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Atlantic Racing Scene

Everything you need to know about TPMS

xBy John Cannell

What in the world is TPMS? This acronym stands for “Tire Pressure Monitoring System”, which, as the name suggests, are on-board devices whose purpose is to monitor air pressure of a vehicle’s tires while it is operating on the highway.

The National Highway Traffic Safety Administration, or, (NHSTA) (more acronyms) is the driving force behind this innovation, their main objective being the drastic reduction of under-inflated tires on the highway, and the tragic results of their subsequent failures. Your first reaction might be, “Who needs it - just something else to go wrong!”

But, think about this fact. With ultra-low profile radial tires now standard equipment on virtually all vehicles, it is almost impossible to tell a fully inflated tire from one that is only 70% inflated. nevertheless, the load rating (GAWR) of that 70% inflated tire is also reduced by 30%.

A German TPMS on a VW

In fact, if the vehicle is carrying a full complement of passengers and luggage, the GAWR could easily be exceeded on one or more tires. Heat is a major factor in tire failures, so add a hot day plus sustained high speed, and well you get the picture.

The original incentive leading to development of TPMS was provided by the invention of “Run - flat” tires which permit the driver to travel at 80 kmh. on a tire with no air whatsoever until service is available. Obviously, there had be a method by which a no-air situation could be detected so the driver would be aware of the need to reduce speed and arrange for tire service.

This was accomplished by sensors mounted in the ‘well’ of the rim which communicated with the vehicle’s onboard processor resulting in an audible and visual warning. These tires required special tire changers and or, demount/mount procedures in order to avoid damage to these relatively expensive sensors. Coats 7060EX and 9024E are good examples of these.

How TPMS works

The principle behind TPMS is similar except the system is simpler, and uses only one sensor per wheel which transmits to an RF receiver interfacing with the vehicle’s ECU and a dashboard indicator warning the driver of a 25% (or greater) drop in tire pressure. Upscale versions such as those used with GM’s On-Star system work with optional satellite receivers so that the driver can be warned by a call from an On-Star advisor.

TPMS, in one form or another has been around since 1990, but by the end of 2007 it should be standard equipment on all North American passenger cars and light trucks with a GVW of 10,000lb (22000kg) or less.

At present, two basic configurations are being used. One is called an “Indirect” system which evaluates relative wheel speed via the ABS wheel speed sensors, based on the fact that decreasing inflation reduces effective wheel diameter thereby increasing wheel speed. This system cannot determine actual tire pressure, or which tire has the low pressure, therefore it will probably be discontinued because it does not meet the federal standards.

The other system is, as you might expect, called the “Direct” monitoring system. Pressure is monitored through a sensor residing in each wheel assembly, which transmits once vehicle speed exceeds 15 mph (24 kph). A dashboard readout indicates current status and warns of any individual tire with low pressure. This system meets with NHTSA approval.

The important thing for all service providers to remember is that any service to the wheels, such as, rotation, changing, balancing etc., will require re-programming or resetting of the wheel sensors so the dash indicators can provide accurate information. Wow, you say! How do you manage this?
Glad you asked. More than one manufacturer has produced a service tool for this purpose, however, the most recommended package is provided by the OTC division of SPX Corporation.

Ordering number of this sophisticated package is 3833 (photo above) . It is available with an accessory kit which greatly enhances various mechanical and other functions required when servicing TPMS. Perhaps more important is the availability of full information and specifications via CD, printed manuals, or quick reference manuals.

Obviously, every shop doing anything with tires, (not just tire dealers) will require a TPMS tester. It is recommended that a shop consider the purchase of only one unit to start. By the time the number of vehicles coming through their doors really begins to increase, all personnel should be comfortable with the TPMS concept, selling it to the customer, changing TPMS equipped tires, and using the tester for rapid and accurate TPMS service.

I know, it was much easier in 1956, but, would you really swap your 2006 daily ride for a 1956?

You can contact John in Saint John, New Brunswick here