TPMS useless?

[DigiMagic]

Yesterday I've got a flat tire, a metal screw punctured it. I've got a warning from an aftermarket TPMS system immediately as the pressure started dropping; the built-in system gave no indication at all that anything was wrong. I've managed to find a place to park when the pressure dropped to about 1/2 of the normal one; aftermarket still beeps, built-in is silent. The pressure dropped to 0, aftermarket still beeps, built-in is silent; I've had to drive a couple of meters like that so that the car could be put on the tow truck. Later at the tire shop when they've removed the wheel, still the same.

Is this normal - is the built-in system simply super insensitive to changes in tire pressure (... as well as to the fact whether the tire is even there)?

 

[vc-10]

The built-in system works by comparing the speeds of each of the 4 wheels. It has to do this over a period of time- the individual speeds change as you go round corners etc. When the tyre deflates, the effective diameter shrinks, and as such that wheel has to turn faster. To avoid false positives, it doesn't react for more minor pressure changes, there has to be a fairly significant pressure loss to be notified. It also has to do this over a distance, and over a decent enough speed. Your few mph for a few metres onto the tow truck wouldn't qualify.

My previous Fabia had a slow puncture which would lose pressure over a couple of weeks. It would normally be at 2.4 bar ('eco' tyre pressures), and then when it beeped it would normally have got down to around 1.7 bar. I wouldn't be surprised if an Ibiza with bigger wheels and lower profile tyres (it had 15" alloys, can't remember the profile but they were comfort orientated!) took more of a pressure loss to trigger an alert.

Edit: It's also not TPMS, but TPWS. M is for monitoring- like your aftermarket system which can tell the individual pressures. TPWS is a warning system, which just alerts you to check the pressures. Sometimes they will pinpoint the wheel in question but not always.

 

[DigiMagic]

Fair enough, now that I understand it better, it seems the system is working as designed. I've had one or two slow punctures earlier on another car, so these things apparently happen often enough to justify the TPWS.

 

[vc-10]

In many ways it's a 'free' system- it uses hardware already there for the ABS system .

 

[camelspyyder]

The built in system has to have a reference set each time you re-inflate the tyres - press the button or select the function on the HU.

It sounds to me like this was not done.

Once set up it is sensitive to about 3 psi (0.2bar) drops on both my Ibiza & Toledo.

 

[KXL]

In my Leon, a pressure difference of 10 psi on the rear tyre, it pinged within 5 mins, in my Ibiza, when it pinged, it was wrong, but it was icy, perhaps too much wheelspin. On my Volvo, it pinged a 2-3 psi difference after 5-6 miles of 40-50mph driving. @DigiMagic how many miles did you travel before your tyre went completely flat?

 

[DigiMagic]

About 0.2 miles - it was a big puncture, an M5 or M6 screw, I think.

I don't remember if I've reset the pressure setting on the HU - generally I do that, and follow the prescribed service intervals. It's possible I've forgot and the system was set to (slightly) lower pressures of the summer tires.