Pump vs Tire Gauge vs TPMS

[figure1a]

apologies if this isnt the right place to ask this, but it feels like it is.
So my TPMS has been moaning on an off for the nearly 2 weeks since i've had the car. so i went to the local garage that has a air pump and filled it to 39 at front and 37 at back as per the documentation.
in the TMPS its telling me its 38 and 36. so i can understand that its maybe slightly off as per all the chat in this thread. but the issue i have now is that in the other part of TPMS where it tells you the deviation, its saying that there is -1, -2, -1, -2 differences... which I'm baffled about given that all the readings are effectivly around -1 from the "recommended"
does TPMS not recalibrate itself and therefore the deviations should all be 0? even if not, should it not all be -1? if the tpms is faulty then surely i'd see that in the actual pressure part of the page and see all of the tyres being different values of pressure?

You are getting those differences because you have filled the tires to different pressures that what is registered in your TPMS. Your TPMS will tell you what your tires should be filled to when you select your wheel size, summer vs. winter, etc.

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[figure1a]

This is a question I have long wondered about. Figured I would get the expertise of this group on it.

I have often seen a significant difference between the tire pressure as measured by my pumps vs my tire pressure gauges vs the TPMS. I am wondering which one to trust. Here is the data from this morning. All tires were cold:

• 38 PSI as measured by the TPMS
• 40 PSI as measured by 2 tire pressure gauges
• 42 PSI as measured by two pumps that have built in gauges

Which one do I trust or am I overthinking it? 4 PSI difference seems like a fair bit. Plus if I trust the pumps, the TPMS may start complaining that the tires are underinflated.

Thanks in advance for the feedback.

My cheapy little digital handheld tire gauges all pretty much line up with the TPMS. I think most of them are pretty accurate nowadays. Also, I think most modern TPMS are pretty good at being accurate also.

 

[Sly_North]

Does anyone know if our TPMS are direct (measure pressure. Larger valves) or indirect (car has a sensor near each wheels to detect when the valve TPMS is close; it counts the number of wheel rotation and compares with the car speed) ?

A quick internet search did not give me the answer. Given we have to drive at 25+ kmph, I would guess we have an indirect one. Which would be the less accurate version (for instance, it cannot know the exact radius of the wheels, which depends on how used they are).
In which case, an external measurement would be more accurate.

 

[Scandinavian]

Why not use the search function here on the forum?
https://www.taycanforum.com/forum/t...sors-tpms-oem-and-aftermarket-resources.6343/

Looks like a very normal sensor to me that measures pressure

 

[thefunkygibbon]

You are getting those differences because you have filled the tires to different pressures that what is registered in your TPMS. Your TPMS will tell you what your tires should be filled to when you select your wheel size, summer vs. winter, etc.

yes but as i say, i did fill it to what the pressure is supposed to be according to the tpms. having it complain that the tyres are not full, when there are different usecases which require different pressures, let alone the fact that any external pump is likely to (and is demonstrably the case) show different pressures than the tpms system is/can. without pumping, going for a drive and then going back to the pump and driving etc ad nauseam, getting the tyres to the exact pressure that the tpms system is wanting, is an unnecessarily long process.

also as i also said, the pressure which it is showing is actually 38 (instead of 39) and 36 (instead of 37) on both the front and back respectivly. therefore, why would it report in the deviation screen that front left is -1 and front right is -2 and the opposite for the rears. that itself doesnt seem to make any sense to me. (i'd 100% get it if it was -1 all around like the pressures actually suggest)

 

[whitex]

This is a question I have long wondered about. Figured I would get the expertise of this group on it.

I have often seen a significant difference between the tire pressure as measured by my pumps vs my tire pressure gauges vs the TPMS. I am wondering which one to trust. Here is the data from this morning. All tires were cold:

• 38 PSI as measured by the TPMS
• 40 PSI as measured by 2 tire pressure gauges
• 42 PSI as measured by two pumps that have built in gauges

Which one do I trust or am I overthinking it? 4 PSI difference seems like a fair bit. Plus if I trust the pumps, the TPMS may start complaining that the tires are underinflated.

