, and, if tyre pressure is lower than it should be, transmitted directly to your dashboard where the indicator light illuminates. A direct tyre pressure monitor usually sends all of this data wirelessly. Each sensor has a unique serial number. This is how the system not only distinguishes between itself and systems on other vehicles, but also among pressure readings for each individual tyre.

Many manufacturers use proprietary technology for these highly specialised systems, so replacing a TPMS in a way that’s consistent and compatible with your vehicle will require an experienced, knowledgeable technician.

Advantages of Direct TPMS

  • Deliver actual tyre pressure readings from inside the tyre
  • Not prone to inaccuracies because of tyre rotations or tyre replacements
  • Simple re-synchronisation after tyre rotation or tyre replacements
  • Batteries inside the sensors usually last for about a decade.
  • May be included in a vehicle’s spare tyre

Disadvantages of Direct TPMS

  • More expensive overall than an indirect TPMS
  • Though simple, re-synchronisation may require costly tools.
  • Battery rarely serviceable; if the battery is drained, the whole sensor must be changed.
  • Proprietary systems make installation, service, and replacement confusing for consumers and auto shops.
  • Sensors are susceptible to damage during mounting/removing

Tyre Pressure and Safety

Although the methods may be different, both systems serve the same purpose and activate the same indicator light. Even though a TPMS can deliver accurate alerts when properly maintained, it’s not a replacement for manual air pressure checks, consider it just another item in your car maintenance toolbox.