Why do we need TPMS?
The purpose of TPMS (Tire Pressure Monitoring System) is basic. Primarily it is intended to save lives. Secondary is increasing tire life.
A little history. Over the years there has been confusion about proper tire inflation and many serious accidents that may have been avoided had the tires been inflated properly according to NHTSA (National Hi-way Traffic Safety Administration).
In the fall of 2000 the US government enacted the TREAD act (Transportation Recall Enhancement, Accountability and Documentation). One after affect is that most vehicles sold in the US since 2007 have TPMS. The NHTSA estimates that this saves 660 lives per year, as well as preventing 33,000 injuries per year and saving over 500 million in fuel.
Love them or hate them it looks like they are here to stay. As a basic rule the system is designed to warn you by turning on the TPMS dash light when one or more tires has dropped by 25% of the manufactures rated pressure.
How does TPMS Work?
There are two methods to get the job done; Indirect and Direct. Most of the vehicles that we see are direct.
Indirect systems work in conjunction with the Anti-lock/traction control system. This looks at the wheel speed of each wheel via the wheel speed sensors. Hence an under-inflated tire has a different role-out than a properly inflated tire and using an algorithm the system will signal you of improper inflation via the dash light. This system is less reliable that the direct system.
The direct system consists of a module inside each wheel that measures the tire pressure directly. This module includes the valve stem that you see on the outside as well as an air pressure gauge, radio transmitter and battery inside the tire. When a drop in pressure is recognized the sensor indicates this to the vehicles computer and then turns on the indicator lamp.
Pro’s and Con’s
We have already covered most of the benefits, lives saved, injuries avoided, increased tire life and fuel savings.
What are the cons?
I will focus mostly on the direct system as they are more common.
The sensors can be ruined if the external valve stem is damaged.
Because of the way that the sensors are tucked inside the tire they can easily be damaged when mounting or dismounting a tire. Because of this design most shops do not accept responsibility of a damaged sensor when changing tires.
OEM sensors can run between $80 and $180 and aftermarket sensors are similar at approx. $50-$150 each. Prices are starting to come down and should continue to some point.
The valve stems are nickel coated and are susceptible to external corrosion and if a brass valve core is accidentally installed, they can rust together.
They are not standardized from manufacturer to manufacturer.
They have to be reset. If the tires are rotated or a sensor is replaced. In the best case you just drive the car over 20mph for 20 minutes to a much more complicated reprograming requiring expensive equipment.
Two more items to address are, batteries and, TPMS sensor replacement.
How long do batteries last in a TPMS sensor?
Battery life with Lion (Lithium Ion) Batteries can range from 5 to 10 years. Older style sensors may only last around 5 years. The battery is included inside the sensor, so when the battery goes dead it needs a new sensor.
Do TPMS sensors need to be replaced with tire replacement?
There is always a chance that when tires are mounted or dismounted that the sensors may be damaged by the tire. One must always consider the age of the sensor and many times when the tires wear out, the sensor in near the end of its battery life.
If the sensors are replaced it can increase the cost of tire replacement quite a bit. However if not replaced and one fails later, you can incur expense due to system testing tire dismount and mounting to replace a faulty sensor. Armed with this information you just have to make a judgement call. Some tire shops strongly suggest that they are replaced every time new tires are installed.
I hope this sheds a little light on that pesky TPMS light and will help in making future decisions. If you have questions or concerns about the system you can give us a call and we are always happy to help.