Stupid Question - Air Pressure Readings??

[Va PANZUR]

I have a 2018 200 Series LC

On the Air Pressure display, I've noticed lately that some of the air pressure values that are shown on 2 - 3 of the tires (to include the spare) are in a yellow/orange "box", as if the numbers are highlighted. The other tire(s) pressure is shown as normal, in white font.

What does that orange/yellow box (aka highlighting) indicate? Said tires' have the correct air pressure.

MANY thanks in advance!

 

[LvCruiserNewb]

I have a 2018 200 Series LC

On the Air Pressure display, I've noticed lately that some of the air pressure values that are shown on 2 - 3 of the tires (to include the spare) are in a yellow/orange "box", as if the numbers are highlighted. The other tire(s) pressure is shown as normal, in white font.

What does that orange/yellow box (aka highlighting) indicate? Said tires' have the correct air pressure.

MANY thanks in advance!

Not sure I've seen that nor know what it means but if you haven't already I'd do the TPMS initialization procedure in the manual and see if it goes away. If I had to guess the highlighted tire pressure means that they are within some threshold of being declared low. Perhaps the last time the initialization was done the tires weren't at equal pressure thus each tire has a different low pressure threshold so not all tires are boxed at a certain pressure.

What I do notice on my 2018 tire pressure display is that the dash reading is a solid 2-3 psi on all tires less than I measure externally. And that's with multiple different pressure readers. Every pressure sensor I use at the valve stem gives the same answer and the dash display is always 2-3psi below that.

 

[AnyMal]

Not sure I've seen that nor know what it means but if you haven't already I'd do the TPMS initialization procedure in the manual and see if it goes away. If I had to guess the highlighted tire pressure means that they are within some threshold of being declared low. Perhaps the last time the initialization was done the tires weren't at equal pressure thus each tire has a different low pressure threshold so not all tires are boxed at a certain pressure.

What I do notice on my 2018 tire pressure display is that the dash reading is a solid 2-3 psi on all tires less than I measure externally. And that's with multiple different pressure readers. Every pressure sensor I use at the valve stem gives the same answer and the dash display is always 2-3psi below that.

Interesting, are you using denso TPMS?

Mine are somewhat accurate to AI digital amazon gauge, but are 1 PSI below my air chuck gauge. My new indeflate analog gauge seems spot on though, but it has less precision.

 

[LTSR]

What I do notice on my 2018 tire pressure display is that the dash reading is a solid 2-3 psi on all tires less than I measure externally. And that's with multiple different pressure readers. Every pressure sensor I use at the valve stem gives the same answer and the dash display is always 2-3psi below that.

my 2018 LC dash read out for tire pressure is dead accurate with my manual air pressure gauge. Maybe you are measuring cold vs hot.

 

[Leaky2014]

The AI bot spit this out.

The orange/yellow box indicates that the tire pressure is at or near the maximum pressure level that is recommended for that tire. This is a safety feature to help prevent over-inflation. If the pressure increases beyond the recommended level, the box will turn red to indicate that the tire pressure is too high.

 

[kcjaz]

my 2018 LC dash read out for tire pressure is dead accurate with my manual air pressure gauge. Maybe you are measuring cold vs hot.

Are you at sea level? I think that is the only way TPS could be accurate.

 

[LvCruiserNewb]

my 2018 LC dash read out for tire pressure is dead accurate with my manual air pressure gauge. Maybe you are measuring cold vs hot.

I'm measuring at the same time. I.e. with the car on and showing values I measure every tire and compare. And with 3 or 3 different manual pressure gauges.

 

[LvCruiserNewb]

Are you at sea level? I think that is the only way TPS could be accurate.

A quick internet search claims that TPMS displays a calibrated or calculated psig based off a seal level ambient calibration not direct psig. So you're right if that is true and there's where my difference is coming from as I'm at about 3000ft MSL. I presumed TPMS was a direct psig sensor without thinking about it first as inside a sealed tire it can't get an outside reference pressure to measure psig against. The owners manual or TPMS manual should probably explain that so high altitude dwellers don't inflate their tires to the dashboard reading.

