The idea of indirect monitoring of tire pressure with sensors already on the car, e.g., wheel speed from the ABS system (just look to see if wheels are spinning at different speeds and/or look at GPS distance traveled versus wheel rotations), seems to be unpopular. In trying to clean up our 2007 Infiniti M35x so that the hulk could be sold (rather than moved to the Florida Free State where one gets no points for being a frugal Yankee driving an old car into the ground), it turned out that the slow leak in one tire was actually being caused by the TPMS sensor itself. Also, the shop said that the systems in older cars usually accumulated programming mistakes that led to the display being inaccurate regarding which tire was at what pressure. An indirect system wouldn’t be subject to these human errors.
A good example of how a system that is great in theory is weak in practice? Direct TPMS is presumably engineered to work well for the three-year standard new car warranty. But the service life of a car is closer to 20 years (average age of a car on a U.S. road right now is 12.1 years).
Would it have been smarter if we’d insisted on indirect sensing that couldn’t be a new source of leaks?
Could it differentiate between tread wear and inflation? I suppose not too difficult given how gradual tread wear is.
@Andrea: It depends on how good the ABS system is and the programming it uses to monitor the “tone rings” that read how fast the wheels are turning. Most systems have some tolerance built into the systems that owners can mount tires that are slightly different in diameter than the OEM tires without causing an ABS system fault to be thrown. How closely that is monitored and reported probably depends on which car you are driving. This also causes speedometer error on cars that do not compensate for it or force the owner to recalibrate the system. My car’s speedometer reads a few MPH “slow” at 70 mph, I’m actually going approx. 67-68 MPH with my new tires and not a peep from the system. If’s 11 years old…and I don’t know how newer models behave.
Sensors for tire pressure is unnecessary. My 2005 Toyota doesn’t have them, and I’ve never missed them. But then again, I bet this car has a whole lot of expensive to repair unnecessary geegaws anyway. Luxury cars are full of them, and it’s one of the main reasons they depreciate so much. Running cost becomes excessive after 5 years, as all this stuff breaks down, and parts and labor are very high.
Tire pressure monitoring has been mandatory in US vehicles for 15 years or so. This was mandated in reaction to widespread failure of Firestone tires on Ford vehicles: https://en.wikipedia.org/wiki/Firestone_and_Ford_tire_controversy
I used to have indirect tire pressure monitoring on one of my SUVs and it was quite unreliable.
I can see the need for direct TPMS on performance cars because if you’re driving fast and develop a leak you really want to know right away, and also you presumably have the disposable income to replace the sensors at regular intervals to keep them “fresh.” That probably involves a trip to the dealership to make them talk nicely with your car’s other systems, or using something good from the aftermarket that works with your car.
I have them on my 2010 FEH and mine are still working properly at 11+ years old. I have a very slow leak in the passenger side tires, a little worse in the front than the rear, which I attribute to imperfect sealing between the tire and rim and/or porosity in the factory cast aluminum wheels.
When the front gets down to about 25 psi the light comes on, and once I saw that I decided to buy a good gauge ( https://www.popularmechanics.com/cars/g36892480/best-tire-pressure-gauges/ ) and I measure them once a week and reinflate to 40 PSI. The tires are good Pirellis that replaced the factory Michelins, which were also 11+ years old when I got the car and were so dry-rotted, cracked up (Michelin Man looked like The Thing) and hardened that driving in the rain was more like water skiing a 3900 pound car than driving.
Anyway, I spend a few minutes once a week to measure and reinflate them with an air compressor I own that has a dessicant dryer to keep the water out as much as possible. It’s a good investment if you care about your cars and very handy for other things also.
I’ve always been intrigued about what a reliable indirect system would consist of.
One thing I don’t like about the 11-year-old TPMS is that even through the CAN network with the car attached to a laptop and code-scanning software I CANNOT find the absolute readings from each tire, which is either my ignorance or by design. In my world you would just hit a button on the computer and bring up the pressures.
Addendum: Also measuring and reinflating once a week means that I’m working the valve stems harder than usual so I can probably expect to wear out the little seals and then have to replace them earlier than usual, unless I spring for new wheels and so forth.
TPMS periodically turn on in error in my 2015 Honda Fit. Resetting the system cures it. Done it since almost new happens a few times a year.
Wife once bought me a set of aircaps that had a green bit that stuck out when tire pressure was OK, never installed them, because they are depressing the valve and relying on the plastic cap to hold the air in. TPMS is just a fancier version of this cleverness.
