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[davemack]

Hi all, my wife just bought a 16 plate Forfour a couple of weeks ago. The tyre pressure warning light has come on so I checked the tyres and filled them all to 30psi.
After a couple of days the warning came back....the manual tells me that the recommended pressure is on a plate on the drivers door arch, no sign of it there so I checked the filler cap...not there. I decided to check on line and some websites say 30/30, some say 30/36psi so I decided to call the Mercedes dealer she bought the car from and the service dept. 'wasn't sure' but transfered the call to the Mercedes main office helpline, they didn't know so passed me to their roadside assistance dept. This department also didn't know but they looked online and announced 29/29. They then suggested I reset the 'colour' computer once I have correctly pumped the tyres up. I then had to point out that to them that this might be resetting to a wrong figure as previously the computer must have been set higher for the warning light to come on, he agread but couldn't suggest any other help. Can anyone on hear give me a better answer? (it's the 0.9 engine)

[Problemchild]

You sure you don’t have a slow puncture?

It doesn’t matter about setting to the wrong pressure as it then sets the baseline and will flag up when it drops below

Some monitor actual pressure and some use the rotational speeds of the wheels to detect a drop in pressure (and thus the circumference of the flatter tyre reduces)

JJ

[steddie eddie]

G'day davemack,  I have my tires always at 35 psi, they last longer that way as they have not that much deflection, but the comfort is a bit less.
Greetings,
Eddie.

[CrazyG]

Ahhh..steedie eddie..you well might run your tyres at 35psi, but
then your car is a (Gen.1) 454, not a (Gen.2) W453

Anway, I've had a 'rummage' online and found more than one tyre
website quoting the front+rear tyre pressures should be 29psi.
HOWEVER...it also depends on what model W453 you have.
Most are 165/65R16 fronts and 185/60R16 rears.
The Brabus has differernt wheels and tyre sizes, but that combination
was/is available as an option on any model.
Assuming yours is 'standard' it has 165/65R16 fronts+185/60R16 rears
and the pressures should all be 29psi.

So, ensure all your tyres are at 29psi, then reset the tyre pressure
monitoring system as per the Owners Handbook. If that doesn't work
then there is either a slow puncture, or a tyre bead not 'sealing' on
the rim of a wheel, or a fault 'somewhere' in the tyre pressure monitoring
system. It it's not a nail or a 'bead seal' fault (which is an easy fix if it is)
then you should take the car back to wherever you bought it from as
it then can only be a fault with the monitoring system.
Tell them of the defect and that it has been like it since you bought it
from them, that you have kept checking and pumping up/down all the
tyres but that the problem is still there. If you have reset the system
as per the handbook then tell them that as well. As it was sold with a
defect then you have every right to expect them to fix it. It should be
covered by the warrantee that they had to sell it with to you.