There was a time when wise older people warned you to check your tire pressure regularly. We never did, and would eventually wind up with a flat or, worse, a blowout. These days, your car will probably warn you when your tires are low. That’s because of a class of devices known as tire pressure monitoring systems (TPMS).

If you are like us, you see some piece of tech like this, and you immediately guess how it probably works. In this case, the obvious guess is sometimes, but not always, correct. There are two different styles that are common, and only one works in the most obvious way.

Obvious Guess

We’d guess that the tire would have a little pressure sensor attached to it that would then wirelessly transmit data. In fact, some do work this way, and that’s known as dTPMS where the “d” stands for direct.

Of course, such a system needs power, and that’s usually in the form of batteries, although there are some that get power wirelessly using an RFID-like system. Anything wireless has to be able to penetrate the steel and rubber in the tire, of course.

But this isn’t always how dTPMS systems worked. In days of old, they used a finicky system involving a coil and a pressure-sensitive diaphragm — more on that later.

Many modern systems use iTPMS (indirect). These systems typically work on the idea that a properly inflated tire will have a characteristic rolling radius. Fusing data from the wheel speed sensor, the electronic steering control, and some fancy signal processing, they can deduce if a tire’s radius is off-nominal. Not all systems work exactly the same, but the key idea is that they use non-pressure data to infer the tire’s pressure.

This is cheap and requires no batteries in the tire. However, it isn’t without its problems. It is purely a relative measurement. In practice, you have to inflate your tires, tell the system to calibrate, and then drive around for half an hour or more to let it learn how your tires react to different roads, speeds, and driving styles.

Changes in temperature, like the first cold snap of winter, are notorious for causing these sensors to read flat. If the weather changes and you suddenly have four flat tires, that’s probably what happened. The tires really do lose some pressure as temperatures drop, but because all four change together, the indirect system can’t tell which one is at fault, if any.

History

The first passenger vehicle to offer TPMS was the 1986 Porsche 959. Two sensors made from a diaphragm and a coil are mounted between the wheel and the wheel’s hub. The sensors were on opposite sides of the tire. With sufficient pressure on the diaphragm, an electrical contact was made, changing the coil value, and a stationary coil would detect the sensor as it passed. If the pressure drops, the electrical contact opens, and the coil no longer sees the normal two pulses per rotation. The technique was similar to a grid dip meter measuring an LC resonant circuit. The diaphragm switch would change the LC circuit’s frequency, and the sensing coil could detect that.

If one or two pulses were absent despite the ABS system noting wheel rotation, the car would report low tire pressure. There were some cases of centrifugal force opening the diaphragms at high speed, causing false positives, but for the most part, the system worked. This isn’t exactly iTPMS, but it isn’t quite dTPMS either. The diaphragm does measure pressure in a binary way, but it doesn’t send pressure data in the way a normal dTPMS system does.

Of course, as you can see in the video, the 959 was decidedly a luxury car. It would be 1991 before the US-made Corvette acquired TPMS. The Renault Laguna II in 2000 was the first high-volume car to have similar sensors.

Now They’re Everywhere

In many places, laws were put in place to require TPMS in vehicles. It was also critical for cars that used “run flat” tires. The theory is that you might not notice your run flat tires were actually flat, and while they are, as their name implies, made to run flat, they also require you to limit speed and distance when they are flat.

Old cars or other vehicles that don’t have TPMS can still add it. There are systems that can measure tire pressure and report to a smartphone app. These are, of course, a type of dTPMS.

Problems

Of course, there are always problems. An iTPMS system isn’t really reading the tire pressure, so it can easily get out of calibration. Direct systems need battery changing, which usually means removing the tire, and a good bit of work — watch the video below. That means there is a big tradeoff between sending data with enough power to go through the tire and burning through batteries too fast.

Another issue with dTPMS is that you are broadcasting. That means you have to reject interference from other cars that may also transmit. Because of this, most sensors have a unique ID. This raises privacy concerns, too, since you are sending a uniquely identifiable code.

Of course, your car is probably also beaming Bluetooth signals and who knows what else. Not to even mention what the phone in your car is screaming to the ether. So, in practice, TPMS attacks are probably not a big problem for anyone with normal levels of paranoia.

