Community Papers

Tire pressure monitoring system indicators can cause confusion

I imagine everyone has their snow tires on by now. I also imagine there are a lot of vehicles driving around with an extra little orange light on.  That light is either indicating a malfunction of your Tire Pressure Monitoring System (TPMS) or maybe you actually have a tire pressure problem.  Do you know what your problem is?  Do you care?  Should you care?

TPMS is an electronic system that monitors your vehicle’s tire pressures and reports to the driver when there is a tire pressure issue (Too low of pressure? yes.  Too high of pressure?  Maybe.)  It also monitors itself and reports malfunctions.

There are a variety of systems out there.  The technology dates back to Europe and the late eighties.  Systems started to show up on luxury vehicles of that era.  For we North Americans the tire pressure gauge would be the TPMS of choice until the mid 2000s.

The late 1990s saw a recall of Firestone tires from so equipped Ford Explorers.  It seemed that people were driving on underinflated Firestone tires on their Ford Explorers and then losing control of their vehicles when a tire blew.

Ford and Firestone argued whether the tires or the vehicles were the problem and I do not remember who got the blame but the U.S. government of the time (Clinton was in charge) instigated the Tread Act as a result.  Remember correct tire pressure saves fuel and decreases tire wear. (very GREEN)

The Tread Act mandated TPMS in all light vehicles (under 10,000 pounds) sold after September 1st. 2007.  Other countries adopted similar laws Canada not being one of them.  Despite that fact many vehicles now sold in Canada have TPMS as standard or optional equipment.

There are basically two types of TPMS.  Direct and indirect.  Indirect TPMS infers the air pressure of a tire by the way it rolls.  Indirect systems use simply the wheel speed sensors that were already being used in ABS systems.

As a vehicle rolls straight down the road all the wheels should rotate at the same speed if they have the same circumference (distance around the tire).  Significantly reducing the pressure in one tire will reduce its circumference and thus cause the wheel to rotate faster than the others.  This system has its faults as it cannot detect a problem when all the tires have lost air equally (an everyday normal occurrence:  a typical tire will lose 3 to 9 psi yearly).  There have been advances in indirect systems recently that use advanced physical properties of a rolling tire to determine an air pressure issue.  These systems require no extra parts on the vehicle, only special algorithms in one of the modules. A reset procedure (usually pressing a button before driving for several minutes)  allows the module to learn how the wheels roll.

Direct systems are more common.  The direct system places a little battery powered radio frequency sensor right in each  wheel.  It is either attached to the back side of the valve stem or strapped on the wheel.  These sensors then broadcast both temperature and pressure to the receiver (TPMS module) in the vehicle.

Each of these sensors has an electronic ID that must be registered to the TPMS module.  The registration process tells the TPMS module where each sensor is located on the vehicle. The module has a required pressure range that must be met for each tire and when the pressure is incorrect, on comes the reminder light.These direct systems have been a source of grief for the service industry.  The sensors are battery operated and delicate.  They are fairly easy to break if you are not careful removing the tire from the wheel.  The battery will wear out and require replacement of the sensor.

If the owner has two sets of wheels and tires (common here in the snow belt) in order to have an operational TPMS all year round requires eight sensors.  As well, each time tires are replaced and or rotated the sensor IDs much be registered to the TPMS module in the correct position.

The registration procedures are not the same for all vehicles.

In general these direct systems have a built in diagnostic system.  If your TPMS light suddenly turns on solid while driving it is indicating a tire pressure problem (some vehicles give data for each tire separately and will indicate the faulty tire pressure location).  On the other hand if your TPMS light starts blinking first and then stays on solid your problem is a malfunction of the system. (bad sensor or fault in the TPMS module and its wiring).

Seems like a lot of trouble to eliminate due diligence with a tire gauge and five minutes of the owner’s time.  Such is progress.

Ron Nutini is a licensed automotive technician and graduate of mechanical engineering from UBC.

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