What does the "Set" button do?

[Ed30DSG]

I had a slow puncture when I first got my car and it only seemed to register a pressure drop on longer journeys i.e. over 10 miles.

[topher]

It monitors it constantly when you are moving because it compares against the other wheel on the other side of the car. I believe it's like that to allow you to set your front tyres lower than the rears in winter/snowy conditions. I assume it has some sort of allowance delay for the tyres warming up which will increase the pressures so it may not notice straight away if you only do very short journeys.

[sambo]

Makes sense but then the question 'why bother setting the tyre pressure with the button' arises. Surely it could conduct the same monitoring just by comparing the rolling radius from turning the ignition on? Ideally if you came out in the morning to find out some one had let your tyre down a bit, you'd want it to tell you (if you hadn't already noticed by the way it drives) but this system won't do that 

Sam

[topher]

rolling radius

[sambo]

  You know what I mean 



























don't you? 

[topher]

the wheels need to be moving for the abs sensors to detect anything - i think in the states they actually got pressure monitors in the wheels on some models of mk5

[sambo]

yep sure so why doesn't the car automatically log the rolling radius at the beginnning of every journey and if it detects a drop within that time, throw up a warning. Therefore completely eliminating the need to set a particular tyre pressure?

The set button appears to be useless 

[topher]

not really sure what you're asking, and i'm crap at explaining stuff It can't measure differences until the car is moving and the tyres are up to temperature - it may even have to reach a certain speed for a certain length of time, not really sure. It will then compare front n/s against front o/s and rear n/s against rear o/s. If there is a discrepancy with either it puts the light on. You can press the reset button without inflating the tyre but in a few miles it'll notice that there is a discrepancy again and put the light on until you get the tyre pressures matching, either front or rear and then reset the system again. I think the steering angle sensor even plays a part in the process.

[mac7]

Sambo, what you are supposed to do is inflate all your tyres to the correct (or your choice) of pressure then use the set button. Now, don't touch the set button again until next time you check/inflate/replace your tyres. Pressing the set button provides a datum for the system to work from.

The system is for detecting a flat tyre, rather than monitoring tyre pressure. It's looking for bigger changes than just a few psi.