The Nuts & Bolts of TPMS

Insight on servicing TPMS.


It’s clear that Tire Pressure Monitoring Systems (TPMS) are here to stay, and by now we have all heard of the new federal mandate requiring TPMS on all vehicles less than 10,000 pounds GVWR. While there have been many articles written about this mandate, I'd like to tackle the technology fueling it.

Indirect Approach

When it comes to TPMS, there are two basic technologies. The first, called "Indirect" TPMS, relies on the use of the vehicle's ABS system. Indirect TPMS is based upon algorithms which compare speeds from wheel to wheel and draw conclusions based on their variations. Engineers have correlated specific tire pressures to the RPMs generated by a tire.

The obvious problem with this approach is a vehicle where all 4 tires have the same pressure (high or low). This limits the system's ability to perform comparative calculations, and draw conclusions. Most of these systems can be re-set with the push of a button on the dashboard.

While the Indirect approach is the most cost effective and desirable for vehicle manufacturers, the Final Rule of the TREAD Act requires that the TPMS be capable of reporting a low pressure condition for a single tire. This is the primary reason why most vehicle manufacturers will be forced to use Direct TPMS.

But there are efforts underway to develop Indirect TPMS that meet these requirements. Recently, laws were amended to allow for up to 20 minutes of "calibration time" in reporting low tire conditions to the driver. This means a TPMS problem may require driving the vehicle for up to 20 minutes in order to identify a problem.

Direct Approach

The Direct approach is more accurate, and it allows the driver to be alerted if a single tire drops below the specified pressure. The proper pressure for each tire is ultimately determined by each vehicle manufacturer, but it must be within 25 percent of the vehicle manufacturers’ cold inflation pressure. The Direct approach to TPMS is the primary technology that will be in use on September 1, 2007, when full compliance is required.

The Direct approach incorporates the use of radio frequency sensors that are either banded to the wheel or directly mounted to the valve stem. The Direct systems that use the valve stem design are typically identified by a metallic valve stem. These sensors transmit a unique ID to the vehicle’s TPMS control system, and are often capable of sending temperature, velocity, and other criteria as well.

To understand how to work with Direct sensors, it's important to understand how they function. In basic terms, all sensors have a response capability. This is the ability for the sensor to communicate with the vehicle’s TPMS control module. As of press time, all sensors in North America communicate on either the 315 MHz or 433.92 MHz frequencies. Communication between the sensors and vehicle's control system can be verified with some of the triggering tools currently available.

Synchronous Sensors

While the capabilities of the Direct type sensors are fairly straight forward, triggering them is a little more complicated. Most are synchronous, which means they are activated by either a magnet, or when exposed to a powerful low frequency electronic signal.

When the vehicle is in motion, the sensors are transmitting to the TPMS control module multiple times a minute. However, when the vehicle is at rest these sensors are designed to sleep in order to conserve battery life ( 7–10 years). They can, however, be triggered on demand with a triggering tool.

Federal law allows vehicle manufactures up to 20 minutes to report a TPMS fault. For this reason it is important to perform a quick check of the TPMS before performing tire or wheel service.

If the vehicle has the standard telltale indicator after replacing or rotating the tires, it will not require re-training, unless a sensor is replaced. If the vehicle has a 4-position graphic indicator on the instrument panel, it will most likely require re-training. While retraining is outside the scope of this article, detailed illustrations and explanations can be found in the Mitchell 1 Guide to TPMS.

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