The tyre pressure monitoring system (TPMS) warns drivers when tyre pressure is low through a yellow indicator light. There are two main types of TPMS: indirect and direct. Indirect TPMS uses wheel speed sensors to detect differences in rotation that may indicate low pressure, while direct TPMS has sensors inside each tyre that directly measure and wirelessly transmit pressure readings. Both aim to prevent accidents that may occur from underinflated tyres, though direct TPMS is more accurate while indirect TPMS is less expensive. Proper tyre maintenance is still important, as TPMS is not a replacement for manual pressure checks.

1. WHAT IS TPMS & HOW DOES IT WORK
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.
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.

2. 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.

3. 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.

4. 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

5. 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 centralized control module
where it’s analyzed, 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 specialized systems,
so replacing a TPMS in a way that’s consistent and compatible with your vehicle
will require an experienced, knowledgeable technician.

6. 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 resynchronization 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, resynchronization 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/demounting
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.
Source: http://www.bridgestonetyre.com/
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