CheckStein

ok, 2017 GLC, I get what it's saying, my RF and RR psi is low.

But if 33psi is "low" then why does it not highlighting the LR tire ??

DDGator

I got a cryptic message like this the other day too. Since when is 33psi low? I put some air in and reset the indicator, but I don't understand why 33 psi is considered low pressure.

midesaka

Just a guess, but 33.4 and 32.6 both round to 33. Perhaps the system maintains an extra decimal place internally, but they look the same because it's rounded for display? Maybe the system is set to alert if pressure isn't 33.0 or greater.

DDGator

Yes, but one of the recommended pressures is 32psi. I wouldn't think the "low" warning would occur until at least 30psi.

Takehold

I got this message yesterday too but the opposite way. My tires were at 38/39 due to the extreme heat we are having and the fact that I was driving a long distance at the time. I recently set them at 33 but did not reset the monitor. I believe that the system uses a baseline setting that we choose and alerts if we are off either way by a certain amount. I think I when I last adjusted my monitor they were set to 32 and it asked me if I wanted to use those pressures as the baseline. So plus 6 or 7 psi may have triggered the alert. Just a thought.

midesaka

Hmm. Maybe it's adjustable from the service menu?

zhenyaf

If you scroll down when on the tire screen, you can set the current values as the baseline. My TPMS was recently triggered at 35psi

CheckStein

I am not sure if TPMS also accounts for ambient air temps while driving. Thus, if its 75F outside and you've driven say 10mi, then perhaps it expects tire pressure to be above 33 because the air in tires should be hot.

But I am not sure if MB TPMS is (or can be) programmed to know if you are using nitrogen or just air.

Anyways, my run flats lose air every few months and I have to add some air.

But, if a decimal point makes a diff in the display, then perhaps I can say the Germans failed at the TPMS task.

glassbloke

It's telling you what the pressure should be, not what it currently is. If you had put a tire gauge on it (hopefully you have already added air), it would have shown a pressure below 33.

protseng

lol if 32 is low why do they recommend 32 psi on the fuel door

DDGator

You interpret this as saying those are the pressures the tires should be? Why are they different in the front?

And, do you think the TPMS system knows which size wheels and tires are on the vehicle?

glassbloke

CheckStein

so yellow means the psi is below min, and the # shown in yellow is the min needed, so 32 front and 33 rear are the minimum psi. isnt it usually the other way around, for front engine vehicles you typically see front 1-2psi higher than rear??? door sill says 35F and 42R. seems like odd rear psi.

so it tells you you have bad psi, but doesnt tell you the actual psi.
is 15psi more urgent than 31 psi? i would say it is.
doesnt seem like a very smart way to do TPMS. just tell me what it is, and if its low or not.