The Limitations of Indirect, ABS-Based TPM Systems
Two types of tire pressure monitoring systems are available on the market today: Direct TPMS and Indirect TPMS. Direct TPMS sensors utilize wheel-mounted sensors that collect accurate pressure and temperature data directly from the tire. Data is then relayed via radio frequency to the vehicle's ECU. The ECU then interprets the data and warns the driver of a potential tire pressure issue IF one or more tires is underinflated by 25% or more.
The indirect tire pressure monitoring system is a bit more tricky, as it interprets tire pressure data based on the rotational wheel of each wheel.
The following excerpt is taken from Schrader International case study, "North American Utilization of TPMS."
Indirect TPMS approximate tire pressure by using data gathered by the antilock brake system (ABS), sending it to the electronic stability program (ESP) ECU for calculation and interpretation.
Once an indirect TPMS system alerts the driver, all tires must be checked to discover which tire or tires are at risk. Then the system must be manually reset- but new tire pressure data will not be available until the car has been driven long enough to generate new data for calculations.
When it comes time to replace tires on a car with indirect TPMS, the choices for tires are limited, because indirect TPMS functions properly only if certain tires are installed. The technology and performance limitations of indirect systems make Direct TPMS an easy choice for OEM's.
Carl Wacker, Vice President of Sales and Marketing for Schrader states, "OEM customers have technology choices for their TPMS system, and our customers choose Direct TPMS systems for good reasons. Direct TPMS technology ensures the safety of their customers and provides flexibility across their product range."