For those who work in the auto repair field I'm sure you've heard about Tire Pressure Monitoring System, or TPMS for short. What most auto repair technicians do not know is that there are two main types of TPMS systems that you need to be aware of, Direct TPMS and Indirect TPMS.
This style of TPMS is what you will find on all new cars that are sold in the United States since 2008. (It is required by law that automakers equip TPMS sensors and a low tire pressure warning indicator light in the dashboard on all new cars and light trucks.) Direct TPMS works by having a TPMS sensor installed in each tire so that it can read the tire pressure and then transmits a radio frequency (RF) signal to a TPMS controller located on the vehcile. Federal law states that if a tire pressure falls below 25% of the auto-manufactures recommend tire inflation level, usually found on the driver's side door or door jamb, a light will alert the driver in the vehicle dashboard within 10 minutes.
This is so you can go inflate your tire, which I know most people reading this probably do not do.
An indirect TPMS system doesn't technically read and measure the tire pressure individually. Indirect TPMS uses wheel speed sensors to determine if a tire pressure is low. How does that work?
Basically it detects different speeds in the wheels. If a tire is underinflated and not at the correct proper PSI (pound per square inch) level it will have a slightly smaller diameter than a properly inflated tire. A smaller diameter tire with low PSI pressure will rotate slightly faster than the properly inflated tires. The indirect TPMS system will take measurements when the vehicle is turning and measures the diagonally opposed wheels. (When a vehicle makes a turn the outside wheel move slightly faster than the inside wheel to make the turn.) If the indirect TPMS makes a reading that shows that the wheels are rotating faster than usual, the TPMS warning light will be triggered on the dashboard.
As it's moniker states it is “Indirect” since the readings are not coming directly from each wheel. Rather a determination is made from wheel speeds.
Indirect TPMS vs Direct TPMS
Indirect TPMS has many advantages over Direct TPMS. Chiefly that these TPMS systems do not need additional and expensive components (TPMS sensors in the tire) to be installed on a car or light truck. An Indirect TPMS systems is inexpensive and easily added since most vehicles today have four wheel speed sensors already installed. The disadvantage? Obviously a TPMS system can't tell you which tire is low and it won't say if several tires have a low PSI, it just illuminites a light in the dashboard letting you know there is a low tire somewhere on the vehicle. Indirect TPMS doesn't meet the Federal Highway Traffic Safety Administration (FMVSS) 138 mandate which states that a TPMS system needs to detect if a tire pressure PSI falls below 25% of the automanufacturer's recommended inflation level.
Direct TPMS, while more expensive for car owners, is more accurate and largely doesn't have the problems associated with Indirect TPMS systems. Generally cars equipped with Direct TPMS will tell you exactly which tire is low. (Not always so be sure to check that spare tire.) It meets the FMVSS 138 mandate which is why it is installed on new cars being sold today. Direct TPMS systems send a continuous intermittent RF signal to the TPMS receiver to let it know which tire is low. It doesn't rely on the vehicle turning to take it's measurements.
This system is “Direct” as it take readings directly from the source.
Why you should be aware of Indirect TPMS
Obviously Direct TPMS is more accurate and what is being used on millions of vehicles being put on the road today. Why should you care about Indirect TPMS?
Indirect TPMS systems have been around for a number of years and it's likely that even if you don't see this style of tire pressuring monitoring systems everyday in the shop, you probably will at some point. You might have a customer, friend, or family member with a vehicle equipped with an Indirect TPMS system on their car and understanding how it works is never a bad idea.
What are your thoughts about Indirect TPMS vs Direct TPMS? Have any questions or thoughts? Please leave a comment below and let me know.