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Old 12-27-2012, 08:32 PM   #7
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Per the owners guide:
Understanding your tire pressure monitoring system (TPMS)
The tire pressure monitoring system measures pressure in your four road
tires and sends the tire pressure readings to your vehicle. The low tire
pressure warning light will turn on if the tire pressure is significantly low.
Once the light is illuminated, your tires are under-inflated and need to be
inflated to the manufacturer’s recommended tire pressure. Even if the light
turns on and a short time later turns off, your tire pressure still needs to
be checked
Pg 246 of version 5.

The car uses direct pressure sensors mounted inside the tire (at the valve stem) not an indirect or imprecise measuring system.

Direct Monitoring Systems

Direct tire pressure monitoring systems measure, identify and warn the driver of low pressure. Because direct systems have a sensor in each wheel, they generate accurate warnings and can alert the driver instantly if the pressure in any one tire falls below a predetermined level due to rapid air loss caused by a puncture. In addition, direct tire pressure monitoring systems can detect gradual air loss over time. Some direct systems use dashboard displays that provide the ability to check current tire pressures from the driver's seat.

Direct systems attach a pressure sensor/transmitter to the vehicle's wheel inside the tire's air chamber. Most Original Equipment and some aftermarket systems attach their air pressure sensor/transmitter to special tire valves. While the presence of a metal clamp-in valve typically identifies the presence of a direct tire pressure monitoring system, special snap-in rubber valves have also been used to support direct system sensors. The transmitter's signal is broadcast to the in-car receiver and the information is displayed to the driver.

Tire Pressure Monitoring System (TPMS) - Overview

The BCM monitors the tire pressure in the 4 road tires with tire pressure sensors that communicate the tire pressure via radio signals to the BCM. The tire pressure sensors are battery operated and mounted to the valve stems. All of the controlling software for the TPMS is contained in the BCM. The IPC illuminates the TPMS warning indicator and the message center displays a message when a fault is present or when the tire pressure falls below the low pressure limit.
TPMS Sensor

Each of the 4 TPMS tire pressure sensors contain a battery, a tire pressure sensor and a radio transmitter. The TPMS sensor radio transmissions are sent approximately once every 60 seconds when the vehicle speed exceeds 32.2 km/h (20 mph).
This last quote from the factory service manual states that the sensors send off signals once every 60 seconds above 20 mph.
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