Home » How ‘Indirect Tire Pressure Monitoring Systems’ Work And Why We Should Celebrate Their Return

How ‘Indirect Tire Pressure Monitoring Systems’ Work And Why We Should Celebrate Their Return

Under Pressure
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Ding! A symbol that looks like a baseball bat dropped into a punchbowl appears on your dashboard. You have a tire that’s low on air, and your car’s alerted you to it far before the heat and friction of underinflation cause odd tire wear or a panel-bending blowout. Well, either that or one of your tire pressure monitoring sensors has died. But what if there was a better way of keeping tabs on tire pressure? One that perhaps trades some precision for being completely maintenance-free? Well, such a thing used to exist, then the federal government killed it, but now it’s coming back. May I introduce you to indirect tire pressure monitoring?

The tire pressure monitoring systems in most American cars feature four sensors and a receiver, or sometimes four sensors and a combination receiver-transmitter unit. See, those expensive little sensors are jam-packed full of tech, from the battery to the pressure sensor to the analog-digital converter to a radio frequency transmitter sometimes used in conjunction with a receiver inside each sensor to share data with the tire pressure monitoring system receiver or receiver/transmitter inside the vehicle. This system of wireless data transmission explains why you could theoretically build a cheap pressure chamber out of pipe and a schraeder valve, dump four sensors in there, pump it full of air, throw it in the trunk, and the TPMS system will be none the wiser. It also explains why these things cost a mint, adding three figures to the cost of, say, a winter tire and wheel package.

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However, indirect tire pressure monitoring doesn’t add any hardware to a car, so they should be cheaper even factoring in the software development (and tire changes should be easier). In fact, an indirect system just takes signals the car’s already generating and uses them to discern if a tire’s low on air. An underinflated tire will have a smaller overall diameter than properly-inflated tire, which should get picked up on the wheel speed sensors used for ABS as a difference in wheel speed. If the difference is great enough, the vehicle’s electronics will notice and illuminate a warning light on the dashboard. In principle, “indirect” tire pressure monitoring is perfect for anyone whose last name isn’t Verstappen or Vettel. There are no expensive wheel-mounted sensors that eventually crap out or may get damaged in the tire mounting process, and winter tire swaps are extremely easy. However, there’s a reason why indirect tire pressure monitoring briefly disappeared from the American market.

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In 2005, the federal government mandated a phase-in of tire pressure monitoring systems in every car, with a firm deadline of September 2007. However, talks about mandating TPMS started years earlier, due to a headline-making scandal involving Ford Explorers and Firestone tires. Underinflation, blowouts, and a whole lot of mud-slinging over a recall prompted Congress to pass the TREAD Act, a piece of legislation primarily focused on recalls and reporting. However, section 13 of the TREAD Act specified that:

Not later than 1 year after the date of the enactment of this Act, the Secretary of Transportation shall complete a rulemaking for a regulation to require a warning system in new motor vehicles to indicate to the operator when a tire is significantly under inflated. Such requirement shall become effective not later than 2 years after the date of the completion of such rulemaking.

Well, so much for the deadline. Eventually, this tiny little section of the TREAD act led to Federal Motor Vehicle Safety Standards section 138, a tire pressure monitoring system mandate with very specific criteria. A critical part of the test certifying tire pressure monitoring systems for use on American vehicles is to:

Stop the vehicle and deflate any combination of one to four tires until the deflated tire(s) is (are) at 7 kPa (1 psi) below the inflation pressure at which the tire pressure monitoring system is required to illuminate the low tire pressure warning telltale.

Sounds like simple criteria, right? Unfortunately, most older indirect TPMS systems can fail this test. From NHTSA:

There are two types of TPMSs currently available, direct TPMSs and indirect TPMSs. Direct TPMSs have a tire pressure sensor in each tire. The sensors transmit pressure information to a receiver. Indirect TPMSs do not have tire pressure sensors. Current indirect TPMSs rely on the wheel speed sensors in an anti-lock braking system (ABS) to detect and compare differences in the rotational speed of a vehicleís wheels. Those differences correlate to differences in tire pressure because decreases in tire pressure cause decreases in tire diameter that, in turn, cause increases in wheel speed.