An iTPMS sensor won’t work on a tire that isn’t moving, so monitoring your spare tire is out. Even dTPMS sensors often stop transmitting when they are not moving to save battery, and that also makes it difficult to monitor the spare tire.

The (Half Right) Obvious Answer

Sometimes, when you think of the “obvious” way something works, you are wrong. In this case, you are half right. TPMS reduces tire wear, prevents accidents that might happen during tire failure, and even saves fuel.

Thanks to this technology, you don’t have to remember to check your tire pressure before a trip. You should, however, probably check the tread.

You can roll your own TPMS. Or just listen in with an SDR. If biking is more your style, no problem.

Once upon a time, a car phone was a great way to signal to the world that you were better than everybody else. It was a clear sign that you had money to burn, and implied that other people might actually consider it valuable to talk to you from time to time.

There was, however, a way to look even more important than the boastful car phone user. You just had to rock up to the parking lot with your very own in-car fax machine.

Dial It Up

Today, the fax machine is an arcane thing only popular in backwards doctor’s offices and much of Japan. We rely on email for sending documents from person A to person B, or fill out forms via dedicated online submission systems that put our details directly in to the necessary databases automatically. The idea of printing out a document, feeding it into a fax machine, and then having it replicated as a paper version at some remote location? It’s positively anachronistic, and far more work than simply using modern digital methods instead.

Back in the early 90s though, the communications landscape looked very different. If you had a company executive out on the road, the one way you might reach them would be via their cell or car phone. That was all well and good if you wanted to talk, but if you needed some documents looked over or signed, you were out of luck.

Even if your company had jumped on the e-mail bandwagon, they weren’t going to be able to get online from a random truck stop carpark for another 20 years or so. Unless… they had a fax in the car! Then, you could simply send them a document via the regular old cellular phone network, their in-car fax would spit it out, and they could go over it and get it back to you as needed.

Of course, such a communications setup was considered pretty high end, with a price tag to match. You could get car phones on a wide range of models from the 1980s onwards, but faxes came along a little later, and were reserved for the very top-of-the-line machines.

Mercedes-Benz was one of the first automakers to offer a remote fax option in 1990, but you needed to be able to afford an S-Class to get it. With that said, you got quite the setup if you invested in the Büro-Kommunikationssystem package. It worked via Germany’s C-Netz analog cellular system, and combined both a car phone and an AEG Roadfax fax machine. The phone was installed in the backrest of one of the front seats, while the fax sat in the fold-down armrest in the rear. The assumption was that if you were important enough to have a fax in the car, you were also important enough to have someone else driving for you. You also got an AEG Olyport 40/20 laptop integrated into the back of the front seats, and it could even print to the fax machine or send data via the C-Netz connection.

Not to be left out, BMW would also offer fax machines on certain premium 7 Series and L7 limousine models, though availability was very market-dependent. Some would stash a fax machine in the glove box, others would integrate it into the back rest of one of the front seats. Toyota was also keen to offer such facilities in its high-end models for the Japanese market. In the mid-90s, you could purchase a Toyota Celsior or Century with a fax machine secreted in the glove box. It even came with Toyota branding!

Ultimately, the in-car fax would be a relatively short-lived option in the luxury vehicle space, for several reasons. For one thing, it only became practical to offer an in-car fax in the mid-80s, when cellular networks started rolling out across major cities around the world.

By the mid-2000s, digital cell networks were taking over, and by the end of that decade, mobile internet access was trivial. It would thus become far more practical to use e-mail rather than a paper-based fax machine jammed into a car. Beyond the march of technology, the in-car fax was never going to be a particularly common selection on the options list. Only a handful of people ever really had a real need to fax documents on the go. Compared to the car phone, which was widely useful to almost anyone, it had a much smaller install base. Fax options were never widely taken up by the market, and had all but disappeared by 2010.

The Toyota Celsior offered a nice healthy-sized fax machine in the 1990s, but it did take up the entire glove box.

These days, you could easily recreate a car-based fax-type experience. All you’d need would be a small printer and scanner, ideally combined into a single device, and a single-board computer with a cellular data connection. This would allow you to send and receive paper documents to just about anyone with an Internet connection. However, we’ve never seen such a build in the wild, because the world simply doesn’t run on paper anymore. The in-car fax was thus a technological curio, destined only to survive for maybe a decade or so in which it had any real utility whatsoever. Such is life!