By Jim Kerr
It all began with a simple question: “I want to put winter tires on my car but it has a tire pressure monitoring system (TPMS). What happens if I use another set of wheels for my winter tires? No one seems to be able to tell me.”
With provinces such as Quebec requiring winter tires during certain months of the year, and drivers in many other parts of the country readily switching to winter tires for the increased traction and safety, I would have thought this question could be answered immediately. Instead, I found that there are many different systems on our vehicles, and each one has their own specifications and setup procedures. The only thing that is consistent is the U.S. Government regulations covering tire pressure monitoring.
The U.S. Federal Motor Vehicle Safety Standards for tire pressure monitoring systems is called the Transportation Recall Enhancement, Accountability and Documentations Act (TREAD Act). The TREAD Act mandates that new vehicles less than 10,000 pounds gross vehicle weight, sold in the United States, be equipped with tire pressure monitoring systems, beginning with 20% of vehicles for the 2006 model year, increasing to 70 percent of 2007 model vehicles and to 100 percent of all 2008 model vehicles. Because most of the vehicles we buy in Canada are built for North American standards, we also drive many vehicles equipped with tire pressure monitoring equipment.
The TREAD Act outlines a number of specific TPMS performance requirements. New vehicles must be equipped with sensors that monitor tire pressure in all four tires. Spare tires are also monitored on some vehicles, but this is not a requirement. The system must work when the ignition is turned on and the TPMS warning light must turn on if a tire becomes under-inflated by 25 percent or more and stay on until the tire is inflated to the proper pressure. The TPMS must also warn the driver if there is a malfunction in the system.
My experience with these systems has not been wonderful. Temperature changes in the spring and fall often cause the low tire pressure warning to turn on. Sometimes they turn off automatically when the temperature warms up. Other times, it means a trip back to the dealer for them to be reset. Many of the newest vehicles allow the driver to reset the system, which may be necessary if the tire pressures are changed or the tires are rotated.
Pressure sensors inside each wheel (usually part of the valve stem assembly) send radio signals to a control module. Individual serial numbers from each sensor are also transmitted to the control module. This is how the module recognizes the tire position (LF, RF, RR, LR). This is also the reason that the sensors require relearning when the position of the sensor is changed during tire rotation. During programming, the Control module learns each individual wheel pressure sensor. This allows it to monitor only the radio signals from your car and not the signals from one driving beside it.
So what does this mean for the driver who wants to swap wheels and tires? If you install a set of wheels and tires without tire pressure sensors, the TPMS will turn on a warning message that the system is malfunctioning because it doesn’t see any signals when the vehicle is in motion. To prevent the warning message, you need to install pressure sensors on your “winter” wheels and these sensors must be the correct type for your vehicle. There are many different system manufacturers, so you need the exact units designed for your vehicle.
Finally, when the wheels and tires are swapped, the system will need to be reset, so the module can learn the new tire positions. One additional caution: when buying an extra set of wheels for those winter tires, make sure the pressure sensor will fit on the rim. Not all sensors fit all rims.
Tire pressure monitoring systems seem like a good idea. However, they only warn when a tire is getting dangerously low, and if the warning light comes on every time there is a large outside temperature change, then drivers begin to ignore the warning and the systems become useless. TPMS is designed to provide safer motoring, but these warning systems do not replace regularly checking the tire pressures.