To meet the four-tire, 20 percent alternative, vehicle manufacturers likely would have had to use direct TPMSs because even improved indirect systems would not likely be able to detect loss of pressure until pressure has fallen 25 percent and could not detect all combinations of significantly under-inflated tires. To meet the three-tire, 25 percent alternative, vehicle manufacturers would have been able to install either direct TPMSs or improved indirect TPMSs, but not current indirect TPMSs.

As I mentioned before, early indirect tire pressure monitoring worked simply by referencing the speed of all four tires against each other using a vehicle’s wheel speed sensors. The theory is that an underinflated tire has a slightly different diameter than a properly inflated tire, and will therefore spin at a different speed. While this eliminates pesky TPMS sensors, it does come with a blind spot — if the pressure in all four tires is reduced by the same amount, the TPMS light won’t come on. After all, if all four tires are rotating at the same speed, a computer only referencing relative wheel speed won’t notice anything’s wrong. Early indirect tire pressure monitoring also has a limitation when it comes to certain staggered tire setups that run different overall tire diameters on each axle. For those vehicles, similar deflation across any one axle may cause a false negative.

Mazda Cx 5 Indirect Tire Pressure Monitoring System

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However, since the advent of FMVSS 138, indirect tire pressure monitoring systems have evolved substantially, with Brake & Front End reporting that “New systems are taking advantage of better wheel speed sensors and modules to make indirect systems work.” A specific example is how Infineon Technologies has a patent on indirect tire pressure monitoring using “an electronic control unit (ECU) coupled to the ABS and configured to process the sensed signals using a multidimensional resonance frequency analysis (MRFA) that includes a spectral analysis identifying at least two tire vibration modes in the wheel speed signal and isolates at least one characteristic affecting the at least two tire vibration modes.” Translation? By using spectral analysis, this system can detect if all four tires are losing pressure simultaneously.

Modern Tire Dealer, a website that’s apparently the “premier source of news, research and market trend analysis,” breaks all this down further, discussing Audi’s new indirect system from supplier Nira Dynamics AB called “TPI.” After noting that it’s easy enough for an indirect system to use wheel sensors to figure out if one, two, or three wheels are deflated (since the tire radius changes, and thus the deflated wheels will have to rotate faster to “keep up”), the issue of four simultaneously-low wheels comes up:

TPI also features other components analyzing high-frequency components of the wheel speed signals stemming from different vibration modes of the wheel. These approaches are applied to each wheel separately, and by combining this with the previously mentioned radius analysis, TPI can detect under-inflation in one, two, three and four tires simultaneously.

The article goes on to say that there’s an “initial calibration or learning phase” to figure out what the normal “characteristic parameter values” should be with fully inflated tires. Those are later used to compare what the vehicle is reading from the sensors as a user drives the vehicle. “The driver should reset the system and re-initialize calibration whenever he or she has made a change to the tires, the tire pressures or, in general, to the wheels,” the article continues before saying:

TPI is designed to give a warning for under-inflations of 25% or more compared to the nominal pressure level, which the system learns during the calibration phase (25% is the required detection level under FMVSS 138).

The best statement regarding the reliability of the system at this point is probably that TPI (has been) in production for the Audi TT since 2006 and for the Audi A5/A4… and has received positive reactions from customers and quality assurance people alike. Further, since TPI is a software-based system using existing and well-proven components of the ABS/ESC system, in particular the wheel speed sensors, it’s optimally simple and robust. The key point here is that software doesn’t break!

These developments allowed these indirect systems to slowly start trickling back into the American market in the 2010s. The first-generation Mazda CX-5 (see above) used an indirect tire pressure monitoring system, as do many Volkswagen Group cars and most newer Hondas. For the vast majority of drivers, it’s simply a better, more cost-effective, maintenance-free solution compared to direct tire pressure monitoring.

