Tire pressure monitor

[PigLick]

Ok, I know this topic has been discussed quite a bit on here, but now that people have been driving around with them for awhile, I have some questions on how this thing really works.

I know thw whole deal that it uses the ABS sensors to monitor wheelspeed, etc. My question is, how sensitive/accurate is it? ie how well does it work?

on the Owner's Lounge boards, someone posted that they checked their tire pressures when they were hot, and two of the tires were 12psi low, yet the sensor hadn't gone off, so his conclusion is that the sensor is not very sensitive and is only good for major "blowouts."

However, this system is designed to work with the runflats. Now, the whole point of runflats is that when they lose pressure, they retain their shape and strength. If that is true, then shouldn't they also retain their diameter and therefore rotational speed, meaning the sensor wouldn't see a difference? This leads me to believe that it must be pretty sensitive in order to detect what must be a relatively minute difference in the speed of a flat (runflat) tire vs the non-flat (runflat) tires (hope that wasn't too confusing, ). Seems to be a bit of a paradox to use tire diameter (indirectly) to sense a flat on tires designed to retain their diameter when flat, doesn't it?

PigLick

[GAJ]

I think that it is rubbish! I had a puncture and my tyre was at 5 psi and the light did not come on!

[KAMAKAZI]

as best as i can determine, the indirect ( based on rotational speed) method of tire pressure monitoring requires a 30% or 10 pound drop in pressure to trigger the dash warning light. Some consumer organizations feel this type of system is inadequate and should not be used. There are better systems available , but mini picked the cheapest, costing about 12 dollars per vehicles. The good systems cost about 60 dollars per vehicle. Practically, this means we should all be manually checking tire pressure at least monthly and not rely on the sensor except for blowouts.

more information is available on the web. search for "tire pressure monitoring"

[BillK]

Well, I think I can answer part of the equation. I don't expect the sensor to tell me when a tire is low, but I do expect it to tell me when I have a runflat with NO pressure. (And I don't know what happened to GAJ, other than the indicator failed.) In fact, since the system can be manually reset, the tires may be inflated incorrectly prior to resetting and the system would accept that and not light up. That may be the reason for the 12 psi low reading - they were low since the system was set. The system does not in fact monitor tire pressure at all.

All the indicator will tell you is that one tire is rotating at a different rate than the others. A runflat with no pressure will hold the car up, but it will loose height and bulge more on the sidewalls than a fully inflated runflat. That will cause enough differential rotation to trigger the alarm.

[GAJ]

I took my car to the dealers to have the system checked as I assumed it was faulty but they assured me that it was working fine.

I had been driving for a few days before realising that the tyre was flat, the runflats deform less than you might imagine - I noticed the tyre looked a bit flat but convinced myself that it was fine and didn't check the pressure for a few days - I trusted the warning light - silly me! Luckily I was only driving around town and wasn't realy pushing the car - could have been messy!

[lightbody]

You also should bear in mind that the system needs set properly with all your tyre pressures correct. Read your manual and reset it yourself carefully.

[KenL]

Don't know how well it works if you have a slow puncture either. It says it won't detect if there is a natural loss over time. If the pressure drops over a week or so it might miss it. It should catch it though if only one tyre softens.

Sounds like a good experiment. If I remember tomorrow I'll let 10psi out of one tyre and see if it lights the warning lamp.

Incidentally this system is not designed just for use with runflat tyres. The 15" tyres on my car are not runflats and it's even more important on them than runflats to be warned that you have a loss of pressure/puncture.