What is TPMS?
The purpose of the tire pressure monitoring system (TPMS) in your vehicle is to warn you that at least one or more tires are significantly underinflated, potentially leading to unsafe driving conditions. The TPMS low tire pressure indicator is a yellow icon that illuminates on the dashboard instrument panel in the shape of a tire cross-section (resembling a horseshoe) with an exclamation point.
This indicator light in your vehicle has a history. It’s a story based on years of uncertainty about proper tire pressures and many serious car accidents that could have been avoided if drivers had known their air pressure was low.
Even now it is estimated that a significant number of vehicles hit the road with underinflated tires every day. However, proper tire maintenance with the help of a TPMS can and can help prevent many serious accidents.
Before this indicator light became commonplace, knowing if your air pressure was unsafe meant getting out, crouching, and using a tire gauge. With a few exceptions, this was the only pressure testing tool ordinary consumers had.
In response to an increase in accidents due to underinflated tires, the US government passed the Transportation Recall Improvement, Accountability and Documentation Act (TREAD). One of the results of this legislation is that most vehicles sold in the US since 2007 include a tire pressure monitoring system.
Not all TPMS work the same way. Illuminating the low tire pressure indicator is the final step in the process of an indirect TPMS or a direct TPMS.
What are the Types of TPMS And How Does it Work?
There are two different types of systems being used today: Direct TPMS and Indirect TPMS.
1. Direct TPMs
It uses an in-wheel sensor to measure the air pressure in each tire. When the air pressure drops 25% below the manufacturer’s recommended value, the sensor transmits this information to your car’s computer system and triggers the indicator light on your dashboard.
Direct TPMS uses pressure monitoring sensors within each tire that monitor specific pressure levels – not just wheel revolution data from the anti-lock brake system.
Sensors in a direct TPMS may even provide tire temperature readings. The direct tire pressure monitoring system sends all of this data to a centralized control module where it’s analyzed, interpreted, and, if tire pressure is lower than it should be, transmitted directly to your dashboard where the indicator light illuminates.
A direct tire 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 tire.
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.
Advantages Of Direct TPMS
- Deliver actual tire pressure readings from inside the tire
- Not prone to inaccuracies because of tire rotations or tire replacements
- Simple resynchronization after tire rotation or tire replacements
- Batteries inside the sensors usually last for about a decade.
- May be included in a vehicle’s spare tire
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
2. Indirect TPMS
It works with your car’s anti-lock braking system (ABS) wheel speed sensors. When the tire pressure is low, it will roll at a different wheel speed than the other tires. This information is recognized by your car’s computer system, which triggers the dashboard indicator light.
An indirect TPMS is usually based on wheel speed sensors that the anti-lock braking system uses. These sensors measure the rate of rotation each wheel is making and can be used by on-board computer systems to compare them with each other and with other vehicle operating data such as speed.
Based on the rate of rotation of each wheel, the computer can interpret the relative size of your vehicle’s tires. If a wheel spins faster than expected, the computer calculates that the tire is underinflated and alerts the driver accordingly.
An indirect tire pressure monitoring system does not measure the tire pressure. It doesn’t electronically process the same type of measurement you might see with a tire gauge. Instead, an indirect tire pressure monitor simply measures how fast your tires are spinning and sends signals to the computer that actuate the indicator light if something is wrong with the rotation.
Advantages Of Indirect TPMS
The TPMS eliminates guesswork by notifying you if a vehicle is low on air or flat. This knowledge can not only help prevent accidents but can also help you improve your gas mileage. Your TPMS can also warn you about existing or impending problems with your vehicle.
- 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 tire
- May be unreliable when tires are unevenly worn
- Must be reset after properly inflating every tire
- Must be reset after routine tire rotation