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So what if you’re a tire pressure gauge-carrying scofflaw with a car from the late aughts and don’t need no stinkin’ TPMS sensors? Well, depending on the make and model of your car, you may be able to “code in” indirect TPMS. For instance, most Bangle-era BMWs will let you code in Flat Tire Monitoring, an indirect tire pressure monitoring system that uses relative wheel speed derived from wheel speed sensor data instead of an expensive tire pressure monitoring sensor in each wheel. Of course, we don’t advise breaking any laws, but we do think it’s funny that tire pressure checks used to be part of normal driving routine, and now many people simply don’t carry a cheap tire pressure gauge in their car.

(Photo credits: Thomas Hundal, Amazon, Mazda)

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My Skoda is the Most Superb
My Skoda is the Most Superb
8 months ago

My 2010 MK6 Golf TDI had direct TPMS sensors and so when I replaced the wheels with 18s from a GLI I didn’t bother also reinstalling the TPMS sensors, meaning I had the light always on in my dash.

Was pleasantly surprised to find out that my 2012 MK6 GTI that I bought once the TDI was bought back from VW had the indirect system.

Considering my family members mostly own Ford products who periodically have TPMS sensors go out, I will celebrate the indirect system’s continued permeation.

Ben
Ben
8 months ago

we do think it’s funny that tire pressure checks used to be part of normal driving routine, and now many people simply don’t carry a cheap tire pressure gauge in their car.

Probably because direct TPMS can tell you the pressure in any tire at any given time. Way easier than pulling out the old school gauge and walking around to all four tires. It’s a quality-of-life improvement that also means I keep closer track of my tire pressures than I would if I had to check them manually.

I will not celebrate the return of indirect TPMS. Sure, direct systems have their drawbacks too, but aftermarket sensors are not that expensive and it doesn’t cost that much to have them installed by your local Discount Tire or whatever. It’s worth $100 every ten years (or more) to not need to carry a pressure gauge in all of my cars.

Adam EmmKay8 GTI
Adam EmmKay8 GTI
8 months ago
Reply to  Ben

The sensors do not work at any given time. They go to sleep to save battery when vehicle is parked. You have to drive a block or 2 for them to wake up and update dash readings.
Otherwise if you just turn on ignition after car was parked for few hours you get higher readings from last time sensor updated dash, about a minute before you turned it off previously.
Every morning my Wrangler says tires are at 41 PSI, by the end of the block tire pressure is down to 36 PSI and climbing the farther I drive (tires heat up and air pressure increases naturally)

Myk El
Myk El
8 months ago

My family had a couple of cars with indirect systems. They were generally correct. I had one issue my dealer couldn’t get to stay cleared but when I needed a tire repaired, the place I took it to got it properly corrected. The one my Nissan any my parents’ Hyundai used were super sensitive to cold weather. You could pretty much guarantee it was going to come on if it dipped below 20.

Adam EmmKay8 GTI
Adam EmmKay8 GTI
8 months ago
Reply to  Myk El

That is because tire air pressure naturally changes with temperature. Air’s molecules physically increase in size the warmer they get and that increases air pressure reading.
That Nissan and Hyunday was correct – tire pressure was low with lower temperature and needed to be filled to air pressure it needs before wheel and tire warms up from driving.

Myk El
Myk El
8 months ago

Perhaps I should have clarified. I wasn’t accusing the system of being incorrect, but more like recommended pressure of 33 psi and it would go off at 31. It wasn’t off by even 10%. But in Colorado where I lived at the time, you can have a high and low vary 30+ degrees F in a given day, by lunch it’d be back in acceptable and the light goes off. I had the Nissan for 7 years, I got used to it. Just a thing that it did.