Well, well, well.
TPMS sensor is integrated with the valve stem. To say “the leak is caused by TPMS sensor” is the same as saying “valve stem is leaking”. On the car of this age valve stem may start to leak and it has nothing to do with the actual TPMS sensor.
Re: programming. Yes, older systems required programming, which is really annoying. Modern ones usually just required press of a button (physical or on screen) once wheels are replaced and more reliable.
@SK: That’s right! I forgot! I’ll guarantee that nobody has ever replaced the seals in my TPMS valve stems, even when they mounted the Pirellis (which was the one and only tire change my car has ever had.)
>TPMS sensor is integrated with the valve stem.
Wrinkles:
@SK: I’m investigating a little more now and apparently that is not strictly true on every car. It’s time to contact the service manuals or a knowledgeable third party. But thank you because you’ve got me very interested in this now.
https://forums.finalgear.com/threads/tpms-tire-pressure-monitoring-system-are-they-integrated-with-the-valve-stem.32934/
@Alex
Of course there are exceptions, but the reason most sensors are on the valve stem is that there are no other mount points inside the typical wheel. This way you can use same rims and tires for both TPMS and non-TPMS systems.
@SK: Just wanted to say I wasn’t trying to be sardonic with you, I really did forget that and I’m asking some people I know about what I might do with my car. 11 years is a long time for a valve stem to last and I’m sure mine are the original ones that came with the car. It would be fantastic to know that my wheels are 100% because I want to strip the (peeling) clearcoat and oxidation off, polish and repaint them instead of replacing them, which can actually be done without dismounting the tires if you get a little creative.
My 2018 Honda Clarity and 2017 Honda Civic both have indirect TPMS. With indirect, you cannot tell which tire is losing pressure. The Clarity sensor goes off every couple of months because one tire has a very slow leak. It takes about a 10 PSI loss to trigger it.
The owners manual says:
The system does not monitor the tires when driving at low speed.
Conditions such as low ambient temperature and altitude change directly affect tire pressure and can trigger the low tire pressure/TPMS indicator to come on.
Tire pressure checked and inflated in:
• Warm weather can become under-inflated in colder weather.
• Cold weather can become over-inflated in warmer weather.
The low tire pressure/TPMS indicator will not come on as a result of over inflation.
The TPMS may not function properly if tire type and size are mixed. Make sure to use the same size and type of tire.
The low tire pressure/TPMS indicator may come on with a delay or may not come on at all when:
• You rapidly accelerate, decelerate, or turn the steering wheel.
• You drive on snowy or slippery roads.
• Snow chains are used.
The low tire pressure/TPMS indicator may come on under the following conditions:
• There is a heavier and uneven load on the tires, such as when towing a trailer, than the condition at calibration.
• Snow chains are used.
My 2019 Audi Q5 has indirect TPS. Has worked great for 2 years, and does point out the specific tire that has a leak. Very happy with it.
@Paul B: Thanks. Leave it to Audi. Their system is smart and sophisticated. I’m learning new things all the time.
https://www.audi-technology-portal.de/en/chassis/brakes-wheels/tire-pressure-monitoring-system
“Audi has used both systems in its vehicles, with Direct TPMS being used in earlier Audi models and an Indirect ABS (Antilock Brake System)TPMS system used in later models. The Indirect ABS based TPMS system makes use of the wheel speed sensors that are part of the antilock braking system and electronic stability control system. Audi is the first vehicle manufacturer that is standardizing the ABS based Indirect TPMS system. All Audi 2011 and up vehicles are to be equipped with the ABS Indirect TPMS system. ”
https://www.tpmsdirect.com/Audi_ABS_Indirect_TPMS_Reset_Procedures_a/702.htm
@Alex – thanks for those links, I had no idea this info existed. I will also say that it’s amazing how complicated and confusing their 500+ page manual is, and hard to find things (even searching a .pdf), especially as it seems it’s almost a direct translation from the original German I would guess. The modern car is a maze of firmware that allows incredible things and can be incredibly frustrating. Like most modern devices it seems.
