Tyre Pressure Monitoring Systems for Dummies: How they work
Do tyre pressure monitoring systems actually need calibration? Is the dealership ripping you off if you’re forced to seek help?
Here’s a question from Tim, regarding the TPMS on his 2017 Subaru Impreza S.
"The manual says that if I get my tyres rotated or if my sensors are reading one value and my own tyre pressure gauge reads another I should take my car to the stealership for calibration. Isn’t this illegal in Australia? The manual doesn’t list any way the end user can reset or re-calibrate the TPMS system. "
- Tim
Our hero Tim says he’s religious in routinely checking the fluids and tyre pressures on his car - which is awesome and more people should do this. In fact, here’s a dead-easy three-step checklist for checking your car’s running fine >>
Tim says he has a bunch of tyre pressure gauges and they all read pretty much the same but the Tyre Pressure Monitoring System reads about 3-5psi higher than all those gauges. In other words, the tyres are actually at 32, Tim says, but the TPMS says 35-37.
The barbed wire enema in this situation for Tim is (quote):
“Having to fork out dollars at the stealership every time I get my tyres changed or rotated to get the TPMS reset.”
Obviously you need a quick reset when you get the tyres rotated - because otherwise the car has no way of knowing the sensor that was formerly on the left front is now actually on the left rear, or whatever. The recalibration process here is merely a matter of telling the car where the sensors are now. It’s, like, a five-minute job with a scan tool.
I’m not so sure - at least in every case - that individual sensors can be recalibrated in the context of adjusting the reading they send to the car, for any given pressure. The ‘recalibration’ process there is likely - at least in some cases - to be replacement of the faulty sensor.
Tim adds:
Are you able to shed light on this at all? And if you made is this far, thanks for putting up with this. Cheers.
-Tim
No problem, Tim. I don’t think it’s illegal for an owner’s manual to say ‘take it to the dealership for recalibration’. There are plenty of servicing jobs outside the remit of the car owner themselves. Carmakers always recommend using their authorised dealer, but they can’t extort this, leveraging your warranty, that would be illegal.
And that didn’t sound like such a demand from Subaru to you, it was an active recommendation - you could certainly find an independent technician who could look at this issue for you. And you’ll still have to pay for it, it’s not a charity, obviously.
Just to put this in perspective: TPMS might save your life by preventing a blowout (we’ll get to that) but the feedback effect - the bad news component - is that occasional calibration is required.
Whoever does it, they’ll need to capture the signals from the in-wheel sensors and plug some scan tool or PC into the car via the onboard diagnostics port and let the mad recalibrating voodoo happen.
Inflation rate
There are two kinds of TPMS: the old indirect kind which measures tyre rotation speed;
And the newer direct kind which uses a pressure transducer inside the wheel. It’s actually part of the valve assembly with the sensor inside the tyre and wheel so you can’t see it.
If any car is displaying real-time pressures, you’ve got direct TPMS - the kind with the pressure transducer in the wheel. It’s essentially a sensor (which is a pressure-voltage transducer - converts pressure to electricity), plus an analog-to-digital converter and an RF transmitter, which an ECU connected to the CAN bus in the car receives wirelessly and decodes.
In the case here, where there’s merely a minor difference between the TPMS indicated pressure and the real pressure, why bother calibrating? Live with the offset.
It’s like speedometres. A speedo reading 106km/h indicated equals about 100km/h actual road speed, we know this. You can confirm it with a GPS receiver, and once you know what the offset is, you can adjust your own calibrations accordingly.
Tim knows that 35-37psi indicated by onboard TPMS equals 32psi actual. Just be aware that ‘normal’ equals a displayed pressure in that region of 35-37. So if there’s suddenly a reading of 28, that’s bad.
So I’d suggest that you don’t need to calibrate your TPMS in this situation. Because you simply don’t need to know your tyres’ exact operating pressures in real time. It’s worthless obsessing about some minor offset.
Point break
The other thing to consider here is the pressure in the tyres rises with operating an ambient temperature. A 10 degrees C rise in temperature is about a 3% increase in absolute temperature (relative to absolute zero). That equals a 3% increase in pressure (ballpark) because that’s how this works.
Absolute pressure int he tyre is about 45PSI above a vacuum, so 10 degrees C is an increase of about 1.5-ish PSI. SO part ofthe discrepancy here, the 35-37 variation could simply be pressure-temperature related for the tyres as they operate.
Back when I was the world’s shittest engineer I worked in a laboratory where we broke stuff. I was most adept at it. Give me anything, I’ll break it for you. Breaking stuff was my thing.
Calibration was an ongoing big-deal because the lab was NATA certified. In the techo world, calibration works like this: you apply a known load to some device and measure the reading on the device. It might read slightly above the amount you applied, so you do it again with a greater load, take the reading and then if it reads slightly higher again, then you know you can apply that known discrepancy to future tests. Rinse and repeat.
So you get an actual result here, despite the reading on the gauge and inaccuracy. You just deal with the offset and it gives you the true value you’re trying to read. This is what we do with our speedos. Measurement: +/- offset factor.
The truth is you don’t actually need to know your exact pressures in real time with a military-style level of accuracy. It’s worthless information. We’re not trying to send men to Mars, and we’re not scientifically measuring your tyre pressures in Formula 1 or in a lab. We’re simply trying to make sure no glaring deflation or leakage occurs while you’re driving to the supermarket.
Deflating experience
What you really need to know is if one tyre starts to lose air significantly. If you’ve got three tyres reading 38 and one reading 24, that’s bad. Do something about it now. This is like a blowout looking for a place to happen - and on a freeway you may not feel anything until it fails catastrophically.
The big strength of TPMS is that it will alert you to that slow leak - which is the major cause of blowouts on the highway. Gradual pressure loss makes the sidewalls sag. They flex a lot more than they’re designed to, and they get hot - thanks to hysteresis, which we covered recently - link above - which is the classic blowout failure mechanism. Overheated sidewall.
I’d suggest one needs a TPMS calibration - and possible replacement of malfunctioning sensors - only if the actual pressure versus the indicated pressure varies wildly.
Definitely keep checking manually with a gauge, every two weeks. That’s a great idea too. I wish everyone did it. In addition to saving your life, potentially, it will certainly also save you money because underinflated tyres wear out fast.
And while you’re down there, you can look for signs of uneven wear, which is a red flag you need a wheel alignment, or mechanical damage to the tyre itself - like a dirty big chunk missing from the sidewall, or something.
Like most things on any car that you drive regularly, it’s important just to keep track of what’s normal and what’s strange, in the context of gauge readings and behaviour generally. ‘Strange’ is usually bad - strange readings, strange vibrations, strange noises, strange pool of ‘whatever’ under where you park the car overnight.
At the very least, this kinda stuff warrants expert investigation. And if you get onto it early, the repair bill is usually a lot less.
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