TPMS: Tire Pressure Monitoring Service for All Shops
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TPMS: Tire Pressure Monitoring Service for All Shops

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Recently, an owner of a 2008 Lincoln Navigator brought their vehicle in for a tire rotation. The tires were rotated front to rear. The customer drove away not knowing he was in great peril.

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A few days later, the right rear picked up a small leak. The TPMS system turned on the dashlight and indicated right front tire was below specifications.  On the way home, the driver pulled into a gas station and topped off the right front tire. The owner was unaware that he overinflated one tire and left the leaking tire underinflated.

In the morning, he awoke to find a flat tire in back. The one he added air to was fine. But, we all know it could have ended in tragedy. By simply looking at a tire one can’t tell whether it is properly inflated. By the time the driver notices that the tire is under inflated the pressure is at 50%, or less,  of its proper pressure. 

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What happened? The shop failed to reprogram or relearn the TPMS system. When the tires were rotated, the sensors in the tires thought they were in their original positions when they were reloctated. The technician did not know about the TPMS or what to look for. To him, it just looked like a fancy valve stem.

Chances are if you have not prepared for servicing Tire Pressure Monitoring Systems (TPMS),  you might have already turned a good customer away to the dealer or a competitor down the street. On the surface, it may seem like one of those new things that automakers have included on their vehicles to get customers to return to the dealer. But, it is an opportunity for shops to sell another service to your customers on a critical safety system.

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One manufacturer of  TPMS tools for OEMs and the aftermarket  claims the average shop can charge between $50 and $150 for a relearn procedure, depending on the system.

TPMS service is critical to any shop’s survival. It is not a service that you can put off like updates for your alignment machine or trying to survive as long as possible with a center post tire changer.

The Law
The National Highway Traffic Safety Administration (NHTSA) issued part one of a two-part final rule requiring tire pressure monitoring systems that warn the driver when a tire is significantly underinflated on May 30, 2002. 

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All vehicles produced after September 1, 2007 must be equipped with the direct pressure monitoring system. The Tire Pressure Monitoring  System (TPMS) contains a Malfunction Indicator Lamp (MIL) that will illuminate when there is an under pressure condition or a fault has occurred within the system.

Tire pressure is critical to the life of the tire and safety of the vehicle.  By simply looking at a tire, one can’t tell whether it is properly inflated.  Running an under inflated tire causes extreme sidewall flex that separates the cord from the carcass and adjacent rubber compounds. 

In other words, a catastrophic failure that can result in a vehicle rollover. The TPMS system contains a Malfunction Indicator Lamp (MIL) Owners should be made aware that tire pressure monitoring is a very important vehicle system in that it is a preventive measure to ensure the safe operation of the vehicle.  There is a required drive time of 20 minutes for the system to calibrate and register a low pressure warning. 

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It is estimated that the average pressure loss in a tire is 1 psi per month.  If a vehicle is used for short trips under 20 minutes over a time period of 12 months the pressure could drop to a condition greater than a 25 percent under inflation and go undetected. 

The system is required to activate a warning when a tire is 25 percent below the pressure that is  posted on drivers side B pillar. The information placard describes the tire size, speed rating and cold inflation pressure. Cold inflation pressure is measured when the vehicle has been stationary long enough where the tire is the same temperature as the ambient temperature. The placard also contains the seating arrangement and maximum load that includes passengers and cargo.

Sensor Construction
The sensor can be apart of the valve stem or attached to the drop center of the wheel rim. The valve stem sensor is secured in the wheel inflation port with a nut and seal.  The nut has an installation procedure and torque as specified by the vehicle application to properly seal the valve stem. A strap and bracket are used to attach the sensor to the rim.

Components: Battery
The battery used for its power source can be either a nickel metal hydride at 1.25 volts per cell or lithium ion at 3.6 volts per cell. The average battery life can range from 5 to 10 years.

