Tire Pressure Monitoring System
 

MY06 Boxsters have an optional electronic Tire Pressure Monitoring System. How does it work? Where are the sensors?

Will the display be a readout of the actual tire pressure of each tire, or is there just a "red light" when the pressure is too low (or too high)?

My Ford Explorer has this too. I think it is some magnetic gizmo. Very accurate though. I'll get a reading on the dash board computer if the tire pressure is "low" or "extremely low". On a cool morning, I'll get a "low" reading until the tires warm-up in a few miles driving.

The tire pressure monitoring system (RDK) uses a 433 mhz radio transmitter to relay pressures from each of the four wheels to a processor, which is then integrated into the display. I assume it can be checked using the OBC stalk control.

Where is the sensor, inside the valve stem?

I've seen these sensors on some American SUV and it was a little black box strapped to the center of the rim.

Interesting our 2006 BMW 530i tire pressure monitoring system monitors the RPMs of each of the four wheels. When one of the wheel's rpms drops due to a change in rolling radius (relative to the others), it indicates a tire pressure drop and the alarm light comes on.

This is a very practical method for monitoring tire pressure change since RPMs at each wheel are monitored anyway for ABS.

Surprised other mfgrs do this any differently. :)

Best regards,
Gordie
Austin, TX

The RPM models are interesting.

Do tires move less in one revolution when at just a psi lower than the tire at pressure?

I would want remote sensing not the rpm style. I hope this is what P uses.

Hi,

These sensors are usually fastened to the inside of the wheel, in the air cavity between the wheel and the tire. It's a pressure sensing device which actually lives in the high pressure inside the tire.

Happy Motoring!...Jim'99

Hi,

No offense, But I'm not sure I buy this. I may simply not understand it.

A system which can measure the minute differences in tire circumference of a couple PSI would be pretty sensitive stuff. But, there are other things which effect the tires circumference such as Tread Wear, Ambient Temperature, Running Temperature and even Barometric Pressure and/or Altitude. Even the Brand of Tire has an effect because there is some variation in the Circumference of a certain size Tire between Brands. That would mean such a Monitoring System would need to sense and eliminate the effect of these variables as well.

Maybe you can enlighten me. Thanks, I look forward to your reply...

Happy Motoring!...Jim'99

Maybe you can enlighten me. Thanks, I look forward to your reply...

Happy Motoring!...Jim'99[/QUOTE]

Hey Jim,
Below is text from BMW Owner's Manual. from Owners' Circle portion of BMW web site. You can read how it works and the "level of accuracy" obtained.

As I previously said, all vehicles with ABS have the capability to detect rpms on each wheel. There is an electromagnetic sensor on each wheel. The roll radius (not circumference as you mentioned) changes as a tire goes flat and increases the rpm of the affected wheel. The system is apparently accurate enough to detect this change and alert the driver.

Best regards,
Gordie
Austin, TX
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Flat Tire Monitor

The concept

The Flat Tire Monitor keeps track of the inflation pressures in the four fitted tires as
you drive. The system provides an alert whenever the inflation pressure drops significantly in relation to the pressure in another tire.

The check of the tire inflation pressure is based on monitoring the relative speeds of the wheels. A flat tire is detected and reported on the basis of a deviation in certain speed ratios.

Functional requirement

So that the Flat Tire Monitor can learn the correct inflation pressure, please perform the following:

1. Check the inflation pressures in all tires
2. Compare with the inflation pressure table on page 170 and correct if necessary
3. Initialize the system.

Limits of the system

The Flat Tire Monitor cannot indicate sudden severe tire damage caused by outside factors and does not detect a natural, even pressure drop in all four tires.

In the following situations, on the other hand, there can be delays in detecting tire
pressure losses, and even system failure:

When you are driving on snow-covered or slippery road surfaces

If you are using a sports driving style: slip on the drive wheels, high lateral acceleration

False alarms and undetected loss of pressure may occur when you are driving with snow chains fitted

When you are driving with the compact wheel, the Flat Tire Monitor is unable to
function.

Initializing the system

Perform the initialization immediately after correcting the inflation pressure,
after changing a tire or changing a wheel or all wheels. A drive is necessary for this purpose.

Control Center, for principle details, refer to page 16:
1. Before beginning driving, start the engine, but do not drive off
2. Open the menu
3. Select "Vehicle settings" and press the controller
4. Select "FTM" and press the controller
5. Select "Set tire pressure" and press the controller
6. Select "Yes" and press the controller
7. Drive off.

The message "Initializing" is displayed. It takes a few minutes before the Flat Tire
Monitor can detect and report a flat tire. After the initialization is completed, the
message "Active" is displayed.

If a flat tire is detected during initialization, a message appears on the Control Display.

When driving with snow chains or with the compact wheel, do not initialize the system.

@Grease,

Thanks for the explanation. After my post, I thought it must work something like this, it's the only way. I was thinking in terms of actual RPMs of the Tires, not Rolling Radius which is actually a measure of the force imparted on a tire due to bumps and lateral forces and transmitted through to the driveshaft. RR is always VERY close to the actual Circumference, but always slightly less. But, most the variables which affect Circumference will also affect the Rolling Radius.

But a couple points. This system is a comparative system, meaning that it cannot detect actual pressures, it merely detects the difference of the force transmitted between one tire and [u]another[/i], not all four as you earlier mentioned. This difference equates to a difference in Tire pressures. I wondered about this especially.

I suspect it compares it's mate on the same axel, or it cannot adapt to differing Tire Widths which many people install (on the Rear to increase Traction, on the Front to induce greater/lesser Understeer).

Overall, it's a slightly less accurate way to monitor the tires. Both Tires could be underinflated to the same degree and the Monitoring System wouldn't detect it. But, I concede this is unlikely.

I think the trade off is that there is no need for a transponder and receiver, and I can see where this might be benefitial, certainly less complex.

Thanks for the enlightenment!

Happy Motoring!...Jim'99

Hey Jim,
I am sure there are more accurate ways to measure drop in tire pressure, but BMW has been using this system since 2004 and I don't see any complaints on the BMW forums.

As you suggest this method has many vulnerabilities, even BMW points out some of them. It does compare tires on same axle, various 3 series. 5 series and Z4s have different width tires front to back.

Basically it's purpose is to alert driver to major drop in tire pressure, not necessarily noticeable since many BMWs now use runflats.....

Quote: If you are using a sports driving style: slip on the drive wheels, high lateral acceleration

Well, that put's me out Jim.