mbmbp

When in Tires Pressure screen on the instrument clusted display and then pressing the down button it says:
"Use Current Pressures as New Reference Values"
What does it mean "reference values"?
TPMS readings are not absolute but reference values?

superswiss

Perhaps what you are confused about is that there is not a single recommended tire pressure. The recommended tire pressure depends on the size/type of tires that are on the car and the load, meaning how many people and how much cargo you loaded into the car. A fully loaded car needs a higher tire pressure than if you are just driving it yourself with an empty trunk, so that's why you need to set the correct reference values after adjusting the tire pressure. Most Mercedes have a sticker inside of the fuel door that lists all the recommended tire pressures. There are typically two basic ones. One is called the normal load pressure and the other one is called the max load pressure. The former is meant for normal driving with just 2-3 passengers and not really any cargo, and the latter is for when the car is fully loaded. On AMG models, the tire pressure even depends on how fast you intend to drive, so you need to adjust your tire pressure for the intended use and then set them as your new reference values. Here's an example of the fuel door sticker in my AMG. Yours will have less options, but the same concept.

bmwpowere36m3

You can program in a "new" set of tire pressures. You set the tire pressures as desired and then activate the "New Reference Values". The system will use these new values as the threshold at which to trigger the TPMS when actual values drop below.

mbmbp

You guys know then what is the threshold for triggering a warning? If I set, for example, 32 PSI overall as a reference value, so the TPMS will be triggered even at 31.5?

konigstiger

superswiss

No, TPMS is programmed to meet the law. In the USA the law states that TPMS has to alert if pressure falls 20-25% below the reference value. So with 32 psi you will not get an alert until the pressure is down to about 25 psi. It's only a safety net to alert drivers of severely underinflated tires. It's not a replacement to regularly check your tire pressure using an old fashioned gauge. Tire pressure should be checked and adjusted at least once a month, more if your ambient temperature changes drastically because for every 10F drop or rise in ambient temperature the pressure changes by +-1 psi. So if you set your tire pressure on a 90F day to 32 psi and the next day the temperatures drop to 60F your tires will be at 29 psi and you need to put more air in them.

mbmbp

In such case, if a tire inflator and a tire pressure gauge both show 32 (measured with the tires being cold) but TPMS shows 34 (right when the first values come up - considered cold?), what should I make of it?
Which one should be more accurate?

By the way, thank you everybody for great replies!

superswiss

Well, that's the other thing. The TPMS sensors are actually not all that accurate and they only have a 1 psi resolution. A good digital tire gauge is highly accurate and if you get a shop grade one they have a resolution of at least 0.1 psi. To be honest, I use the TPMS mostly just as a smoke check. If the numbers at the beginning are within 1-2 psi of the desired cold pressure then everything is good, and as I drive if the pressures make sense based on the current tire temperatures then I also know I don't have a leak. I don't know if regular Benz models show the tire temperatures, but AMGs do, so I can quickly do the math in my head and determine if everything makes sense. Mine consistently show about 1 psi higher than actual and my right rear is the least accurate. Point is don't rely on TPMS. Use it as a piece of mind. If you see the pressure of one tire being significantly lower than the other tires, then that's a good sign you have a leak, but if everything is about the same and around what it should be then everything is good.