 

[kcjaz]

A quick internet search claims that TPMS displays a calibrated or calculated psig based off a seal level ambient calibration not direct psig. So you're right if that is true and there's where my difference is coming from as I'm at about 3000ft MSL. I presumed TPMS was a direct psig sensor without thinking about it first as inside a sealed tire it can't get an outside reference pressure to measure psig against. The owners manual or TPMS manual should probably explain that so high altitude dwellers don't inflate their tires to the dashboard reading.

Yes, I went down this rabbit hole the first time I took the 200 to Colorado. TPMS sensors are absolute pressure measurement devices. You are right that they have to be given their location in the tire. I have actually done the math and have verified the difference from my home 925' elevation to Ouray's 7800' and checks within the precision of the TPMS display (no decimal). I don't rely on TPMS when I air down when I care about accuracy.

What would be nice is if the car's computer would altitude compensate rather than simply using 14.7 to convert absolute P to psig, though practically speaking, a few psi over inflation, really isn't going to be an issue. Very few people are this anal about tire pressure or need to be. The difference from seal level to Ouray is 14.7 to 11.0 psi or 3.7 psi. Its 5 psi at the top of Engineer Pass (see what I did there?).

 

[AnyMal]

Yes, I went down this rabbit hole the first time I took the 200 to Colorado. TPMS sensors are absolute pressure measurement devices. You are right that they have to be given their location in the tire. I have actually done the math and have verified the difference from my home 925' elevation to Ouray's 7800' and checks within the precision of the TPMS display (no decimal). I don't rely on TPMS when I air down when I care about accuracy.

What would be nice is if the car's computer would altitude compensate rather than simply using 14.7 to convert absolute P to psig, though practically speaking, a few psi over inflation, really isn't going to be an issue. Very few people are this anal about tire pressure or need to be. The difference from seal level to Ouray is 14.7 to 11.0 psi or 3.7 psi. Its 5 psi at the top of Engineer Pass (see what I did there?).

This is interesting info i never considered being in TX…

I just don't understand how the internal PSI reading would be inaccurate though in a closed system inside the tire. Changes in pressure internally by altitude yes, but i don’t get the above readings being off in the gauges.

Surely we would here about this discrepancy everywhere around tire info?

 

[kcjaz]

This is interesting info i never considered being in TX…

I just don't understand how the internal PSI reading would be inaccurate though in a closed system inside the tire. Changes in pressure internally by altitude yes, but i don’t get the above readings being off in the gauges.

Surely we would here about this discrepancy everywhere around tire info?

PSIG (gauge) sensors measure pressure with a little diaphragm. One side of the diaphragm is exposed to the pressure being measured and the other side is open to the ambient pressure. The deflection of the diaphragm produces the pressure measurement. So, what you are really measuring is the differential pressure between two things. When you put your hand held gauge on your tire you measuring the differential pressure between the ambient (barometric pressure) and pressure inside the tire. The reading is called gauge pressure (psig). If you put a pressure gauge inside a tire, it would read zero because the pressure on both sides of the diaphragm would be the same. TPMS sensors are "absolute" sensors and have one side of the diaphragm sealed to a little chamber that is at vacuum (as in zero) pressure. The TPMS system simply assumes sea level barometric pressure and subtracts 14.7 from the reading.

 

[Johnny396BB]

TPIS sensors have batterys inside also. Special tool needed to test them i think. Not worth repairing really if broken our out of battery the sensors. Rather useless system on cars..better to check cold once a week with a good gauge in my opinion. on Big aircrafts yes they are usfull

 

[lx200inAR]

TPIS sensors have batterys inside also. Special tool needed to test them i think. Not worth repairing really if broken our out of battery the sensors. Rather useless system on cars..better to check cold once a week with a good gauge in my opinion. on Big aircrafts yes they are usfull

Useless is a pretty strong word. Not sure what would work better for detecting a slow leak than the active TPMS.

