• 2017

The Pressure is Off

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Replacing FCA US LLC Tire Pressure Monitoring Sytems
Tire pressure monitoring (TPM) systems automatically check and record tire pressure for all four wheels on the vehicle. Federally mandated since the 2008 model year, TPM systems were first offered on top-of-the-line models such as the Crossfire, Viper and Prowler. As we all know, tire air pressure has a direct impact on fuel mileage and tire wear. And, since checking tire pressure is not an ingrained maintenance habit for most car owners, the TPM system will alert the driver when tire pressure drops below a pre-determined pressure. This allows the vehicle owner to adjust tire pressure, as required.

TPM System Basics
The TPM system is designed to monitor tire pressure in all four road wheels (spare tire pressure is not monitored). Pressure sensors, one mounted in each wheel, transmit RF (radio frequency) signals indicating the tire pressure in each tire. The signals are collected by a receiver located in the TPM module. These RF signals are sent about every minute when the vehicle is traveling over 15 mph.

Upon detection of a low pressure condition, the TPM module will send a request to the module that controls the indicator lamp and text display, via the vehicle bus system, to (1) illuminate the warning lamp, (2) sound the chime and (3) display a graphic with the low pressure value and position of the tire (the graphic display will flash in the EVIC, the vehicle information center).

Note: The base TPM system does not have the graphic display; thus, the pressure value and then the location of the tire with low pressure is unknown to the driver; this display option is only available with the premium TPM system.

A low pressure warning is determined by the difference between the placard pressure (the tire pressure when the tire is cold) and the actual pressure measured by the sensor. If the difference between the two pressures is such that the actual pressure has reached the low pressure ON threshold, the warning request is sent by the TPM module. The TPM system will continue to warn the driver of low tire pressure as long as the condition exists. Once the tire pressure increases to an acceptable level, known as the low pressure OFF threshold, the warning (lamp, chime and graphic display) will turn off.

These three values (placard, ON threshold, OFF threshold) for the tires installed on the vehicle are learned by the TPM module during the initial Manufacturing Plant Process. The placard and threshold values for a particular vehicle are listed on the Tire Inflation Pressure (Placard) Label found on the driver’s side B-pillar. For example, on a 2014 minivan, if the placard temperature is 32psi, the low pressure ON threshold is 25psi.

Cold Weather Issues
In locations where the temperature variation is great (for instance, in the Great Lakes states, summertime temperatures reach into the 90s and winter temperatures fall below 0), low pressure warnings can be displayed during the winter months. This is due to the fact that tire pressure decreases as the outdoor temperature decreases and vise versa. The rate is 1psi per 12 degrees.

If the temperature one day is 46 degrees and the next day it drops to 10 degrees (36 degree change), tire pressure will drop 3psi. Taking these numbers into consideration, let’s look at our 2014 minivan. If tire pressure on the first day is 27psi (2 degrees above the low pressure ON threshold of 25psi), the next day, the tire pressure will be 24psi (1 degree below). When the vehicle is used the next day, the low pressure warnings will be turned on. As the vehicle is driven, tire pressure increases (2psi to 6psi) and the warnings are turned off.

If tire pressure is not checked and maintained on a particular vehicle (this is more common than you think), low pressure warnings as described above occur regularly. Then, the warnings turn off after the vehicle is driven for a while. The same scenario might occur a second day in a row. Often, these vehicles end up at the dealership with complaints that the TPM system is malfunctioning, when, in fact, it is not. It might be helpful to explain this scenario to your customers if they show up at your shop with this complaint.

TPM System Diagnosis

Problems with the TPM system can be diagnosed rather easily. When diagnosing a tire pressure issue, first check the TPM indicator lamp during ignition key ON. From the OFF position, turn the key ON and check the TPM indicator to observe one of the following:
1. If, after 10 seconds, the lamp is illuminated continuously, check the tire pressure in all four road wheels; if the wheels all check out OK, or a problem is found and fixed and the light stays on, look for a system fault
2. If, after 10 seconds, the lamp flashes on/off, then remains solid, a system fault has been detected; if a premium system is installed, the message Check TPM System will be displayed; refer to the appropriate diagnostic information

Replacing TPM Sensors

The TPM sensor is a rather durable component, especially since the valve stem material was changed to rubber from aluminum. Each sensor has an internal battery that lasts up to 10 years, but it is not serviceable. The entire sensor must be replaced if the battery is bad. Some parts are serviceable, though, such as the valve stem, core and cap. If the sensor must be replaced, or serviced, keep a few things in mind.
1. TPM sensors are designed for OEM-style wheels
2. Valve stem caps and cores are designed specifically for TPM sensors; do not confuse these components with standard cores and stems
3. TPM sensors are not interchangeable with other vehicles; the 2014 minivan transmits signals at 433 MHz, while other designs transmit signals at 315 MHz TPM sensor replacement is straightforward.

 

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Remember, care must be taken when dismounting the tire from the wheel. To avoid damaging the sensor, do not use a bead breaker in the area of the sensor on both sides of the wheel. Before dismounting the tire, carefully insert the mounting/dismounting tool 280° (+/-10°) from the valve stem (1) (Figure 1).

(Figure 2) Remove the sensor-to-valve stem retainer screw (3), then remove the sensor (1) from the valve stem (2). Remove the valve stem (2) from the wheel (4). If a new sensor is being installed, the new valve stem is pre-mounted on the sensor as an assembly. If replacing the valve stem only, always use the new mounting screw.

Insert the valve stem through the wheel and pull it through to seat it with a standard valve stem installation tool. If replacing only the valve stem, verify that the flat sides of the brass extension on the bottom of the valve stem are at a 90° angle with the wheel. Position the pressure sensor over the brass extension, then install the mounting screw. Tighten the screw to 12 in.-lbs.

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