Is your low tire light on?

Most of you who own a newer vehicle probably have experienced a low tire light at one time or another. In fact, that is our most common request on the service drive. I felt that it was enough of a common question that it was deserving of a few minutes of conversation in our blog.
The TPMS or tire pressure monitoring system is actually a government mandated system that began phase-in in October of 2005 with about 20% involvement and reached 100% in models produced after September 2007 and is simply a system to monitor or calculate tire pressures and alert the driver to a potential problem. Depending on when your vehicle was built, the system may be very basic or somewhat advanced. Earlier models calculate tire pressure variances based on individual wheel speed. In this system, each wheel has a speed sensor which is typically associated with the anti-lock brake system as well. The computer looks for a variance in one wheel’s speed as compared to the other three. The theory here is that if a tire loses pressure, it will shrink slightly in size and a smaller tire will need more speed to keep up with the other three larger tires. In the event that this happens, the low tire light is illuminated thus alerting the driver to the problem. One issue with this type of system is that the driver was not normally alerted as to which specific tire was low rather, just a warning to check the tires in general.
Fast forward to today’s vehicles and we see a slightly different technology. The sensors are actually mounted inside each tire just below the valve stem. This sensor determines the tire pressure to a very accurate measurement and sends a wireless signal to a receiver in the vehicle. The vehicle then displays the tire pressures per wheel on the DIC or driver information center. This newer technology allows the driver to actually see the pressure reading for each individual tire and if an out-of-line measurement occurs, the driver is alerted with a “low tire” light and/or a “service tire monitoring system” message on DIC.
Although this system is far more advanced than the prior system, it is not without its share of pitfalls: First, when your tires are rotated (as part of your regular preventative maintenance program) the system needs to be reprogrammed so that the receiver recognizes the new positions for each sensor. This is because there is a unique sensor ID that is broadcast to the receiver along with the tire pressure information. Which wheel each sensor is mounted to is programmed into the computer so when it sees the sensor ID from say, the left front wheel, it knows to display “LF:” and the pressure reading. This is why when an identical car pulls up to a traffic light next to yours, that person’s low tire does not display on your dash….the ID broadcast from his sensors is different than yours.
Next, this system is extremely sensitive to tire pressure fluctuation due to temperature. A variance of only 3 PSI will illuminate the light. Most of us who have experienced this situation have seen it happen on the morning of the first frost. This is because the compressed air (which is what your tires are filled with) shrinks when it gets cold and expands when it gets hot due to the H²0 molecules within the air. On that cold morning when your low tire light is on, if you begin driving to work you’ll often notice the light goes out then returns after your car sits all day while you work and gets cold again. This is because the friction created within the tire while driving heats up the air and it expands causing the pressure to go up and the light to go out.
This is one of the many reasons why Nitrogen is becoming a popular choice for filling tires rather than compressed air. In a tire filled with 98% pure Nitrogen, there is little, if any moisture and therefore the tire is not very susceptible to size fluctuation due to temperature change. Because of the stable tire pressures, many users of vehicles with Nitrogen filled tires have also noted an increase in fuel mileage. Here in our service drive at Outten GM, Nitrogen has become very popular and many are already enjoying its benefits including less “low tire” warning lights.Please do not hesitate to give us a call if you’ve been experiencing a recurring low tire light or if you simply want to take advantage of a Nitrogen tire fill.