Tech Stuff

Tire pressure monitoring systems

We've discussed the subject of TPMS (tire pressure monitoring systems) in the past, but this is a topic that can always benefit 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.

Typical components of a “clamp-on” style sensor. The sealing washer seats against the inside surface of the wheel’s valve hole. The seal is placed at the outboard side of the hole, captured behind the hex nut.
<p>Typical components of a &ldquo;clamp-on&rdquo; style sensor. The sealing washer seats against the inside surface of the wheel&rsquo;s valve hole. The seal is placed at the outboard side of the hole, captured behind the hex nut.</p>


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 25% of the recommended cold inflation pressure (placard pressure). A direct style tire pressure monitoring system (most widely in use today) features a pressure sensor/transmitter installed inside each wheel. The sensor constantly reads inflation pressure. The sensor then sends a radio wave signal to a receiver located in the vehicle. When inflation pressure falls below a predetermined level, a warning light illuminates on the instrument cluster. Depending on the year, make and model of vehicle, this warning may simply indicate a low-pressure warning, leaving it up to the owner or technician to then determine which tire location is underinflated; or the warning system may indicate which location (LF, RF, LR, RR) is underinflated.


Instead of monitoring actual inflation pressure, an indirect system relies on the vehicle’s wheel speed sensors for the 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, one challenge is  if all four tires are underinflated by the same amount, the system may not detect a difference, and no alert warning may be provided. A direct TPMS is much faster and more accurate by comparison.

Indirect systems can also cause false tire pressure warnings if/when tires spin on slippery surfaces. If this happens often enough, the driver may be accustomed to ignoring the warnings and may ignore actual low-pressure alerts.

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-in. 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-in 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 that allows the valve stem to be pulled through the hole and “snapped” into place similar to a traditional valve.

Snap-in sensors feature a “conventional” valve housing that pulls through the valve hole in the wheel.
<p>Snap-in sensors feature a &ldquo;conventional&rdquo; valve housing that pulls through the valve hole in the wheel.</p>

NOTE: When servicing any TPMS sensor, do not use a brass valve core. This can result in dissimilar metal reaction and can lead to corrosion issues. Use only the valve cores provided with specific TPMS service kits, which are special nickel plated brass. Whenever a valve core has been removed, always replace it with a new core. Also, only install a valve cap that is designed for use with TPMS. This should a plastic cap and will feature an internal O-ring for better sealing. The use of a steel or brass cap can also lead to dissimilar metal reaction.


Vehicle relearn and sensor programming are completely different procedures. Confusion is common due to the availability of aftermarket TPMS sensors that are offered as “programmable,” “universal” or “cloneable” sensor units. These sensors must be programmed with the proper protocol/software application information for the vehicle before they can be installed on the wheel. An exception is VDO’s REDI-Sensor which does not require sensor programming prior to installation (these sensors are already loaded with protocols needed to function with the vehicle’s system).

Regardless of the brand of sensor, once programmed and installed, the sensors must then be relearned to the vehicle, even if only one sensor has been replaced. It’s not uncommon for shops to overlook the necessary vehicle relearn step, proceeding under the assumption that the sensor programming was all that was required. This misunderstanding can result in comebacks and customer complaints.

Vehicle relearn is a standard and mandatory procedure whenever a sensor has been replaced, and that includes both OE or aftermarket sensors, in order to ensure that the TPM system functions properly. Some vehicles such as specific models from Chrysler and Mazda automatically learn replacement sensors during an initial drive cycle. Other than these exceptions, a TPMS scan tool is required.

NOTE: Since relearn procedures vary among automaker brands, it’s necessary to determine the correct procedure for any given vehicle before starting the relearn process.

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