Adam EmmKay8 GTI
Adam EmmKay8 GTI
8 months ago
Reply to  Myk El

set it to 33PSI when it is the coldest and enjoy better MPGs with more rattles!

Madewithgenuineparts
Madewithgenuineparts
8 months ago

Counterpoint: indirect TPMS is bad and annoying, and easy to defeat

Had a ’13 CX-5 and a ’16 SportWagen with it and a ’19 G70 and a ’23 Mazda3 with proper TPMS, and the wheel-by-wheel direct system is significantly easier to diagnose/live with as far as knowing which tire is low, and getting it fixed in a timely manner with proper verification.

Adam EmmKay8 GTI
Adam EmmKay8 GTI
8 months ago

My 2022 GTI tells me which tire is low when it is low (for example right rear) instead of just showing that some of the tires are low.
My mom’s Rav4 just says that at least one tire is low on pressure without telling which one.

My G8 and Wrangler had direct tire pressure system. In G8 I had to teach it which wheel was on which corner without any tools, Wrangler knows which tire is on what corner by itself.
My cousin’s F150 and Chevy SS need special tools to teach car which wheel is on what corner.

Madewithgenuineparts
Madewithgenuineparts
8 months ago

Turn-of-the-bankruptcy GM systems are the worst, yes (also own a 2008 Aura, have to teach it the corners and my Chevy dealer never does)

Adam EmmKay8 GTI
Adam EmmKay8 GTI
8 months ago

It worked fine for 13 years.
I like how me doing oil change voids my bumper to bumper warranty but dealer’s trained certified by manufacturers “technicians” do not know how to rotate tires on an Aura properly.

James Mason
James Mason
8 months ago

Poor people who can’t afford to have 4 tires of the same type and/or size will continue to have to set the TPMS system to ‘ignore’ by applying a square of electrical tape over the warning light. Of course, it is unlikely they would have had direct TPMS sensors installed anyway.

Sam I am
Sam I am
8 months ago

Setting aside cost/complexity, direct is far superior as far as the information it suppplies. I can push a button on the steering wheel and see the real-time pressure in all the tires including the spare. Of course I carry at least one gauge as well. With indirect, no information is gathered until the car is in motion for some amount of time. If a tire has leaked down overnight on the side you don’t see when you get in the car, you can drive far enough to damage the tire and wheel (not that far, actually) before you realize there’s a problem. *Not a hypothetical..

Adam EmmKay8 GTI
Adam EmmKay8 GTI
8 months ago
Reply to  Sam I am

The sensors do not work at any given time. They go to sleep to save battery when vehicle is parked. You have to drive a block or 2 for them to wake up and update dash readings.
Otherwise if you just turn on ignition after car was parked for few hours you get higher readings from last time sensor updated dash, about a minute before you turned it off previously.
Every morning my Wrangler says tires are at 41 PSI, by the end of the block tire pressure is down to 36 PSI and climbing the farther I drive (tires heat up and air pressure increases naturally)

Cheap Bastard
Cheap Bastard
8 months ago

“The key point here is that software doesn’t break!”

Oh?

https://imaginovation.net/blog/most-software-is-broken-but-here-is-how-to-fix-it/

Phantom Pedal Syndrome
Phantom Pedal Syndrome
8 months ago

That’s OK

Phantom Pedal Syndrome
Phantom Pedal Syndrome
8 months ago

Ba da be bop bop

Phantom Pedal Syndrome
Phantom Pedal Syndrome
8 months ago

No.. it’s:
Um ba ba be
Um ba ba be
De day da
Ee day da –

Phantom Pedal Syndrome
Phantom Pedal Syndrome
8 months ago

Nice! I got you:
Chippin’ around – kick my brains around the floor
These are the days it never rains but it pours
Ee do ba be
Ee da ba ba ba
Um bo bo
Be lap
People on streets – ee da de da de
People on streets – ee da de da de da de da

Phantom Pedal Syndrome
Phantom Pedal Syndrome
8 months ago

Ok!

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