Clarification of tyre pressure monitor system

[Beanee687]

I had a major puncture last Saturday 24th.
I have a full size spare BUT it is a 16in not a 17in wheel.
I fitted it. Adjusted the pressure in all four tyres to 2.5 bar. Then I drive off to visit a friend. After a rew minutes the warning flashed upon the screen about tyre pressure. I pulled over and checked the tyres and then reset the system on the touchscreen. However the trouble spanner illuminated along with the amber tyre symbol with exclamation mark in it. Then the auto brake amber light came on. A few seconds later a message appeared on the dash spare wheel fitted certain systems inhibited. All rather worrying to be honest.
Driving was ok but with many aids inhibited care was needed.
Fast forward to Wednesday and a trip to tyre repairers. Had to have two new tyres as the punctured one was beyond repair, failed side wall. The other front also worn but still legal. So two new ones fitted. Whilst the tyre was off I asked the fitter where the tyre pressure sensor was on the wheel, he said it's not on these cars it uses the ABS sensors.
So basically the ABS sensor senses the wheel rotation rate. When all is ok all wheels rotate at the same rate over a set distance. However when a wheel locks up it doesn't travel the same distance as the others so the brakes get released. Also if it spins same thing but this time the brakes get applied. When you have a puncture and the tyre deflates the distance again changes but this time it decides it's a puncture. When the spare was fitted it worked out it was a different size and told me so.
Once the new tyres were fitted I reset the tyre pressure system on the screen. Then I drive off, however the spanner and the Auto brake warnings were still present. These reset automatically after about ½ mile. Car is now back to normal

[Juan Sheet]

Yes they dropped direct TPMS years ago and use the indirect system now. The ESP ECU plays such a huge role in the car's operation now from Hill Start Assist, Stability Control, Anti Skid, ABS, EBA, EBD, Active Safety Brake, Grip control option etc. as well as the indirect TPMS as you've found out

Though I'm not sure what's going on about the spare - is this one you have added yourself? The spare should be the same size, but won't be an alloy, just a steel one.

Edit: Just confirmed they are indeed 16" Spares for those vehicles fitted with one. It's the space saving one, so as the spare wheel changes the vehicle’s ground clearance, the speed and distance to be travelled are limited to 80 km/h (50 mph)!

So they also recommend replacing the spare after use each time - about £126 inc. VAT! That's not surprising if it's the wrong size - it probably wrecks the tyre.

[routemaster1]

The indirect system is not a real time system. It takes a while for a puncture to register as the system needs to recognise some kind of average rotation speed over a period of time. This is so it doesn't give false positive warnings. When you need to reset the system after a warning you are not resetting, you are starting a reset. The car needs to be driven for a short distance for the system to collect the rotation speed data, and this may explain why the warning takes a while to stop a warning.

The tyre pressures are not actually measured. As it relies on the rolling radius of the tyre, if you fit a spare of a different size, even if it is inflated to the correct pressure, there isno guarantee that the rolling diameter will match the other tyres and a warning may result.

If you have a warning, put on a spare of a different size, then reset, when a new correct tyre is fitted you will probably get a warning and need to reset again. If you get w warning and get te puncture repaired if the tyres are set correctly you should not get a warning.

One last thing. When most tyre fitters mount a tyre, they almost inevitably just pump the tyre up to a standard setting, usually ~35psi. On my C4 Picasso, which needed a pressure of over 45psi, this inevitably meant a visit to a petrol station to reset all of the tyres, then reset the TPMS.

[Beanee687]

My car came with a 50 mph space saver spare. I bought a 16 full size spare fir my C3 MK3 hatchback but removed it when I traded it for the Aircross.
My normal wheels are 17 inch wheels. So when I fitted the full size I had 3, 17 inch wheel and 1, 16 inch.
A 17 full size won't fit in the wheel well in the boot.
The new car I have on order a C3 Aircross Shine Plus automatic is fitted with 16 inch wheels so the spare will go in my new boot.