redcupr

Did some searching, both here and in the manual, but didn't find an answer.

My TPMS is reading about 2 PSI lower than when I check the pressure manually. Is there a way to calibrate the system?

I've checked with two separate gauges. The TPMS display in the dash with read 34, then I check it with the gauge and I'm getting 35.8.

RTIS250

iTrader: (9)

Unfortunately there is no way to calibrate the system. Figure there is probably +/- 1 PSI accuracy in both the gauge you're using and the TPMS system...

Obscene14

One of my TPMS is off too. The left rear reads 1 psi high. I use a racing gauge that reads down to a .10 psi to top off my tires. The TPM is deffinately off. My M35 TPMS were a lot worse. They all were off by 2-3 psi so Lexus has Infinity beat as far as that goes.

Bass Mech

FYI the barometric pressure will directly affect your readings with an external gauge. Here in colorado at 6500 feet our pressures are off by 3psi

Obscene14

Why would it matter what altitude your at? If your Tire pressure gauge reads 35psi at 6500 asl then your TPMS should read 35psi at 6500 asl. Isn't a TPMS just an analog pressure gauge that converts the signal to digital so the ecu can display the psi on the gauge cluster?

Bass Mech

well since you asked here is the technical answer.

first off there are 2 different terms you need to know. absolute and relative

the TPMS reads absolute. meaning barometric pressure at standard day temperature is 29.92 inches of mercury or about 14.7 PSI

TPMS sensor uses a sensor that reads 0 in a vacuum then calibrated to read 0 at 14.7 psi because if your tire is flat the internal pressure will be 14.7psi (1 bar)

now let's talk relative pressure. relative pressure is pressure relative to the outside pressure. to have 30 psi in the tire it has to have a pressure that is 30 psi greater than the external pressure (1 bar)
when you measure pressure at the valve stem and a manual pressure gauge the gage is literally comparing the pressure inside the tire with the pressure outside the tire.

if you put 30 psi in the tire at sea level then drive to Colorado and assuming the temperature remains constant, the same tire has the same amount of air inside it, but the barometric pressure outside the tire has now dropped by 3psi.

so if you check it with a manual pressure gauge, it's going to read a pressure difference of 3psi more than at sea level.

now with all of that info out of the way it really depends on what type of pressure gauge your measuring the tire with, if your using a digital gauge it can either be absolute or relative. most are relative because we want to know the difference in pressure between inside and outside the tire. this directly corresponds to how much pressure is acting against the inside of the tire to keep it inflated.
the problem with a TPMS is it's located totally inside the tire. it has no way to reference the outside barometric pressure when you take the reading. it's reference 0 is held constant at 14.7 so any pressure gauge you check the pressure with is going to reference the barometric pressure outside the tire.

this is also one of the reasons they tell you to measure your pressure when the tire is cold (its also a good idea to perform this in the morning when the temps are low and barometric pressure is the least affected by temperature. because if the air in the tire is hot it will make the pressure greater. we need to have some form of reference or standard if we are going to take a measurement.

so in a nutshell if you want to validate the accuracy of the TPMS you have to check a tire at sea-level with the outside temperature at 59 degrees Fahrenheit. this is what's known in the aviation industry as (standard day conditions) when these 2 conditions are met the barometric pressure will be exactly 29.92 inches of mercury or 14.7 psi atmospheric pressure.

under these conditions everything should line up exactly from a relative standpoint, and an absolute standpoint assuming all of your gauges are calibrated correctly..

you'll find at the temperatures raise the density altitude increases meaning the density of the air becomes less. just like if you increase altitude the pressure decreases.

in the case of a tire since the air is trapped inside the tire, if you heat it the pressure will increase because it has no place to go!

does this clear it up for you?

Obscene14

Thanks for the detailed explanation. I guess one of my sensors wasn't calibrated correctly since it constantly reads 1 psi higher than the other three.

Bass Mech

that is entirely possible.

so it reads 1 psi different per the gauge than is displayed on the dash?

check it in the morning before you drive anywhere. make sure the car is kept in the garage because of the sun heats a tire up the pressure will vary.

overinflate each and using a digital gauge bring each just down to the preset 32 psi or whatever and then start the car.
read the 4 tires. they should all be pretty close.

Obscene14

My car lives in an underground parking garage. Very little temp fluctuation to speak of. I only top up the tires in the morning after the car has sat unused all night.

My top off routine consists of overfilling the tires by 1 psi so all are at approx 37 psi then bleeding each tire down in small incriments to 36 psi as stated on the door jam placard.. When I start the car and drive off the left rear will always read 35 while the others read 36. After the tires are thoroughly warmed up (10 or so miles) the left rear may or may not read the same as the others. More often than not it still reads low. It's not really a big deal and not worth a dealer visit imo. It's just annoying because I'm **** about setting them all to the same psi and then one reads low.

Bass Mech

Got it! Well if I were you I would put a little extra in the tire, you'll never notice it!
And forget the fact you checked them all and just pretend all 4 are inflated the same..