Tire Pressure Monitoring Systems
Monitoring the state of operation of the automobile began with simple instrumentation such as the speedometer, the voltmeter and the temperature gauge.
Monitoring the state of operation of the automobile began with simple instrumentation such as the speedometer, the voltmeter and the temperature gauge. But when the onboard processor was introduced, the number of sensors rose dramatically to the point where the amount of oxygen in the exhaust stream is being measured to optimized combustion. Such technology was hard to imagine as recent as the 1970s.
Today, the modern car knows if someone is sitting in the passenger seat, what direction the car is traveling and the outdoor temperature. Voice controls are used to operate the radio, CD and DVD players along with cell phones. With all this neat technology being utilized, it was just a matter of time before a vehicle could monitor the air pressure in the tires.
The Tire Pressure Monitoring (TPM) system is a very helpful technology for the simple fact that most tires are under inflated. This can lead to improper tire wear and reduced fuel mileage. So anything that can help a car owner maintain proper tire pressure is practical. Also, in the 2008 model year, this technology, first offered as an option in 2001, became required by Federal Law.
TPM SYSTEM BASICS
The TPM system monitors air pressure in the four road tires. Spare tire air pressure is not monitored. Two tire pressure monitoring systems are available—the base system and the premium system. The main difference between the two is the ability to detect the number of tires with low air pressure and where those tires are located. The base system just tells the driver one or more tires has low air pressure. On the other hand, the premium system indicates which tire has low air pressure.
On vehicles equipped with a TPM system, the traditional tire valve stem is replaced by a tire pressure sensor, such as that shown in Figure 1. The sensor (6) is designed for original style factory wheels (5). It is not recommended to install a tire pressure sensor in an aftermarket wheel (tire sealing and system performance issues can occur).
The serviceable components of the sensor are the metal washer (1), the sensor-to-wheel seal (2), the valve stem nut with a pressed-in washer (3), the valve stem cap (4) and the valve stem core.
Each sensor has an internal battery that can last up to 10 years. The battery, however, is not serviceable. When the battery fails, the entire sensor must be replaced.
The base system consists of the tire pressure monitoring sensor, a central receiver module (known as the Wireless Ignition Node, or WIN) and an indicator lamp on the instrument panel. The premium system has the sensor, WIN and the indicator lamp, plus three wheel sensor transponders located in three of the four wheels and an electronic display.
The receiver circuit for the TPM system is integrated into the WIN. The WIN decodes the Radio Frequency (RF) signals transmitted by each tire pressure sensor. This information is used to determine if a warning or fault condition exists. When a warning or fault is detected, the WIN will send a request to the module that controls the indicator lamp (and electronic display if the vehicle has the premium system) via the vehicle bus system to illuminate or flash the lamp. Also, the electronic display will send a request to sound the chime.
The WIN will store all warning and fault conditions, placard pressure values and low-pressure threshold values (lamp on and off) in memory that can be accessed through diagnostic communication. If a new sensor is installed on the vehicle, the data stored for the sensor being replaced will be deleted. Also, all wheel sensor IDs and locations and all other data values transmitted from each active wheel sensor will be stored in the WIN.
TPM SYSTEM OPERATION
Tire pressure sensors, mounted in each road wheel, transmit an RF signal indicating the individual pressure values to a receiver located in the WIN. These transmissions occur about every minute at vehicle speeds over 15 mph. If the base TPM system detects that the tire pressure in any one of the four road wheels is going low, below what is known as the low pressure (lamp) ON threshold, the TPM system will continuously illuminate an indicator lamp on the instrument panel. If a system fault is detected, the lamp will flash for 75 seconds, then go solid.
When these events occur with the premium TPM system, a chime will sound, the indicator lamp will illuminate and a graphic display will be shown on the Electronic Vehicle Information Center (EVIC). This display will flash the tire pressure value(s) and the position of the low tire(s).
The TPM system will continue to warn the driver of the low tire pressure as long as the condition exists. The indicator lamp will turn off when the tire pressure is at or above the low pressure (lamp) OFF threshold. The system will automatically update and the TPM indicator lamp will turn off once the updated tire pressures have been received.
Tire pressure will vary with temperature by about 1 psi for every 12° F. In other words, when the outside temperature decreases, tire pressure will decrease (and vise versa when the temperature increases). Tire pressure should always be set based on cold inflation tire pressure (also known as placard pressure). This is defined as the tire pressure after a vehicle has not been driven for more than 3 hours (in outside ambient temperature). The tire pressure will also increase as the vehicle is driven. This is normal and there should be no adjustment for this increased pressure.
For both the Base and Premium TPM system, the recommended placard pressure, the low-pressure threshold (low pressure ON) and the hysteresis pressure values, for the tires installed on the vehicle, are learned by the WIN or the TPM module from the Front Control Module (FCM) configuration during the initial manufacturing plant process. The values can also be learned during a service procedure. The WIN, upon learning the placard pressure via the bus, applies the appropriate low pressure threshold and hysteresis values.
To determine the pressure thresholds for a particular vehicle, refer to the Tire Inflation Pressure (Placard) Label on the driver side B-pillar. Then, apply the placard pressure to the TPM Threshold Pressure Table that can be found in the Chrysler Group LLC Service Manual for the vehicle in question—Section 22, Tires and Wheels. The low-pressure OFF threshold is defined as the low-pressure ON threshold plus the hysteresis pressure value.
Let’s look at a representative tire pressure in this chart. For a placard pressure (cold) of 30 psi, the low-pressure ON threshold pressure is 24 psi. This is the tire pressure that will illuminate the indicator lamp on the instrument panel, telling the driver that there is a tire with low air pressure. The low-pressure OFF threshold pressure is 28 psi. This is the tire pressure at which the indicator lamp will be turned off after the pressure has dropped to the low-pressure ON threshold pressure.
