Does anyone know how the FTM actually works? There are two systems out there, one checks for pressure against predetermined thresholds, and the other checks for a pressure variance from the pressure at reset (i.e. 5lb drop). I suspect that the BMW system is the latter, therefore if your tire pressure has gradually dropped since the system was reset, then you would get an "alert" if your pressure varied from the predetermed variance. Also, if the tire pressure in one or more of your tires is hovering around this pressure variance, then this would explain the intermittent alerts. So, if you check your tire pressure frequently, then you need to reset the system each time you change your tire pressure.

I recently checked the tire pressure on my '05 545i for the first time, since I got a compressor for Christmas, and found the cold pressure was a bit low, maybe 2 lbs, so I added air, and reset the FTM.

I also have a C5 Corvette, and the tire monitoring system is even more sophisticated. The C5 is equipped with run-flats, and has a system that monitors tire pressure. This system displays the current pressure in each tire, by location. It also will issue an alert if tire pressure goes above a predetermined threshold. This system has minimum and maximum thresholds, and will alert if any tire falls outside of these thresholds.

 

My understanding of the systems (on the UK E60) is that there are no pressure sensors - it is done on differential wheel rotation speed - ie - a flat tyre (even the run flats) will have a reduced rolling circumference and hence need to rotate faster than the opposite wheel for a given engine/road speed, the ABS sensors detect this and flag it up via the system.

I might be wrong but this is how i have had it explained to me and as the wheels do not have sesnors within the wheels themselves it makes sense..

cheers
dave

 

Question: If FTM is initialized while one tire is actually a flat, would the system monitor the slower turning flat tire as being normal after initialization? If so, it makes a good case for a physical pressure check of the tires on every FTM warning before initializing.

 

The BMW system does not measure pressure. There is no sensor in the tire. What it does is use the ABS wheel speed sensors to determine if there is a sustained rotational speed differential from the nominal established when resetting the system. A tire with low pressure will have a different circumferential dimension than one properly inflated so it will generate a deviating wheel speed through the ABS sensor. On my current car (2002 M Coupe) this system doesn't indicate an issue until there is a fairly substantial pressure loss (6-10 psi). I have no idea how sensitive the E60 system is but it is probably trying to use a more sophisticated algorithm given the software intensive nature of the car and that attempt to flag pressure differentials with finer granularity is likely problematic given that pressures are not even being measured. Instead it's the effect of pressures on the physical dimensions of the tire that are being observed but then a lot of things can affect that or wheel speed in general.

 

My front off-side dropped 10psi on two occasions before sounding the alarm.

 

I had original run flat (before my change to winter Dunlop non run flat) but ,with my great surprise I found a spare emergency wheel in the trunk, and I didn't specially order that.