enzo thecat

The damn "Check Tire Pressure" light comes on every time I drive. I reset it, and the thing comes on again as soon as one of the tires change by one 1lb. So is there a way to make a range of about 5lbs per tire before the warning comes on?

e

Diesel Benz

I did not check for the W212 but what sort of tyre pressure do you have? The system used to have an absolute lower limit which of course does not change from the "reset". You may be hitting the absolute lower limit instead of the relative limit, relative to the reference from reset.

Some models had a too high absolute limit and this was programmed to a higher value. I would have thought the W212 would not have this issue though.

El Cid

You need to contact the dealer as it appears you have a malfunction in the system. Most (all?) TPMS systems do not come on unless there are several lbs. of lost pressure.
The only time mine has come on in almost three years is when the tech forgot to adjust pressures after rotating tires. This resulted in about 10# under pressure in rear tires.

ttoE550

Mine does not behave that way. I always have the instrument display set to show tire pressures. You might also make sure nothing is wrong with your tires. Mine once alerted me to a problem even though the tire pressures had not changed significantly. It took some looking, but we eventually found a screw in the affected tire. I'm not sure how it knew when the pressures hadn't changed much...

enzo thecat

It started doing it at 39 lbs. Now I'm down to 34 lbs on all for tires bc its getting so cold now. But where I live we go from a 75 degree high to a 50 degree high the next day... and the tire pressures change. So my tire pressure warning is always own. I reset it, but if one tire changes 1 degree up or down, the warning comes back on.

FYI, all my tires are within one lb of each other. Usually, they are all the same, but intermittently one will deviate from the other 3 by a lb.

ttoE550

Mine will sometimes change upwards 3 lbs just on a trip to the grocery store, and then fall back a couple lbs while I'm inside. It will change up to 6 lbs or so on long drives. My cold temp dropped also dropped 5 lbs from summer to the cooler weather now.

I've gotten exactly one warning - when I got the screw I mentioned before. The mechanic I took it to insisted that we closely examine the tire, even though the first look revealed nothing. He said that, in his experience, the system is never wrong (it was not clear to me if he meant in general or just Mercedes'). But yours might be malfunctioning.

BenzV12

Mine is set at 30 psi for all the tires but to avoid faulty messages I am running 36 psi fronts , 38 psi rears , system is registered at 30 and no fault caveat

Malfunction is also possible but I have checked OP's car is 2012 model year so it is too new

Karl901

Ok, if you set the pressure when tires are cold the sensor should not come on.
If it is the same tire, then you have a leak, or a bad sensor. If the car feels like you have a tire balance issue too, the sensor fell off inside the tire.

enzo thecat

Its not the same tire. Its all four. I am not sure of this, but I think the warning light comes on when a tires goes UP in pressure as well as when it goes DOWN. No I'm not sure after reading the posts here.

Yeah...I'm sure of it. B/c I reset my tires at 34 psi and THEN pumped them up to 40 psi and the warning light came back on.

e

Karl901

I have never had tire pressure sensor go off when PSI goes up.
Time to go to the dealer.

enzo thecat

CRAP!!!!

Diesel Benz

You might need to adjust the settings as in the attached figure.

I have no idea where your current 34 lbs goes in respect to typical values set at production but one should note that the temperature effect only concerns the absolute low pressure limit. I would expect the absolute limit however being lower than 34 lbs (but I would consider 34 low for the car, unless it was only temporary because of the very cold tyre).

The sensors measure temperature and the control unit has ambient temperature info available too. The system compensates the pressure change from temperature and the compensated pressure values are used against the stored reference (from reset). The instrument cluster would still show the real values. As long as the absolute low limit is not hit, a warning should not appear if the pressure drops because of an extreme temperature drop.

A soft warning is issued after the journey (ignition switched off) if a drop of 0.25 bar is detected. A soft warning is issued during the journey if tyre pressures drop 12% for a specified period. A hard warning is issued at 20% pressure loss (5 minute detection period). Also if a gradient of 0.2 bar per minute pressure loss is detected, the warning is given within a shorter detection time (2 minutes).

European cars have automatic tyre fill detection but US cars should not have that. Tyre fill up after TPMS reset should then give additional headroom while it would not do it for Euro cars.

enzo thecat

Talked to MB service manager. He told me to reset the tire pressure with the tires cold at a low setting, which I had already done. Then, fill the tires to max recommended pressure (41psi) and leave the pressure warning setting low, ie do not reset to 41psi. It seems to have worked.

He said something to the effect that if your tire pressure changes a certain percentage of the recommended psi, the warning system will act as I described above. Now i'm wondering what would happen id I'd change to a tire that needed a lower psi. Probably have to have the dealer reprogram the "recommended" values

El Cid

I run mine at 32 (F) and 37(R) which is 2 lbs. over recommended for regular load. Every couple of months check with a gauge and adjust TP's if necessary. If I change TP's, I will check TPMS display after about a mile and then tell system to use displayed TP for standard (whatever it is called). Never had a problem even when displayed TP goes up on trips at 70+ MPH in mid-summer.
I have 17" all-season tires and that may make a difference.