Tyre Pressure Monitoring

[danregs]

So, here's an interesting one. I have just had my rear driver's side 19" 296 alloy wheel crack for the second time in my first 4 months of owning the car.... but it's highlighted something interesting....

A friend of mine who works at a professional wheel repair centre (and is an absolute wizard with wheels) has stuck a replacement wheel on my car while he welds and straightens the 296 wheel for me. When I got into the car the tyre pressure monitoring system (TPMS) said that the rear drivers wheel was "flat" - no surprise as the wheel was not fitted so there would have been no signal from the pressure sensor!..... HOWEVER, when I "initialised" the system, it then reset everything and showed all FOUR wheels as green on the iDrive screen. So, my questions are...

1) How is that possible that it detected that all four wheels were fine when one wheel & sensor wasn't even present! (and before you ask, I was a couple of miles from my friend's workshop when I initialised the system, so it was defintely out of range!).
2) Surely this means if any of the sensors were to die or run out of battery then the system would not know and would just report to me that all pressures were fine, showing all wheels as green on the TPMS.... and actually they could be flat for all I know!

[Buckz]

I think you'd know if a tyre was flat.

it's just a tool for the stupid and oblivious.

[danregs]

Thanks for the patronising comment! If I was running non run-flats then I'd agree with you.

It's got run-flats though. The TPMS is there for a reason - it's not that easy to feel a flat on a RF tyre. Not unless the wall has broken down after repeated use on low pressures.

[Buckz]

Not aimed at you directly, more a personal annoyance with all things going auto.

lights, wipers etc and then you see people driving around without headlights on etc because hey "it's an auto"

I agree on RFT it makes sense but from memory, the manual states that the system is inactive if a wheel without pressure monitors is fitted. So even thought it is reporting all green, going by the manual it will simply be inactive/de-activated.

I'd check your manual to clarify.

[Paulr]

I thing the TPMS works on monitoring the rolling circumference of the wheel using a chassis mounted device, there is no pressure monitor on the wheel sending signals to a receiver. By resetting the system with the button on the dash you are telling it that the circumference it is currently measuring is now the default. If you had a flat from any tyre the circumference alters and the light will come on again.

[Vonlipvig]

When you reset it on the stalk (don't have idrive) it thinks whatever is currently there is the new norm. The previous owner of mine had done this and masked a slow puncture to the dealer. Also the wife did the same on her 3 series, right pita.

As far as I know, it needs to be reset. Not sure if possible to do it with a plugin or not mind you, and happy to be corrected

[danregs]

Not aimed at you directly, more a personal annoyance with all things going auto.

lights, wipers etc and then you see people driving around without headlights on etc because hey "it's an auto"

I agree on RFT it makes sense but from memory, the manual states that the system is inactive if a wheel without pressure monitors is fitted. So even thought it is reporting all green, going by the manual it will simply be inactive/de-activated.

I'd check your manual to clarify.

Agree with people relying too much on technology, and then pleading ignorance or being at a total loss of common sense if their auto system fails!

I thing the TPMS works on monitoring the rolling circumference of the wheel using a chassis mounted device, there is no pressure monitor on the wheel sending signals to a receiver. By resetting the system with the button on the dash you are telling it that the circumference it is currently measuring is now the default. If you had a flat from any tyre the circumference alters and the light will come on again.

Now THAT would make total sense, based on how the system is behaving! You may have solved the mystery, thanks!

[metal licker]

I thing the TPMS works on monitoring the rolling circumference of the wheel using a chassis mounted device, there is no pressure monitor on the wheel sending signals to a receiver. By resetting the system with the button on the dash you are telling it that the circumference it is currently measuring is now the default. If you had a flat from any tyre the circumference alters and the light will come on again.

That's bang on mate the in wheel sensors only came in on later models

[freemadw]

The earlier cars (pre 2012 I think) use the ABS sensors, if a tyre is flat, the rolling circumference decreases, so the wheel rotates faster and it detects that.
The later cars use a pressure sensor/transmitter in each wheel.
I have the earlier version, when I have inflated the tyres and forgotten to reset the monitoring system it hasn't complained.
Cheers, Dave