Sensor

The sensor has multiple functions.  Pressure is measured using a resistive or capacitive type circuit. The reference pressure can be 1 bar or 0 KPa dependant on the manufacturer. 1 bar is equal to 14.5 psi. 0 KPa is equal to a vacuum and 100 KPa is equal to 1 bar. Temperature is measured with a thermocouple or thermistor. As temperature increases, pressure increases. For every 10ºF (5.6°C) change in air temperature, the tire’s inflation pressure will change by about 1 psi. The pressure sensor is compensated for the change in temperature. The analog pressure and temperature signal is transmitted to the micro controller.

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Controller
A Micro Controller Unit (MCU) is used to process the analog sensor data to digital for the transceiver to communicate with the in car receiver unit and Body Controller. The MCU can contain an Electronically Erasable Programmable Read Only Memory (EEPROM) for the programmed sensor identification, location and pressure information.  A crystal used to set the frequency of the sensor. The antenna is attached to the valve stem.  A low frequency oscillator is used for MCU wake up and sensor programming.

Antenna
The antenna can be located on the back of the sensor printed circuit board and connected to the valve stem.  For the valve stem-type sensor, it is necessary to use the core valve and cap used for that specific sensor to maintain proper antenna operation.

Sensor modes of operation

The sensor can be in an off mode for shipping or storage to protect the life of the battery. A test or activation mode is for the initial setup on the assembly line.

A park mode is for the conservation of battery life when the vehicle is shut down.  A learn mode is for reprogramming the sensor. A vehicle drive mode is to transmit pressure information to the receiver.  Follow the installation procedures provided by the manufacturer.  Sensors attached to the drop center of the wheel will not activate until the tire is pressurized.

Receiver

The receiver contains an antenna to receive the serial data from each of the sensors. An encoder in the receiver places the information on the BCM data buss.  The information will activate a warning indicator in the instrument cluster if a low tire is sensed.  If the vehicle has a Driver Information Center (DIC) the driver can display the tire inflation pressure by location.  The programming tool or scan tool can retrieve other information such as trouble codes and battery life. The TPM receiver can be an integral part of the Remote Keyless Entry (RKE) receiver per GM vehicles.  

Programming the Sensor  
The sensor requires a programming tool or magnet to identify the sensors and their location to the receiver. The vehicle ignition must be in the run position to program the sensors.  Programming begins at the left front wheel. The tool initiates an low frequency activation signal  at 1.25 KHz to the sensor. Upon activation, the sensor produces a serial data stream identifying the sensor, its position as Left Front, pressure data and verification check sum. This information is stored in the tool and may be stored in the BCM. When the tool is placed at the Right Front wheel and the sensor is activated the same serial data stream is produced with the location identified as Right Front. The process is continued for the Right Rear and Left Rear sensors. Each time a sensor is programmed, the vehicle and/or the tool will respond with an audible sound alert. It is necessary to follow the manufacturer’s instructions to complete the programming procedure. Reprogramming must be performed when the tires are rotated to change the position location in the sensor.

Wheel and Tire Service:

The construction of the sensors is varied ranging from a monolithic sensor with or without the valve stem permanently attached to a sensor where the valve stem is a separate component. A sensor valve stem can be attached to rim inflation port with a seal and nut or a conventional rubber valve stem that is referred to as a “snap in.” A sensor can be attached with a strap and bracket to the drop center of the rim. Particular care should be used when mounting and dismounting the tire when the pressure sensor is mounted in the inflation port. The sensor is particularly vulnerable when breaking the bead of the tire.  The changer should disengage the tire bead from the rim without contacting the sensor body. Once the bead has been broken, the sensor should be removed before removing the tire form the rim.  When reinstalling, the sensor, a new seal, nut core and cap should be installed. 

Your customer should be advised to have their tire pressure checked at least every six months. If the vehicle is equipped with a DIC, that will easily display tire pressures. Show them how easy it is to check their tires on a regular basis. The Warning indicator (MIL) should never be ignored especially when the vehicle is being driven at speed. There are a fuel savings and long tread life to be realized when the tires are properly inflated. 

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