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Tire Pressure Monitoring System

Sounds comforting, doesn't it. Kind of like a burglar alarm or baby monitor. An electronic device to relieve us of some of the work load of operating a vehicle. But it doesn't relieve us of the responsibility.

Like any tool, it has its limitations. The operator must know those limitations and depend on the tool only up to that point. A baby monitor will let you know the baby is crying but it won't change a diaper.

What is TPMS? TPMS is a federally mandated system beginning with all 2007 passenger vehicles. The system is supposed to notify the driver when tire pressure is low and prevent catastrophic loss of tire pressure (blowouts). We'll see later that it does a good job when it gets around to it. Picture an indifferent store clerk who doesn't acknowledge you until ending a cell phone call with "I love you". You're pretty sure he wasn't talking to his boss.

How does it work? TPMS consists of an air pressure sensor inside each tire that transmits a signal to an onboard receiver that in turn illuminates a warning light on the instrument panel. There are currently two general types of systems; direct and indirect. The direct or discreet system will indicate which tire is low. The indirect will sound a general alarm. Then it is up to the driver to determine which tire is low.

What are the parameters? Federal law says that the system must send an alarm when tire air pressure falls 25% below the recommended pressure. BUT it has twenty minutes to send that alarm. You could run over a piece of glass when leaving the dealership with a brand new car. You could then drive for twenty minutes before the TPMS detected and set the alarm. Running a tire that is seriously low or flat for twenty minutes at freeway speeds will likely destroy it. See the limitations of the system?

Maintenance and repair. There is limited maintenance and no repair; only replacement. The maintenance consists of reprogramming the sensors when tires are rotated so the receiver will know where on the vehicle that particular tire is located. Seal kits are available should the valve leak at the wheel hole.

Replacement will be necessary if the sensor is damaged by a flat tire or careless service person. It is not always carelessness that renders a sensor useless. Corrosion is a problem with all the different metals in contact; aluminum, steel, brass, magnesium, stainless. Some are damaged by simply trying to remove the valve core. They also require replacement when the internal battery runs down which is now estimated to be 5.7 years.

There is no standardization. Not all manufacturers use the same system. A manufacturer might not even use the same system on all its vehicle lines. There are almost 100 part numbers for sensors. At a cost of $60 to $150 each, you can see that a dealer can't stock them all when he would need 4 or 5 of each. In some cases a sensor costs more than a replacement tire.

Can I disable the system? Yes, but federal law makes it a crime for anyone to disable or make inoperative a TPMS. That presents a problem for custom wheel installers. Your sensors may not fit the new custom wheel. The rim profile or valve stem location may not match. If you replace you tire/wheel which has a max pressure of 35psi with one that has a max of 50psi, your sensor will require replacement or reprogramming. Your sensor would not set an alarm until 26psi, nearly half what the new tire would need.

Some unscrupulous installers simply load your old tires and wheels into your vehicle. They argue they haven't disabled the system. Maybe not, but is certainly rendered useless. They know you will go home and unload the tires. You are now the criminal who has disabled the system.

What about mail order tire/wheel packages. Will they come with sensors installed that match your system? This is not a condemnation but a question you should ask.

What does the future hold? Standardization is the first step. If every sensor were identical, the price would come down due to economics of scale. The more you make of a particular item, the less it costs to make each one.

Sensors embedded in tires would be possible after standardization. They are already used in commercial truck tires to record pressure and temperature.

Onboard inflation systems. Maintaining pressure by continuously inflating a leaking tire while driving.

Airless tire/wheel made of molded plastics and rubber. No inflation; no flat.

Possible repeal of the TPMS law? There will be an outcry from motorists when in five years it is necessary to replace millions of sensors. And a revolution in ten to fifteen years when it must be done again and again when the vehicle is near the end of its life. Politicians  must be able to show marked reductions in accidents and injuries attributable to TPMS to justify continuing such a costly program.

Information believed to be accurate as of 1/20-07