Recently, the low tire-pressure warning indicator on the console of my car—under two years old and with less than 15,000 miles—came on (Figure 1). It’s an indicator for a specific problem, so it is not as panic-inducing as the ill-defined, vague “Check Engine” light which can mean anything from a trivial problem such as a loose gas cap to a major impending catastrophe. I pulled over, checked the tires visually, and they all seemed fine.
Figure 1 Either of these two standard console indicators tells you that the tire pressure monitoring system (TPMS) has determined you have a problem with tire pressure. Source: Tirebuyer.com LLC
Later, after allowing the tires to cool in my driveway, I re-checked them again using two different but reasonably accurate digital-pressure gauges and, again, all tires were very close to specified values. Finally, after a few more days, I checked one more time, and the readings were still on spec and unchanged. I concluded the tires were all fine and it was problem with the tire-measurement system instead.
But I used this as an opportunity to look into this entire tire-alert system—formally called the tire pressure monitoring system or TPMS—and how it works. What I found was both impressive and worrisome. In short, each tire is fitted internally with a pressure sensor, A/D converter, microprocessor, RF link, and coin cell (Figure 2). Due to the harsh operating-environment conditions, the entire unit is potted as a single, solid module.
Figure 2 The TPMS has many mechanical parts for the valve stem and air-tight tire mounting along with all the electronics in the sealed module at the left. Source: CariD
The TPMS approach is that each tire, and often the spare tire as well, directly measures and reports its pressure periodically via either 315-MHz or 433-MHz link—depending on TPMS vendor—to a receiver unit in the car. The car, in turn, determines if the pressure is in the danger area of 25% or more below nominal value; if there’s a problem, the driver is signaled by the console indication.
However, as with many presumably good ideas, the “devil is in the details” and that’s especially true for the TPMS. My research reading indicated that there are a lot of issues associated with the TPMS, including:
- The RF link is very low power to extend battery life and is subject to internal and external EMI-induced errors as a consequence of the weak signal and the automotive environment.
- The very weak RF signal can be blocked if something metallic, such as lunch in an aluminum-foil wrapper, is put in the wrong place, which varies by car; for mine, it’s under the driver’s seat.
- The battery in the TPMS unit in the tire has a life of only 5-10 years, depending on various factors, and is not replaceable. So, the entire module must be replaced. In reality, the units for all four tires should be replaced at the same time, so we’re looking at several hundred dollars. The replacement must be the same type and frequency as the originals, of course.
- If you have to replace a tire due to failure or routine wear, the same replacement scenario applies as for end-of-battery. The TPMS module itself can also be damaged by hitting a curb or pothole.
- Not surprisingly, the sensor and entire TPMS need to self-calibrate after module replacement, meaning keep things steady for a few hours to a few days. Going on the highway and heating up the tires—and thus increasing their pressure—too soon after replacement can corrupt the calibration process. As experienced engineers know, the sensors are often exposed and are often the least reliable link in the data-acquisition signal chain.
- Transient shifts in tire temperature occurring when going from a heated garage to a cold outside can also cause false readings.
Note that these are only a few issues I found; there could be many more.
TPMS: a little background
A little background helps in understanding how the TPMS came to be mandated in cars in the United States and much of the world. In the late 1990s, a sudden surge of rollover crashes occurred primarily in Ford Explorer SUVs with Firestone tires. It was estimated that there were over 3,000 injuries and approximately 250 deaths as a result of Explorer-Firestone-related crashes. Firestone blamed Ford and Ford blamed Firestone, but either way, the accidents and deaths received a lot of publicity and demands for politicians to “do something.” So, they did the high-profile and obvious thing, and in 2000, mandated that all cars must be equipped with a TPMS indicating pressure 25% below nominal by 2007.
The various automotive-related communities had differing views on the need or wisdom of TPMS. The auto manufacturers were not happy about this since it would raise the cost of every car by about $200 to $300 and add to their BOM and manufacturing complexity. Safety advocates were strongly in favor, using the almost irrefutable argument that “if it saves just one life, it’s worth it.” Although TPMS doesn’t help in the dominant cause of tire-related accidents when there’s a sudden blowout due to hitting obstructions in the road. Average consumers probably had no idea of the upfront or ownership costs of TPMS, but it sounded like a good idea. Semiconductor and related electronics vendors were ecstatic about it, since it guaranteed a market of at least 50 million units per year for new cars and a large number for ongoing replacements.
Note that there is also an indirect, lower-cost TPMS approach, but it’s less popular due to issues of inaccuracy and inconsistency and possibility of missing a problem. This method measures the rotational speed of each tire using existing or new body-mounted sensors—no batteries or RF link needed. Tires with lower pressure will have a smaller diameter and thus higher rpm compared to the other tires, but the difference is small. Also, if several tires are underinflated, the standard of comparison itself may be invalid.
The more I looked into the reality of TPMS, the more issues I found, along with a very large list of nuisance/misleading readings, maintenance, and long-term cost concerns. In fact, my car’s voluminous owner’s manual discusses TPMS from different perspectives in five of its chapters, with all sorts of do’s and don’ts, cautions, caveats, warnings, and reminders. Mechanics can even buy a specialized instrument which reads, resets, and resyncs the TPMS units (Figure 3).
Figure 3 An instrument such as the Autel TS401 TPMS Relearn Tool can check, reset, reprogram, activate, and restore the individual tire sensors. Source: Autel/Intelligent Technology Corp.
The future of TPMS?
So, is the TPMS an inherently good idea, or is it well-intentioned idea but where the law of unintended or unforeseen consequences applies, and to a high degree? Was it just a knee-jerk “we have to do something” reaction to unfortunate events, done without understanding its broader implications? Is it a case of a partial solution that is perhaps as bad as no solution?
Maybe it would make sense to instead expect car drivers to show some responsibility and just walk around the car every once in a while, and look at the tires in a “pre-flight” check, as airline pilots do.
As for me, I’m happy with the in-warranty TPMS replacement for my car’s module which the dealer installed. If you want to more about the history, development, design, or headaches of TPMS, just Google it: it’s a very distinctive keyword.
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