A Tire Pressure Monitoring System, or TPMS, helps protect a vehicle by warning the driver when one or more tires are underinflated. Correct tire pressure matters because it affects braking, steering, fuel economy, tire wear, and heat buildup. A low tire can flex too much as it rolls, which can damage the tire and make the vehicle harder to control.
The dashboard warning light gives the driver an early signal to check the tires before the problem becomes dangerous.
Key Facts
- Direct TPMS uses a pressure sensor inside each wheel to measure air pressure.
- Indirect TPMS estimates low pressure by comparing wheel speeds using the ABS sensors.
- Gauge pressure is the pressure above atmospheric pressure, which is what tire gauges usually display.
- Many TPMS warnings turn on when tire pressure is about 25% below the recommended cold inflation pressure.
- Pressure changes with temperature: lower temperature usually lowers tire pressure.
- Percent pressure drop = (recommended pressure - measured pressure) / recommended pressure x 100%
Vocabulary
- TPMS
- A Tire Pressure Monitoring System is an electronic system that warns the driver when tire pressure is too low.
- Direct TPMS sensor
- A direct TPMS sensor is a small device mounted in or near the valve stem that measures tire pressure and sends data by radio signal.
- Indirect TPMS
- Indirect TPMS is a system that uses wheel speed data to infer that a tire may be low on pressure.
- Cold inflation pressure
- Cold inflation pressure is the recommended tire pressure measured before the tire has warmed up from driving.
- Dashboard warning light
- The dashboard warning light is the symbol that alerts the driver when the TPMS detects a possible tire pressure problem.
Common Mistakes to Avoid
- Checking tire pressure after a long drive, because warm tires can show a higher pressure than the true cold inflation pressure.
- Ignoring the TPMS light, because it may mean a tire is losing air or is already low enough to affect safety.
- Assuming all TPMS systems measure pressure directly, because some vehicles use indirect TPMS based on wheel speed changes.
- Inflating every tire to the number printed on the tire sidewall, because that number is usually the maximum pressure, not the vehicle manufacturer's recommended pressure.
Practice Questions
- 1 A vehicle's recommended cold tire pressure is 36 psi. The TPMS warning threshold is 25% below this value. At what pressure should the warning turn on?
- 2 A tire is measured at 27 psi, and the recommended pressure is 36 psi. Use percent pressure drop = (recommended pressure - measured pressure) / recommended pressure x 100%. What is the percent pressure drop?
- 3 Explain why a direct TPMS sensor can warn the driver about low pressure even when the vehicle is parked, while an indirect TPMS system usually needs wheel motion to detect a problem.