Buzzer479

Hi everyone !
I have a 2015 RX450H and was wandering how to properly read our tire pressure numbers. As we know they go in line and read something like this 39 38 38 39. I always thought they represent LF RF LR RR. Was adding air yesterday due to cold weather and realized that that was not the case. Upon searching the forum it looks like the numbers do not have any logical order and are pretty much random, based on how they were programmed. With this being said, would everyone agree with this ?
Also, what do you think about how precise these numbers are ( I check the PSI with my pump and all numbers read around 4PSI lower) ?
What would be ideal PSI in your opinon ?

Thanks !

Nad1370

I dont have a 450H but,
Yup, its random.
Manufacture specs is 33psi all around.
I set my tires at 38 since mostly highway driving and gets me a tad better mileage.
TPMS not only measure pressure, but air temperature in the tire as well.
So you probably wont get an exact psi reading but it will be close.( pending air temp, humidity, barometric press... ect.)
I'd trust a known good tire pressure inflator or gauge over the info screen. lol

ColAngus

The reading your "pump" (compressor) is giving you is garbage. Use a tire pressure gauge to compare to the tpms.

salimshah

I am waiting for an expert to chime in ..

Till someone corrects this ..

4 transmitters in the tires transmit the pressure level (not not the psi, just level). The tires can be rotated or moved around and there is no way to discern which tire is sending which order you see in your instrument cluster.

There is calibration process where the tech calibrates the signal level to the actual psi by taking measurement with a calibrated gauge and that value is then displayed in the instrument cluster. This is why the tire shops charge extra labor with tpms. This is in addition to the cost of the sender.

When you see different number then it becomes the issue of which is telling you the truth ... the compressor, the display or the gauge you have in hand. Ironically call can be bad.

Salim

Salim

Nad1370

Im not sure what you mean by pressure level only. (it has to have a value right?) (correct me if im wrong please, i learn stuff everyday lol)
I thought pressure level is measure in form of psi, KPA or whatever value that is transmitted from each TP sensor to the TPW antenna receiver?
Without a value level, what would it transmit?

Im quoting this from Lexus' description on TPMS
" DESCRIPTION OF SYSTEM

(a) Each tire pressure warning valve and transmitter is equipped with a tire pressure sensor and transmitter and is installed in each tire wheel. The sensor measures the tire pressure. The measured value and transmitter ID are transmitted to the tire pressure warning antenna and receiver on the body as radio waves and then sent to the tire pressure warning ECU from the tire pressure warning antenna and receiver. If the transmitter ID has already been registered, the ECU compares the measured tire pressure value with the standard value. When the value is less than the standard value registered in the tire pressure warning ECU, the warning light on the combination meter comes on.

When it is necessary to change the tire pressure setting, use the tire pressure warning reset switch to initialize the system."

As for charging extra to "calibrate" signal levels, any Toyota/Lexus owner can do this via tire pressure warning reset switch, if equipped. (RX450H lower part of drivers dash)
This will change the warning threshold in the TP ECU.
They might charge extra to replace the seals in the TP sensors.

Hope i didnt go off topic from the OP's original question.

Buzzer479

Thank you all for your inputs !
Here is why I had the question in the first place:
When I got the winter tires installed the screen read 39 when cold and up to 42 after an hour or so on the road. When the temp. dropped much lower it read 34 on cold and 39-40 on the road. I thought that that those numbers were a little high checked with my "compressor" it read 32 and 35 respectively. What would be most efficient and precise way to measure the tire pressure and what should we go for in terms on PSI ?

Thanks

salimshah

OP is hopefully satisfied so we can indulge ..
What I meant (again what i say needs to be verified) is that signal from the pressure sensor is uncalibrated. The signal output needs calibration to indicate the pressure is say 32psi. So reference pressure is filled and measured by hopefully a calibrated pressure gauge and then the vehicle is programmed to recognize the signal level from sensor to be say 32psi. This is done to reduce the production cost of the sensor and extra human effort once in say 45thousand miles.

