How Does a Tire Pressure Monitoring System Work?

A red car.

A tire pressure monitoring system is a valuable safety feature in most automobile models since 2000. It can detect changes in tire pressure either directly or indirectly, alerting you to potential problems reliably. Here is what you need to know about how tire pressure monitoring systems work.

Tire Pressure Monitoring Systems

The tire pressure monitoring system on your car is a network combining sensors in your car's wheels with software in the car's computer systems. A direct pressure sensor system sends continuous signals from the wheels while the car is in motion to the monitors to detect changes in tire pressure. When the car is stopped with the engine off, direct pressure sensors send hourly updates to the tire pressure monitoring system. An indirect pressure system works only when the car is in motion by comparing the diameter of the tires at average speeds to the diameter as pressure drops.

Direct Pressure Monitoring

In a direct pressure monitoring system, battery-operated transponders in each wheel send signals to the monitoring software. These sensors are highly accurate and can detect pressure changes of 2 pounds per square inch. They can be mounted either on a steel band in the wheel center or attached to the valve stem.

Disadvantages of Direct Pressure Monitoring Systems

When the wheels are rotated, the direct pressure system computer must be reset. They are more expensive than an indirect pressure monitoring system and must be replaced when the batteries are drained. When installing new tires on rims equipped with direct pressure sensors, great care must be taken not to dislodge the sensors from inside the wheel or near the valve stem. If you replace a wheel on your car following an accident, you will need to install a new direct pressure sensor of the same type as was used before.

Indirect Pressure Monitoring

The ABS (antilock braking) wheel speed sensors provide the data for the indirect pressure monitoring system. The monitoring system learns the average speed that the wheels travel at, and the diameter of the tire at those speeds. It then compares the tire diameter at lower inflation pressure to the average, and signals the monitoring system that a loss of tire pressure has occurred. These systems are less expensive than direct pressure monitoring and have no batteries to wear out. Mounting and removing tires from the wheels has no negative effect on an indirect pressure monitoring system.

Disadvantages of Indirect Pressure Monitoring

While these systems have fewer parts, they are less accurate. A tire can lose 10 psi of pressure on a hot day before its diameter will shrink by 1 mm, the smallest change detectable by the indirect monitoring systems.

Other Aspects of Tire Pressure Monitoring Systems

Be sure you reset the onboard computer for any pressure monitoring system after rotating the wheels on your vehicle. A computer system error due to loose wires or other causes will invalidate the readings of the pressure monitoring system or may shut it down completely. Check your tire pressure manually once a month with a good-quality pressure gauge even if your car has a tire pressure monitoring system.