Tire Pressure Monitoring Systems
In 2001, the United States Department of Transportation (DOT) passed the Transportation Recall Enhancement Accountability and Documentation (TREAD) act. The details of this law are many but one thing it required was that all vehicle manufacturers install Tire Pressure Monitoring System (TPMS) devices on their vehicles. This allowed owners and drivers to monitor the air pressures in their vehicles without using cumbersome pneumatic tire pressure gauges.
This was the first time the systems were required, however, various manufacturers started installing the vital safety systems earlier. This meant the technology wasn't "new." In 1986, Porsche deployed one of the first factory installed tire pressure warning systems. By the late 1980s, many European car companies started installing low pressure warning systems.
How do they work?
There are two different types of systems being used today: Indirect TPMS and Direct TPMS.
Indirect TPMS works with your car's Antilock Braking System's (ABS) wheel speed sensors. If a tire's pressure is low, it will roll at a different wheel speed than the other tires. This is detected by the car's computer system which triggers the dashboard indicator light.
Direct TPMS uses a sensor mounted in the wheel to directly measure air pressure. When air pressure drops 25% below the manufacturer's recommended level, the sensor transmits a signal to the car's computer system which triggers the dashboard indicator light. In theory direct systems are more accurate.
What should I do if my TPMS light illuminates?
If your TPMS light illuminates, check the air pressure in all four of your tires and inflate any tire that is low. By the way, use the pressure number that is stated in the owner's manual, not the maximum pressure stamped on the tire. When the tire is at the appropriate pressure, the indicator light should go off.
If the light doesn't go off, visit your local dealer. When someone comes in with a TPMS problem, the service department at Star Chrysler of Big Spring, TX, explains that have a way of testing the whole system to see where the problem is.
Car makers program a pressure threshold limit into their control modules on the assembly line. Some manufacturers shoot for a very low number, such as in the 15 psi range. Other manufacturers notify the driver in the 20 to 25 psi range. The higher the threshold number the greater the chances that the driver will become annoyed with frequent warnings of low tire pressure.
The reason that this happens is that air density changes in relation to temperature. On a cold subfreezing morning, your tire pressure can differ 5 to 8 psi from the normal operating temperature of a warm tire. This may indeed be enough of differential to turn your tire pressure indicator light on. The solution, of course, is to simply drive. The tire will warm up and the pressure will increase.
By the way, a solution exists for this problem. The answer is to fill the tires with nitrogen gas. A nitrogen filled tire only fluctuates 1-2 psi instead of the 5 to 8 of an standard air filled tire.
Batteries can fail
Many of the cars of the last decade use tire stem mounted air pressure sensors which are powered by lithium-ion batteries. This power cell lasts about 10 years. Unfortunately, on most models the battery cannot be replaced.
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