 

[Johnny396BB]

Ok yeah maybe not useless. But expensive to repair when they fail after 8-10 years.

Check tire pressure with a good gauge 2-3 times per week. Takes 5 minutes maximum

 

[manofthewoods]

I have a 2018 200 Series LC

On the Air Pressure display, I've noticed lately that some of the air pressure values that are shown on 2 - 3 of the tires (to include the spare) are in a yellow/orange "box", as if the numbers are highlighted. The other tire(s) pressure is shown as normal, in white font.

What does that orange/yellow box (aka highlighting) indicate? Said tires' have the correct air pressure.

MANY thanks in advance!

The box around the numbers means that sensor is reading a pressure difference from the previous re-set.
This happened recently on mine when our cold temperatures dropped enough to affect the pressure in the tires. I readjusted each tires' pressure, then, followed the reset procedure.
Problem fixed. Wish everything was that easy.
Happy holidays

 

[TheGrrrrr]

Ok soooo..... My LT285/75R17 have a RCTIP of 37 and I generally set them exactly at that number according to the TPMS. I've always noticed variance versus my other external gauges, but not consistent enough to think about the sea level calibration being the culprit. Now I'm down the rabbit hole.

According to the calculation, if my TPMS reads 37psi at my home (1,234ft elevation) then the actual pressure in the tire is 35.43psi, and in Moab (4,026ft elevation) the actual would be 32.07psi and in Ouray (7,792ft elevation) the actual would be 27.95psi.

So if I want the correct pressure in the tires when I air up after a trail or even when arriving to the area, in these places I would need to have the TPMS show:

Home (1,234ft elevation): 39psi
Moab (4,026ft Elevation): 43psi
Ouray (7,792ft elevation: 49psi

Those pressures in the TPMS would equate to actuals of 37psi. Am I getting this right?

 

[TCH2FLY]

Ok soooo..... My LT285/75R17 have a RCTIP of 37 and I generally set them exactly at that number according to the TPMS. I've always noticed variance versus my other external gauges, but not consistent enough to think about the sea level calibration being the culprit. Now I'm down the rabbit hole.

According to the calculation, if my TPMS reads 37psi at my home (1,234ft elevation) then the actual pressure in the tire is 35.43psi, and in Moab (4,026ft elevation) the actual would be 32.07psi and in Ouray (7,792ft elevation) the actual would be 27.95psi.

So if I want the correct pressure in the tires when I air up after a trail or even when arriving to the area, in these places I would need to have the TPMS show:

Home (1,234ft elevation): 39psi
Moab (4,026ft Elevation): 43psi
Ouray (7,792ft elevation: 49psi

Those pressures in the TPMS would equate to actuals of 37psi. Am I getting this right?

I didn't check your actual math but the real problem is that you are assuming the "standard" sea level pressure of 14.7psi (29.92 inHg) based on a "standard" temperature (15c). The actual pressure at any altitude can and will vary based on temp.
So you really have find the local baro pressure and calculate from there.
For example at the moment I write this the Telluride airport pressure (close to Ouray and easy to get) is 29.56 inHg but the airport elevation is 9070' and the "standard" pressure should be closer to 21.4 inHg so you can see that using the standard values will give poor results.

IMO the TPMS values are great to validate that you have no significant variation in pressure across all the tires (i.e. no slow leaks) but not great for precise inflation. I just use a good gauge and note the variance for reference to the TPMS.

 

[bloc]

IMO the TPMS values are great to validate that you have no significant variation in pressure across all the tires (i.e. no slow leaks) but not great for precise inflation. I just use a good gauge and note the variance for reference to the TPMS.

Agreed. We can’t even use the “idiot light” on trail because when airing down the minimum pressure is below the lowest alarm threshold, so the light will be on unless we disable things.

But we can at least select the pressure readout and watch for one tire getting below the others.

 

[GordJ]

So the pressure in your tires changes with the altitude?

 

[TCH2FLY]

So the pressure in your tires changes with the altitude?

It can, how much depends on outside pressure which is going to vary.