Tire pressure monitoring

Here's a question for the electronics experts: how do the tire pressure sensors work?

We have an '09 that has this feature. My wife drives the car almost exclusively, and yesterday the tire pressure light came on. She went into semi-panic mode about it, so today I checked the tire pressures. (So I need an idiot light on the dashboard to remind me like I need a nagging wife, right?) Typically I over inflate the tires, but I hadn't checked them in a while. I found the lowest at 28 lbs. (hardly under-inflated) and the highest at about 34 lbs. I inflated them all to about 40 lbs. and the light went out.

So what is this system monitoring? Actual tire pressure? Or divergence between pressures in the tires based upon standing height?

from my understanding, it is supposed to measure 2-3 lbs each way from the recommended tire pressure. So over inflating would trip a light just like under inflating. I hated / hate my light, it would come on at 29 lbs (according to my gauge), which was any day it was under 55 degrees in the morning, and then if i would fill them and drive on the highway the light would come back on, couldn't win.

in your case i am honestly surprised it is not on at 40 lbs. But i cannot recall what is recommended.

the sensors are on the valves inside the wheels, and uses a radio signal type this to verify the pressure. so it is not height, although i could see it being the divergence between two tires?

the system is federally mandated, because americans are lazy and stupid. so if you google search how it is supposed to work, that is how they all should work.

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Originally Posted by petrified.rabbit

from my understanding, it is supposed to measure 2-3 lbs each way from the recommended tire pressure. So over inflating would trip a light just like under inflating. I hated / hate my light, it would come on at 29 lbs (according to my gauge), which was any day it was under 55 degrees in the morning, and then if i would fill them and drive on the highway the light would come back on, couldn't win.

in your case i am honestly surprised it is not on at 40 lbs. But i cannot recall what is recommended.

the sensors are on the valves inside the wheels, and uses a radio signal type this to verify the pressure. so it is not height, although i could see it being the divergence between two tires?

the system is federally mandated, because americans are lazy and stupid. so if you google search how it is supposed to work, that is how they all should work.

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Apparently there are no sensors on the wheels or valve stems. That's the "direct" type and this clearly isn't that. The following is from Wikipedia:

"Indirect TPMS do not use physical pressure sensors but "infer" air pressures by monitoring individual wheel rotational speeds and other signals available outside of the tire itself. Most indirect TPMS systems operate under the theory of using an under-inflated tire’s slightly smaller diameter (and hence higher angular velocity) to determine inflation. Later developments of indirect TPMS can also detect simultaneous under-inflation in up to all four tires using vibration analysis of individual wheels or analysis of load shift effects during acceleration and/or cornering, which can be realized in software using advanced signal processing techniques. The vibration analysis technique may require the use of additional suspension sensors which result in increased complexity and cost of the overall system as long as vertical chassis movements are concerned. That is why most current advanced indirect systems use the spectral content of the wheel speed sensor signals so no additional sensors are needed and the computations can also be carried out by existing computing power for example in usual ABS or ESC control units.

Indirect TPMS are realized in software algorithms in combination with wheel-speed sensors for anti-lock braking systems, and electronic stability control systems. An advantage of the indirect TPMS is that it needs no sensors thus decreasing weight and cost as well as increasing customer satisfaction because sensor-related problems are eliminated. A disadvantage of indirect TPMS is that the driver must calibrate the system by pushing a reset button on the dashboard via an on-board computer and if this is performed when any tire is in an under inflated condition then the system will report erroneously."

This would explain why the light never came on before, although I have always over-inflated the tires since the day I got the car, brand new. Also there is no reset button. Apparently the system resets itself automatically when the car is re-started.

Try inflating all your tires to about 35 PSI and see if it helps. I agree that this whole system is just mandated gadgetry that adds to the production cost of the car. The effect is that of shielding manufacturers from potential lawsuits stemming from the ignorance of the vehicle owner to maintain adequate tire pressure.

Check your owners manual, if it is like mine and says to adjust tire pressure back and forth for the tpms reset, then you have a direct system, Like I have. I think (haven't seen the tires off the rim) that our cars TMPS sensor is attached to the rim via a band that holds it in place. If the light hasn't come on at 40 PSI try driving around and get the speed up to at least 20 MPH. This is the speed that most systems activate and start scanning your tire pressure.

I was just going to say, we have the direct monitoring system.

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Originally Posted by daug1502

Check your owners manual, if it is like mine and says to adjust tire pressure back and forth for the tpms reset, then you have a direct system, Like I have. I think (haven't seen the tires off the rim) that our cars TMPS sensor is attached to the rim via a band that holds it in place. If the light hasn't come on at 40 PSI try driving around and get the speed up to at least 20 MPH. This is the speed that most systems activate and start scanning your tire pressure.

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Nope, it doesn't say anything about it. Last time I rotated the tires I didn't notice anything added to the rim. I have a 2009.

Out of curiosity I checked the web to see what the part would look like (click here). Apparently the sensor is part of the valve stem assembly and is internal, located inside the tire itself.

yeah, thats it.

Direct system have been required in the US since the 2008 MY.

Thanks Ford & Firestone!! :angryfire:

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Originally Posted by wylee

Direct system have been required in the US since the 2008 MY.

Thanks Ford & Firestone!! :angryfire:

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Ah, but it took Congress to pass the legislation and the president to sign it. :evil3:

That's probably true, although this might have been done with an administrative rule; I don't know. I'm pretty sure the politicians would not have had it on their radar without the high profile Explorer mess.