By John Cook
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 tires on your vehicle. When a wheel starts spinning faster than expected, the computer calculates that the tire is underinflated and alert the driver accordingly.
So, an indirect tire pressure monitoring system doesn’t measure tire pressure. It’s not electronically processing the same kind of measurement you might 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 actuate the indicator light when something in the rotation seems amiss.
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
- Cannot measure or display absolute pressure values
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 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 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.
- 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.
For more information on TPMS systems and parts visit us at www.johndow.com.