A tire pressure monitoring system (generally referred to as TPMS or TPM) is relatively basic in terms of the number of involved components. The pressure sensor transmitters (one transmitter mounted inside each wheel) monitor inflation data and send an FM radio signal to the system’s antenna and receiver, which then sends a digital signal to an ECU. Note that some vehicles may be equipped with one central antenna, while others feature individual antennas in the wheelwells at each corner.
A ”direct” tire pressure monitoring system (where a dedicated system exists that monitors actual tire inflation pressure) is generally calibrated to alert the driver when one or more tires have lost at least 20% to 25% of the programmed/recommended inflation pressure.
An ”indirect” system relies on tire diameter changes (via ABS wheel speed sensor readings) to alert the driver of tire pressure changes. The indirect system is long-antiquated. All new vehicles (2007 and newer) are federally mandated to feature TPMS in vehicles of 10,000 GVW or lighter.
If a “fault” signal is processed by the ECU, the in-dash tire pressure warning light will illuminate. Generally speaking, if a tire pressure problem is indicated, the warning light will illuminate constantly. If the light blinks (in most OEM systems), this indicates a system fault that must then be diagnosed with the proper diagnostic tool.
It’s important to note that whenever sensors are moved to new locations (during wheel rotation, etc.), that the ECU must be re-programmed (re-set/re-learn) in order to maintain correct location information for the system ECU. Otherwise, each sensor may transmit correct inflation data, but the ECU will then assign the pressure data to the wrong corner(s) of the vehicle. For example, if a vehicle is equipped with dash information that identifies each specific wheel location, you may see a warning that the left front tire is low, when in fact the low pressure problem may be found at the right rear (because the wheels were rotated and the sensors were never re-set). Each sensor has a unique ID imbedded in its pulse signal. The ECU receives this pulse signal and assigns (remembers) the sensor’s wheel position.
Three basic “styles” of TPM sensor/transmitters are available (NOTE: The correct OEM term for the sensor is ”wheel mounted sensor”): banded, snap-in and clamp-on. Banded sensors are affixed to the inside of the wheel using both a positioning adhesive (peel off) and a large diameter worm-drive clamp (the wheel valve is the standard valve style that has been in use for decades). Both clamp-on and snap-in sensors feature the valve stem as an integral part of the sensor. A clamp-on sensor mounts via the wheel’s valve hole and is secured with a mounting nut and is sealed with a separate grommet. The snap-in style features a traditionally mounted rubber valve style.
NOTE: It’s important that you’re able to quickly identify the presence of a snap-in style TPM sensor. The valve stem caps are longer than a traditional stem cap, and (with the cap removed) you’ll notice a beveled brass surface at the base of the cap mating area.