@Paul B: Quite true and sometimes I don’t know what’s worse or better. On the balance it’s better. I used to own a 1987 Audi 5000 CS Turbo Quattro (and several others from that approximate timeframe over the years…you could buy them for a tiny fraction of their value due to the bogus CBS 60 Minutes “unintended acceleration” smear/fearmongering/fiasco which destroyed their value for about a decade) and it was top-of-the-line that year. It had manually locking front and rear differentials and one of the first Bosch ABS systems that was plumbed into a very complicated and failure-prone high-pressure hydraulic booster system. Then there was the engine electronics – oh my dear. The Bentley service manual for that car was $125, about 3 inches thick, IIRC and it had all the wiring diagrams. I learned how to curse in German while drinking German beer quite well. On the balance I prefer plugging in a laptop computer or suchlike and letting it interrogate the network and give me DTCs and PIDs, even with all the attendant complexity. Vorsprung Durch Technik Fo’ Evah! as the Audi Rednecks from Jersey say.
Sorry manually locking center and rear differentials. This was before Audi started using TORSENs.
https://www.mcall.com/news/mc-xpm-1987-10-17-2607337-story.html
That car was almost unstoppable in the snow, particularly with a set of Nokian Wheels not included with purchase of tires Nokian Hakkapeliittas with studs. Folks in Jeeps and such would get stuck and you’d roll up with your bright red Audi four door sedan and yank them out of trouble, and they’d be like: “What?” Fun days, but no TPMS of any kind yet (no airbags or infotainment systems either, but they did have a cool computer for mileage, etc.) but I loved that car when it was running well. I think I paid $3500 for mine.
@Alex – thanks for all of the history and you could work on my car anyday! And too funny about the Audi rednecks 🙂
@Paul B: You’re welcome. And I’m sorry to the editors but I would like to add one more thing to this history for the Journalism Review:
The allegedly murderous “unintended acceleration” highlighted by the crack journalists at 60 Minutes was caused by THEM because they doctored a car to make the problem much more severe than it actually was. If I recall everything correctly, the root cause of the problem was the Idle Stabilization Valve on some of the cars. It existed primarily to boost the idle speed when the (rather small, 2.3 liter) engine was under strain at idle due to the air conditioning compressor and/or alternator+hydraulic brake booster pump (which operated at a very high pressure.) Those combined loads were a big drag on the engine at idle, so Audi had a valve that would let the system adjust it precisely using a pulse width modulated duty cycle.
Once in a while that valve would stick open because of various types of contamination – particularly if the owner didn’t care much about maintenance – and the idle would increase to around 1500-2000 RPM and stay there. This was a little bit of an annoyance with automatic transmission cars because it would cause the car to “creep forward” a bit more than usual. If you put your foot on the brake while shifting out of Park or simply stepped a little on the brake pedal, it was no big deal, you took it to the dealer, they cleaned the valve and that was it. It happened on one of my cars and I cleaned it myself with some solvent, problem solved. It was certainly never an “out of control acceleration” full throttle thing until 60 Minutes decided to dramatize it for their cameras.
After their “Courageous” fearmongering report destroyed the value of the cars, Audi put a lock mechanism on the shifter that required the driver to have their foot on the brake firmly while shifting out of Park, which is now standard on many other makes and models of cars, including my Ford Escape Hybrid.
I’m amazed this website still exists, but some people knew what was really going on and capitalized on it by selling things like replacement chips with different ECU boost maps and wastegate springs to make them accelerate much, much harder. Fun days for real.
http://www.intendedacceleration.com/
Firestone had another scandal, similar in that the problem was shedding tread. It was the 500 radial, introduced in 1971, built in Decatur, Illinois, too. The wikipedia article google found also noted that in 1950, GM, Standard Oil and Firestone were convicted of criminal conspiracy for buying streetcar companies and converting the mass transportation to buses. Just crime after crime with Firestone, but they had stores that put out good Christmas vinyl albums 60+ years ago, too.
Indirect TPMS uses the wheel speed sensors (for ABS, traction control, etc) to detect a rotational speed difference between the wheels. That’s great because it’s cheap, easy to maintain and works regardless of the specific wheels on the car. On the other hand, it sucks because if all the tires lose pressure at the same rate (like when temperature changes), it doesn’t tell you. Also, in my experience, when it does give you a warning, it doesn’t tell you the specific tire having the problem.
Direct TPMS (I’ve never heard it called “direct”) uses actual pressure sensors in the valve stems and can relay that actual pressure to the car. That’s great because you know the actual pressure (important) and you know exactly which tire is having a problem. It sucks because it’s more costly, you have to buy sensors if you get another set of wheels and the batteries in the sensors wear out. Yes, they can also leak but all valve stems can leak.
I have a motorcycle that has direct TPMS and it displays the actual pressure on the bike’s display. It’s fanstastic.