Tire pressure

In my GM vehicle, how does it measure the tire pressures and ultimately readout on the instrument panel? What I've found so far that it counts the
rotations and the pressure is proportional to the diameters.
Or, are there pressure sensors in each tire that sends signals to the computer?
Dave C.
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Dave C. wrote:

Different vehicles use different methods to measure tire pressure. GM makes many different types of vehicles.
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There are a couple variations, the system that displays each individual tire pressure has pressure sensors in each tire. The sensors transmit a frequency, this is interpreted through a module and displayed as a pressure. These need to be relearned when positions change. There is another system that determines low tire by wheel speed, it will just give you a general low tire warning light, not respective to position.
Gerald
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wrote:

There are a couple variations, the system that displays each individual tire pressure has pressure sensors in each tire. The sensors transmit a frequency, this is interpreted through a module and displayed as a pressure. These need to be relearned when positions change. There is another system that determines low tire by wheel speed, it will just give you a general low tire warning light, not respective to position.
Gerald
In the case of pressure sensors, wouldn't there be a need for some sort of a power supply for the sensors? Or is there a method of converting rotation of the wheel to a voltage for the sensor? That might be too exotic for production purposes. I find this interesting.
In my present vehicle, I get a readout of each of the tires in psi. In my former vehicle, I would get a warning light if any one of the tires was low as you mentioned.
Dave C.
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Document ID# 703284 2004 Cadillac DeVille
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Tire Pressure Monitor Description and Operation
The tire pressure monitor (TPM) system warns the driver when a significant loss, or gain of tire pressure occurs in any of the 4 tires and allows the driver to display all 4 tire pressures and the tire locations on the Driver Information Center (DIC) while the vehicle is being driven. The system uses the remote control door lock receiver (RCDLR), DIC, dash integration module (DIM), powertrain control module (PCM), a radio frequency (RF) transmitting pressure sensor inside each wheel/tire assembly, and the serial data circuit to perform the system functions. When the vehicle is stationary for more then 15 minutes the sensors go into stationary mode. In this mode the sensors transmit once every 60 minutes to minimizes sensor battery consumption. As vehicle speed increases to 32 km/h (20 mph) centrifugal force closes the sensors internal roll switch, causing the sensor to go into drive mode. In this mode the sensor transmits once every 60 seconds. The RCDLR receives and translates the information within each sensor's RF transmission into sensor presence, sensor mode and tire pressure. The RCDLR sends the tire pressure and location data to the DIC via the serial data circuit where tire pressures and their locations are displayed. When the TPM system detects a significantly under, or over inflated tire, the CHECK TIRE PRESSURE warning message is displayed on the DIC. When the DIC is scrolled to display the tire pressures, additional messages are also displayed as follows, depending on that tires pressure condition:
TIRE OK TIRE HIGH TIRE LOW
The CHECK TIRE PRESSURE, TIRE HIGH, or TIRE LOW warning messages can be cleared by adjusting tire pressures to the recommended kPa/psi. Refer to Tire Inflation Pressure Specifications in Maintenance and Lubrication, or to Tire Placard in General Information. The sensor's pressure accuracy from -10 to +70C (+14 to +158F) is plus or minus 14 kPa (2 psi). The TPM system can also compensate for high and low altitudes using the PCMs manifold absolute pressure (MAP) sensor input via the serial data circuit. The RCDLR has the ability to detect malfunctions with in the TPM system. Any malfunctions detected will cause the DIC to display the SERVICE TIRE MONITOR warning message. For more information on other functions of the RCDLR, refer to Keyless Entry System Description and Operation in Keyless Entry.

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Good description of the operation. Sounds like each transmitter has its own little battery (probably tiny) and the transmitter is probably attached to the rim inside somewhere or in the valve stem. I think it is a nice feature. Almost every function on this vehicle goes through a computer instead of "hard wired relay contacts."
My previous vehicle (Jeep) only had an output of "check the tires, one is low" but did not tell which one. I alerted me to a slow leak that I had and did not know it.
Thanks,
Dave C.
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