Thanks in advance for the feedback.

The fact that your 2 tire pressure gauges and two pumps don't agree, means your equipment is not very accurate or not calibrated correctly. I have a cheap digital gauge, and a built in pump gauge, they are actually within 1psi of what my Taycan TPMS shows. Of course, all 3 could be wrong (I never calibrated them myself), but they are consistently wrong the same amount.

Generally I go my Porsche TPMS, add or remove air based on the recommended differential pressure.

 

[Sly_North]

Why not use the search function here on the forum?
https://www.taycanforum.com/forum/t...sors-tpms-oem-and-aftermarket-resources.6343/
Looks like a very normal sensor to me that measures pressure

This thread doesn't directly answer my question. But checking several links and reading further online: it seems most direct TPMS uses 433MHz communication (like ours); indirect TPMS doesn't. Thus we do have the direct TPMS.
- Wiki: "Most direct TPMS systems use ultra high frequency (UHF) radio [...] often around 433 MHz in Europe"
- a PDF listing systems for many cars, either as with the communication frequency (for direct TPMS), or as "Indirect TPMS"

Weird that it requires driving at 25+ km/h though. Probably a moving part producing electricity inside the sensor?

n the TMPS its telling me its 38 and 36. so i can understand that its maybe slightly off as per all the chat in this thread. but the issue i have now is that in the other part of TPMS where it tells you the deviation, its saying that there is -1, -2, -1, -2 differences... which I'm baffled about given that all the readings are effectivly around -1 from the "recommended"

The -1 vs -2 are probably rounding errors: like one tire at 38, the other at 37.5: 37.5 would be displayed as 38, but the missing 1.5 would be displayed as -2.

 

[Scandinavian]

Weird that it requires driving at 25+ km/h though. Probably a moving part producing electricity inside the sensor?

They need to wake up! They do not transmit at all when standing still for some time.
You can also easily wake them with a TPMS tool/ clone the adress and install programmable aftermarket sensors. Much lower cost than Porsche offer.

 

[whitex]

Weird that it requires driving at 25+ km/h though. Probably a moving part producing electricity inside the sensor?

The electronics enter low power mode when not in use - to save the built-in battery. Min speed wakes up the circuit. You can also wake them up with diagnostics tools, and some TPMS will also wake up if you start inflating the tires for some number of seconds.

 

[figure1a]

yes but as i say, i did fill it to what the pressure is supposed to be according to the tpms. having it complain that the tyres are not full, when there are different usecases which require different pressures, let alone the fact that any external pump is likely to (and is demonstrably the case) show different pressures than the tpms system is/can. without pumping, going for a drive and then going back to the pump and driving etc ad nauseam, getting the tyres to the exact pressure that the tpms system is wanting, is an unnecessarily long process.

also as i also said, the pressure which it is showing is actually 38 (instead of 39) and 36 (instead of 37) on both the front and back respectivly. therefore, why would it report in the deviation screen that front left is -1 and front right is -2 and the opposite for the rears. that itself doesnt seem to make any sense to me. (i'd 100% get it if it was -1 all around like the pressures actually suggest)

Are you filling to what the door sticker says or the TPMS values?

 

[thefunkygibbon]

Are you filling to what the door sticker says or the TPMS values?

for some reason i dont have a door sticker. but yes i'm doing it to the numbers in the TPMS / pdf i found on here.
I'm awaiting delivery of a battery powered air compressor with digital readout and can use that to inflate. its just a huge faff to get the right numbers within a car garages air compressor since it means a fair bit of time being put aside to do it plus its a quid each time i go back to it to add/remove/read the psi readings.

 

[figure1a]

You have to set it to what your particular cars tpms settings are which you can find in the pcm. Or just add the amount of air your car is telling you that you are under by. When they are all 0, that’s what your tpms is set to which could be wrong. I would go into the pcm and dial in your wheel/tire particulars and the car will tell you what your tire pressures should be set at and it will also set those values in the TPMS.

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