Hot on the heels of another thread saying how great we are at answering questions, I have one today. I am in Spain in the ZM. It’s hot; about 37 degrees. While out today and on a terrible road (but not as terrible as the one I tried earlier which went from a proper B Road to a completely destroyed two laner (think Africa). We had to turn back - and this was the main road into the area). Anyway I digress. Short story is the Red Flat Tyre binged into life. Stopped. Checked tyres - remember not run flats. Okay. Carried onto next petrol station about 40 miles later - yes that uninhabited. Checked with an airline - 2.4Bar all round; maybe slightly more rear offside. About right as the tyres were hot. What’s going on please? I pressed the centre dash button to remove the light/bong when I go my to the hotel 100 miles after that.
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Red Tyre Indicator
- Thread starter CliveN
- Start date
Thank you. Possibly. I have no other warning lights lit up. It has happened once before when I was driving and the front left caliper seized on leading to groaning noises and the same light coming on. I limped home three miles. The wheel was so hot I had to leave it for an hour before I could touch it. I assumed then it bonged because the tyre air temperature had risen so much that it INCREASED the diameter, thus leading to a mismatch. Possible? The other thing I thought was that it might be yet another symptom of my slightly iffy GM5 module (due to be replaced by a part from UMFAAN when I get back). Is that likely?
Not sure it would be anything to do with the GM5 tbh?CliveN said:
Checking NewTIS it says the tyre failure indicator is controlled by the DSC unit.
It also says it only flags an issue if the calculated pressure in a tyfre drops by 30%. Increase in pressure does nothing.
When you press the button you are telling the system to learn the current calculated pressures as a datum. it doesn't care what the figures actually asre, just takes that as it's starting point.
enuff_zed said:
I've triggered the light hitting surface water at speed on the motorway - checked the pressures and all fine. If you've been down dry dusty roads - as Enuff-Zed suggests loss of traction in the dust might well spin one wheel at a different speed to the rest resulting in the tyre warning
If you have already pushed the button to get the light out and you are certain the pressures are ok, then nothing more to do.CliveN said:
If any doubt, check the pressures in the morning, replenish as necessary, then do the normal reset by holding the button until the yellow tyre warning comes on, then drive off.
37 degrees will have expanded the air in the tyres quite a bit, increasing the pressure and, with it, the rolling circumference. If the DSC has stored wheel speeds with much colder tyres then that can have thrown the front to rear wheel speed difference out of tolerance, perhaps, if you have staggered wheels so they expanded at different rates.
enuff_zed said:
Yes, but it's comparing individual wheel speeds to each other. Remember, the only way it knows how fast the car is going is also from a wheel speed sensor. So if a front tyre has expanded less than a rear tyre, because it has a lower volume of air, then it will be rotating faster relative to a rear wheel. And, if it takes the vehicle speed from a rear wheel, then that front will also be going faster than the datum. As least as far as the module is concerned.
So the sensor reacts when there is a drop of 30% of pressure but it does this through wheel rotational speed. A flat tyre has a smaller circumference. The wheel will therefore be rotating quicker than the others. Certainly the backs had expanded slightly more than fronts (one up to 2.6bar from the original 2.2bar; the others were at 2.4bar). Perhaps then the 2.6bar back was going slower than the others triggering the “bong”. Yes??!
Noted. But from what I have read on here it’s not doing it through measurement of pressure (as per the new technologically connected valves in modern cars) but through the speed of the wheel. There is no means that my car can measure tyre pressure as the valve is a “simple” shraeder valve. It must do it be measuring the fact that the wheel is rotating differently than it was when it was measured at the outset. I think we are saying the same thing though.
We are! That is why I keep referring to calculated pressure.CliveN said:
In fact, to be exact, the system has absolutely no idea what the pressure is. All it knows is that you have told it that the current diameter/circumference/rotational speed etc is the one you want it to accept as the norm. It then merely shouts out if that drops by an amount equivalent to a 30% drop in pressure (how that is worked out I have no idea, by the way)
If you put 50 psi in one tyre and 10 in another, then initialised the system it would happily accept that.
That is why I cringe every time I hear of people who see the red warning, press the button and are happy it's gone away, without checking the tyres. What you are actually doing is telling the car "Thank you very much for telling me my tyre has lost pressure. I'll accept that as the new norm."
This is interesting. I've attached the wording from NewTIS, but it seems something triggers it with a large increase too.mmm-five said:
Was it a red warning or a yellow one? If yellow then that could be due to the ABS/DSC system picking up a fault? For example the sticking calliper slowing that wheel in relation to the others.
Could have been either, and it was about 10 years ago.enuff_zed said:
I just know it was concerning enough for me to pull over on the motorways and feel the heat coming off the wheel.
Luckily I was about 10 miles away from the office, and there was a BMW dealer up the road.