When to store tyre pressure in MMI

[LuisM]

Hi all,

I keep reading you should store the tyre pressure (via the MMI) every time you inflate your tyres. Is that so? And why?

An example. Let's consider you stored your ideal pressure (say 2.2 bar) in the MMI. A few weeks later you check the tyres. Pressure is a bit low (but not very much, because the car hasn't warned you yet). So you inflate tyres to your ideal pressure (2.2, the same that was stored). At this point, why should you store again? You will be storing the same value that was already stored.

It would make more sense to me if you had to store only when you change pressure. For example, you inflate to 2.4 for a long highway trip with heavy baggage. Since the target value for the MMI is no longer the previously stored 2.2, now it makes sense that you should store the new value.

Any ideas?

 

[veeeight]

Because it keeps "learning" as you drive, so as your tyres naturally deflate over hundreds of miles, the average stored value will be lower.

Then when you re-inflate your tyres to the correct pressure again, this will trigger an alarm as the new/correct pressure will be different to the values already accumulated.

 

[LuisM]

Thanks for your answer!

So if the system adapts to the tyre naturally deflating, in which cases does it warn you that something's wrong (pressure too low)? I mean, the event that it's supposed to detect is precisely the tyres naturally deflating, isn't it?

Because it keeps "learning" as you drive, so as your tyres naturally deflate over hundreds of miles, the average stored value will be lower.

Then when you re-inflate your tyres to the correct pressure again, this will trigger an alarm as the new/correct pressure will be different to the values already accumulated.

 

[veeeight]

No, it is not a perfect system. You still have to check your pressures manually, at regular intervals. There is a disclaimer in the handbook that says it is the drivers responsibility to check pressures regularly. This is because this particular system is an indirect system, not a direct system using pressure sensors in the wheels.

It will not detect gradual pressure loss through natural diffusion, for example, so if all 4 tyres lose pressure gradually, you won't get a warning. It will detect a sudden loss of pressure in one or more tyres successfully.

 

[Vertigo1]

The key thing here is that it's not actually measuring pressures at all - it's using the ABS sensors to measure the rotation of the wheel. If a tyre deflates, the rolling circumference of the tyre decreases and thus the wheel turns faster and it detects this.

Of course, over time, the tread on your tyres will wear down so the circumference will decrease due to this also but the system obviously shouldn't detect this as a deflation.

 

[LuisM]

Got it. Thank you both!

No, it is not a perfect system. You still have to check your pressures manually, at regular intervals. There is a disclaimer in the handbook that says it is the drivers responsibility to check pressures regularly. This is because this particular system is an indirect system, not a direct system using pressure sensors in the wheels.

It will not detect gradual pressure loss through natural diffusion, for example, so if all 4 tyres lose pressure gradually, you won't get a warning. It will detect a sudden loss of pressure in one or more tyres successfully.

 

[LuisM]

Just one more thing, to make sure I understood:

In the hypothetical case that all tyres naturally deflated or wore at exactly the same rate, the rolling circumference would still be the same for all tyres, and the system wouldn't detect anything, even if the stored reference was outdated.

So storing the new reference when you inflate tyres has the purpose of preventing the system from detecting natural differences in wear (uniform wear would not be detected anyway).

Am I correct?

The key thing here is that it's not actually measuring pressures at all - it's using the ABS sensors to measure the rotation of the wheel. If a tyre deflates, the rolling circumference of the tyre decreases and thus the wheel turns faster and it detects this.

Of course, over time, the tread on your tyres will wear down so the circumference will decrease due to this also but the system obviously shouldn't detect this as a deflation.

 

[veeeight]

.
Volkswagen Tyre Pressure Monitoring System (TPMS) Indirect and Direct | my-gti.com

 

[The Challinor]

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Volkswagen Tyre Pressure Monitoring System (TPMS) Indirect and Direct | my-gti.com

All in all it's not all that useful really, it's now standard on a lot of models so no longer have to pay for it. Direct would be a hell of a lot better as really you should be able to tell when 1 tyre is really low I.E flat.

 

[cuke2u]

So can a driver no longer feel when their tyres are flat, or is it a case that they're only flat at the bottom ;-)