Tire Pressure Sensor: Description and Operation

TIRE PRESSURE SENSOR

DESCRIPTION

CAUTION: Be sure that an original equipment valve stem cap or a valve stem cap of the same sealing capability is always used if the valve stem cap requires replacement.

A tire pressure sensor/transmitter is installed in each wheel of this vehicle as part of the tire pressure monitoring system. A special valve stem incorporated into each tire pressure sensor/transmitter attaches it to the wheel.

Valve Stem Core and Cap
On this vehicle, the valve stem is part of the tire pressure sensor/transmitter and is used as its antenna. If corrosion were to develop in the valve stem, it would affect the ability of the tire pressure sensor/transmitter to send the radio frequency signal to the TPM module.

The valve stem used on this vehicle is made from aluminum. Corrosion of aluminum will be caused by putting a dissimilar metal in contact with it. To prevent this type of corrosion from developing within the valve stem, the core of the valve stem is made of a compatible metal and is specially coated.

For this reason, a standard valve stem core must never be installed in this valve stem. If the core of the valve stem is removed when servicing the tire, the original valve stem core must be reinstalled. If the valve stem core needs to be replaced, it must be replaced with an original equipment valve stem core.

The valve stem caps used on this vehicle are specially designed for their use on the valve stem of the tire pressure sensor/transmitter. Each valve stem cap has an O-ring in the upper end of the valve stem cap to provide a positive seal at the end of the valve stem. This keeps dirt contamination and corrosion out of the valve stem caused by moisture. As listed earlier, these conditions can affect the ability of the tire pressure transmitter to send the radio frequency signal to the TPM module.

OPERATION
Each tire pressure sensor/transmitter serves three functions:
- It is used as a valve stem to inflate and deflate the tire.
- It periodically measures tire pressure. (Approximately every 30 seconds.)
- It transmits the tire pressure value to the low tire pressure warning module in the vehicle. (Approximately every 60 seconds or if tire pressure changes by one psi or more between transmissions.)

The system operates by monitoring a radio frequency transmission from the sensor/transmitters located in each wheel. When the vehicle reaches a speed of approximately 32 km/h (20 mph), centrifugal force created by the rotating wheels closes a roll switch inside each sensor/transmitter, powering up the circuitry.

The sensor/transmitters will only send out information when vehicle speed is above 32 km/h (20 mph). To facilitate transmitting the radio signal to the TPM module, the valve stem acts as the antenna for the tire pressure sensor/transmitter. When the TPM module senses that pressure is below one of two calibrated thresholds, (low pressure or critical pressure) the TPM system will set a diagnostic trouble code and indirectly operate the "Low Tire" pressure indicator lamp and the chime tone through the Body Control Module (BCM).

The tire pressure sensor/transmitter is powered by an internal battery. The battery is not replaceable separately. In the event of battery failure, the entire tire pressure sensor/transmitter at that wheel will require replacement. To extend the life of the battery, the sensor/transmitter is not activated until the vehicle reaches a speed of approximately 32 km/h (20 mph) as stated earlier.

Each tire pressure sensor/transmitter has a unique identity code. This will allow the TPM system to indicate to a technician, through diagnostic trouble codes in the module, which tire of the vehicle has lost air pressure. This code is also used to identify to a technician which pressure sensor/transmitter has set a diagnostic trouble code. For this reason, if a tire pressure sensor/transmitter needs to be replaced, the new sensor/transmitter must be programmed to have its identity code recognized by the receiver.