Indirect Monitoring Systems
In the interest of providing a lower cost Original Equipment system, indirect tire pressure monitoring systems were developed by vehicle manufacturers wishing to comply with the law while minimizing development time and cost. Indirect systems use the vehicle's anti-lock braking system's wheel speed sensors to compare the rotational speed of one tire to that in another position on the vehicle. If one tire is low on pressure, its circumference changes enough to roll at a slightly different number of revolutions per mile than the other three tires. Reading the same signal used to support ABS systems, the vehicle manufacturers have programmed another function into the vehicle's on-board computer to warn the driver when a single tire is running at a reduced inflation pressure compared to the others.
Unfortunately, indirect tire pressure monitoring systems have several shortcomings. Indirect systems won't tell the drivers which tire is low on pressure, and won't warn the driver if all four tires are losing pressure at the same rate (as occurs during the fall and winter months when ambient temperatures get colder). Additionally, our current experience with indirect systems indicates that they can generate frequent false warnings. We have found that false warnings may occur when the tires spin on wet, icy and snow-covered roads. In these cases, the false alarms would train the driver to disregard the tire pressure monitoring system's warnings, negating its purpose completely.
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=44
