W211 Tyre Pressure Monitoring System

[D-18]

As part of my weekly maintenance schedule, I usually check tyre pressures. Today, one the rears was down to 25psi (from the 32psi I normally use). Suspecting a slow puncture I jacked up the car and had a quick look. Sure enough, a screwnail was embedded in the tread. The TPMS hadn't given any indication and the tyre still looked well enough inflated.

I swapped it over for the new (full-sized) spare and headed off to get the puncture repaired. About 1 mile down the road the red TPMS warning illuminated on the dash. Interestingly, it had picked up the small difference in rotational speed / circumference between the 6mm of tread on the other wheel versus the 8mm on the new spare tyre, but the pressure differential between the punctured tyre and good tyre wasn't enough to trigger the warning.

I wonder how much of a loss of pressure is required to activate the warning? Good system though, as the red warning light in the dash display is certainly an attention getter.

 

[C240Sport97]

is this tyre pressure warning, which is standard on 211/219 or monitoring which is optional (and which shows actual pressures)?

 

[D-18]

Sorry Sean, I should have been more specific. This is just the plain vanilla system which I think is standard on the W211, not the one displaying actual pressures. Thought it was pretty good though.

 

[markjay]

Presumably the answer is that it will pick it up when the low pressure causes the wheel rotation to slow down enough...

I don't think there's a precise figure as such.

But keep in mind that it was designed to work with run-flat tyres (i.e. to make the driver aware of a puncture) and it is not really geared towards detection of slightly or partially deflated tyres.

 

[Mercator]

I recently suffered a tyre sidewall burst due to a pothole and the warning light did not come on until I had been running with the tyre completely flat for about 300 yards ( I was making for a nearby KwikFit). Does not seem a very useful system unlike the one on my wife's Citroen which comes on if any tyre is under about 90% of recommended pressure.

 

[joecash]

these tyre pressure monitoring systems must be very hit or miss - my wife also had a citroen system - doubt if it ever worked properly - just something else to cause problems rather than be of benefit -glad to hear your wife's did Mercator

 

[C240Sport97]

I hit a kerb in London (caused by an idiot nearly hitting me) .. rear off side tyre went bang, I drove the 1/2 mile home slowly, no warning, nothing.

 

[markjay]

these tyre pressure monitoring systems must be very hit or miss - my wife also had a citroen system - doubt if it ever worked properly - just something else to cause problems rather than be of benefit -glad to hear your wife's did Mercator

The basic TPMS simply analyses the input that the ESP system collects anyway, and as such does not involve any additional equipment on the car so there's nothing to go wrong.

The 'proper' TPMS that actually measures tyre pressure (and sometimes also temperature) through TPMS wheel sensors built into the tyre valves is a different matter, and can indeed go wrong.

The basic TPMS is a bit hit and miss as you say, but because it costs nothing - ultimately it is just a software feature in the ESP/ABS system - I would say it is better to have it then not to.

 

[SL300-24]

Strange....I picked up a nail in the rear and I got a warning well before it was flat.