Tire pressure monitoring systems By Mike Mavrigian
W
e’ve discussed the subject of TPMS (tire pressure monitoring systems) in the past, but this is a topic that can always beneft from an update. Considering the proliferation of TPMS in today’s market, it’s vital that we gain as much understanding of these systems as possible.
location is under-infated; or the warning system may indicate which location (LF, RF, LR, RR) is under-infated.
INDIRECT TPMS
Instead of monitoring actual infation pressure, an indirect system relies on the vehicle’s ABS to monitor tire rotational speed (since a lower-pressure tire will rotate at a different speed due to the decrease in overall tire diameter). When the ABS detects a different rotational speed, it alerts the driver. This works as designed as long as the system detects a difference in rotational speed as compared to the remaining tires. However, if all four tires are under-infated by the same amount, the system may not detect a difference, and no alert warning Typical components of a “clamp-on” style may be provided. A direct TPMS is much sensor. The sealing washer seats against the inside surface of the wheel’s valve hole. more accurate by comparison. The seal is placed at the outboard side of the Indirect systems can also cause false tire hole, captured behind the hex nut. pressure warnings if/when tires spin on slippery surfaces. If this happens often DIRECT TPMS enough, the driver may be accustomed A “direct” tire pressure monitoring system to ignoring the warnings and may ignore (where a dedicated system exists that actual low-pressure alerts. monitors actual tire infation pressure) is To my knowledge, the indirect system was generally calibrated to alert the driver when no longer is use in new vehicles as of 2007, one or more tires have lost at least 20% when automakers switched over to direct to 25% of the programmed/recommended systems. infation pressure. A direct style tire presThree basic “styles” of TPM sensor/transsure monitoring system (most widely in mitters are available (NOTE: The correct use today) features a pressure sensor/ OEM term for the sensor is “wheel mounted transmitter installed inside each wheel. The sensor”): banded, snap-in and clamp-on. sensor constantly reads infation pressure. Banded sensors are affxed to the inside The sensor then sends a radio wave signal of the wheel using both a positioning to a receiver located in the vehicle. When adhesive (peel off) and a large diameter infation pressure falls below a predeterworm-drive clamp (the wheel valve is the mined level, a warning light illuminates on standard valve style that has been in use the instrument cluster. Depending on the for decades). Both clamp-on and snap-in year, make and model of the vehicle, this sensors feature the valve stem as an intewarning may simply indicate a low-pressure gral part of the sensor. A clamp-on sensor warning, leaving it up to the owner or mounts via the wheel’s valve hole and is technician to then determine which tire secured with a mounting nut and is sealed
14 | March/April 2014
