The tire pressure monitoring system (TPMS) in your vehicle is to warn you that at least one or more of your tires are under-inflated. Under-inflated tires can cause unsafe driving conditions. The TPMS low tire pressure indicator is a yellow symbol in the shape of a tire cross-section with an exclamation point.
Before this indicator light, knowing if your air pressure had reached unsafe levels meant getting out,and using a tire gauge on each individual tire. With some exceptions, this was the only pressure-checking tool known to the public. In response to a surge in accidents due to underinflated tires, the US government passed a legislation that most vehicles sold in the United States since 2007 include a tire pressure monitoring system of some kind.
Not every TPMS works the same way. The illumination of the low tire 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 of each wheel and can be used by on-board computer systems to compare with each other and other vehicle operation data such as speed.
Based on the rate of revolution of each wheel, the computer interprets the size of the tires on your vehicle. If a single wheel starts spinning faster than expected based on the other tires, the computer calculates that the tire is underinflated and alerts the driver accordingly.
So, an indirect tire pressure monitoring system doesn’t truly measure tire pressure. It is not electronically processing the same kind of measurement you see with a tire gauge. Instead, an indirect tire pressure monitor simply measures how fast your tires are rotating and sends signals to the computer that will initiate the indicator light when something in the rotation seems irregular.
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 tire
- May be unreliable when tires are unevenly worn
- Must be reset after properly inflating every tire
- Must be reset after routine tire rotation
DIRECT TPMS: WHAT IS DIRECT TPMS & HOW DOES IT WORK?
Direct TPMS uses pressure monitoring sensors within each tire that monitor specific pressure levelsi0.
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, and interpreted. If tire pressure is lower than it should be, this data transmits 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.
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