Salim

salimshah

There can be calibration issues at all points including your compressor gauge.
Either get a calibrated tire pressure gauge [or borrow 5 or say 10 gauges and get an average reading .. has its problems too].
Over fill the tire by about 5 psi but do not exceed the max psi on the wall. Let the vehicel sit overnight in the garage. IN the morning let out the air to get to the psi stamped on the door jamb or the manual.

But I have seen most folks set 3 to 5 psi over the specified value. So much for the calibrated equipment.

If your display is showing some other value than what you set, then have the TPSM system reset .. flow the instruction posted by much respected member NAD or go to the dealer.

Hope it is all clear like a mud.

Salim

MellonC00

through trial and error I can verify that the sensor sits with each wheel. they are not random. by pumping different pressure to each wheel you can actually find out which number responds to which wheel. one caveat is that they move around with each wheel. in other words, when you do your tire rotation, the order in which the numbers show up on the display screen will be changed. have fun trying to figure that out.

so far, the my numbers on the display is about 0.5 psi off from my manual tire tire gauge. I'm OK with it. I'm running about 34 fronts and 30 rears. They were higher in the summer but the cold pressure reading in the mornings have dropped my psi due to frigid weather (for CA) lately.

Nad1370

CL needs friendly indulging to garner more info on our vehicles and how these old/new tech work.

From what i know, Toyota/Lexus TP sensors are already calibrated.
Maybe on some other car companies , but i dont think for Toy/Lex.
Thats why if you put 32psi of air in the tires, your display if you have one, will show 32psi or close to it without the "calibration" theory.
MellonC00 just confirmed it (last comment prior to mine)
It wouldnt make sense if you need to calibrate each sensor if you adjust tire pressure even if it went flat.
For all i know, my GX has no calibration for the TPS.
It does have a tire reset button though.
I did however needed to replace one TP sensor (tire light flashing, diagnosed as a "dead" sensor)
All they did was register the new sensor so the TPS ECU can recognize it. Kinda like hooking up to a WIFI. (each sensor has a 6-7 digit code, not really sure how many)

And yes i agree its clear like muddy water. Lol
Same respect if not more goes to you as well SALIM.
And where's the d@mn expert...! LOL

Rosscoe83

Agree and I have empirical evidence. I have one tire that has a really slow leak (like it drops 1lb/month). It was on my left rear tire. I was able to verify that they did indeed rotate my tires at my 65K maintenance when it is now the front left that goes low.

takeshi74

Do they change order on the display or is it that a number in a given position on the display now refers to a different location?

riredale

I think the order on the screen simply represents which sensors are picked up soonest. But I hear that on some makes the system is smarter and actually shows which tire is low. Not that important to me--if the tire pressure light illuminates it won't take long to find the low tire.

To my knowledge, the sensors are calibrated from the factory. They also draw power from a built-in cell, which eventually fails, which is why the sensors need to be changed out every now and then. The sensors are a sealed unit and are replaced as a whole, not just the cell inside. There are no adjustments.

There is a reset button under the dash that allows one to tell the computer when it should turn on the "low tire pressure" light. I think it looks at the current reading of the four sensors and then triggers if pressure drops 5psi or more from those levels.

salimshah

You may be correct ... but if the signal from the sensors is calibrated then there is no need to reset at the vehicle level .. it should simply display it and light is triggered at preset (known value).
On the other hand if sensors give out 1v, 0.9v, 1.1v and 1.2v for 32 psi fill level [I am not sure it is voltage or some other digital information], a reset button in the vehicle accepts the levels to indicate 32psi from each sensors. Clearly I am theorizing here.
Maybe one of these days, i will let out air and set them to 25psi and hit the reset button. If the display shows 25psi then my theory is proven wrong.

Please note that I am not claiming that I am right and you are wrong. We are just stating our point of views and trying to reason it out.

Salim

gadiehl

Yes, the TPMS on my old Acura RL displayed a nice picture of a car chassis with 4 tires and the pressure indicated for each one. It was a big disappointment when I discovered my Lexus could not do the same.