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Interesting how tire pressure monitor works.. (Story about a flat tire)

hanl1

Member
Sep 2, 2015
101
10
Stamford, CT
I was lucky enough to get a flat tire this evening after 1 week of delivery. While driving I saw a warning 'Tire Pressure Very Low - PULL OVER SAFELY' so I did. I verified with my own eyes that the rear-right tire was flat and proceeded to call roadside assistance. The guy on the phone was very helpful and soon afterwards got a tow truck to deliver me a spare wheel. I could keep it for 3 days so I could either get my own tire fixed or buy a new one from Tesla (they don't do repairs apparently). I put the flat tire in my trunk and drove on.
However, my dashboard still said that wheel has very low pressure and kept asking me to pull over. I verified that the spare wheel is good (at least visibly), but I called Tesla to ask about it anyways. Their answer is interesting - they say that it is normal and the car is not able to monitor pressure of the spare wheel. This could be a noob question but how does this work? There is something unique in each wheel that is somehow related to pressure monitoring? And the spare wheel doesn't have it so the old status didn't get reset which is why it still reports low pressure?
The only difference I can see between the spare and original wheel is that the spare wheel is missing the center caps with the Tesla logo on it. But that should be purely cosmetic no? Any thoughts?

Also, I am thinking about just getting it patched tomorrow, unless there is compelling reason I should buy a new one instead?
 

trils0n

2013 P85
Feb 12, 2013
1,527
1,947
SF Bay Area
Your wheel/tire has a TPMS sensor that is paired with the car. The new wheel/tire isn't paired, so the car isn't receiving pressure data.
 

martinwinlow

Member
Jun 18, 2012
360
-27
Isle of Colonsay, UK
The sensor is normally part of the tyre valve and is installed in the wheel rim before the tyre is fitted. They use radio communication to the car to send the data. Some require car-specific pairing (Tesla) others do not. The battery is usually good for 5 years+ (tho quite why they can't generate their own power given all the moving around they do is a mystery to me! Coming soon, apparently)... MW

tpms.jpg
 

yobigd20

Well-Known Member
Oct 28, 2012
5,929
532
Skaneateles, NY
The battery is usually good for 5 years+ (tho quite why they can't generate their own power given all the moving around they do is a mystery to me! Coming soon, apparently)... MW

View attachment 95758

all 4 of my TPMS batteries went dead and had to be replaced, out of pocket. lasted about 2 years. each set costs $200. not cool. they should last longer than that. so this is an expense people should expect to be paying periodically...
 

CHG-ON

Still in love after all these miles
Jun 24, 2014
3,079
636
Santa Cruz Mountains, USA
When I deflated a tire in order to reinstall my AlloyGator, the warning did not go off until the tire was almost completely out of air. I would certainly expect a warning much earlier than that, say at a 20% decrease in pressure. Not 80% or so. That seems like a big time safety issue to me.
 

supratachophobia

Active Member
Sep 24, 2014
3,856
2,683
Columbus, Ohio
The thresholds are there because in a given day, the PSI can fluctuate as much as 15psi just by driving and the day warming or cooling. And the readings from the TPMS are not realtime, so as not to drain the batter faster. You probably caught it on the minimum thresholds at the max time between reading levels.
 

CHG-ON

Still in love after all these miles
Jun 24, 2014
3,079
636
Santa Cruz Mountains, USA
Wow. I don't think I have ever seen that big of a PSI swing before.

The thresholds are there because in a given day, the PSI can fluctuate as much as 15psi just by driving and the day warming or cooling. And the readings from the TPMS are not realtime, so as not to drain the batter faster. You probably caught it on the minimum thresholds at the max time between reading levels.
 

JPP

Active Member
Feb 4, 2013
3,060
1,287
SF Bay Area, CA
The thresholds are there because in a given day, the PSI can fluctuate as much as 15psi just by driving and the day warming or cooling. And the readings from the TPMS are not realtime, so as not to drain the batter faster. You probably caught it on the minimum thresholds at the max time between reading levels.

FWIW I have the FOBO Plus system installed on both of our cars (OEM 19" wheels, OEM Goodyear tires on the S85, OEM Primacy on the 70D). If the car has been parked overnight and the tires are cool (65 degrees F), I have pressures around 44.5-45.5. If it is a hot day (100 degrees F) and I have been highway driving at 70 MPH, I see tire temps up over 100 degrees F and pressures at 50.5-51.5.
 

CHG-ON

Still in love after all these miles
Jun 24, 2014
3,079
636
Santa Cruz Mountains, USA
That's been my experience also.

FWIW I have the FOBO Plus system installed on both of our cars (OEM 19" wheels, OEM Goodyear tires on the S85, OEM Primacy on the 70D). If the car has been parked overnight and the tires are cool (65 degrees F), I have pressures around 44.5-45.5. If it is a hot day (100 degrees F) and I have been highway driving at 70 MPH, I see tire temps up over 100 degrees F and pressures at 50.5-51.5.
 

SPXMike

Member
Oct 1, 2015
110
96
La Habra Heights, CA
Thanks for the info! I picked up my CPO P85+ Tuesday. Yay I'm a Tesla driver! Less than 24 hrs later I got a flat on the 105 in Compton. Am driving around with my flat in the trunk. Same experience. Hopefully I can get it fixed so I don't have to drop $525 on a replacement tire.
 

JPP

Active Member
Feb 4, 2013
3,060
1,287
SF Bay Area, CA
Where do you see the tire pressure info after updating to the new software???

If you have a relatively recent build (about VIN 55000 or later) with the new TPMS system, you can use the roller wheels on the steering wheel to select the Status monitor (hold the wheel until you see the various choices, and roll to your choice). Those of us with older builds do not yet have support for TPMS display (...glad I have a FOBO Plus system).
 

dhanson865

Active Member
Feb 16, 2013
4,562
7,014
Knoxville, Tennessee
Wow. I don't think I have ever seen that big of a PSI swing before.

Try setting the tire pressure in the dead of night winter time then driving to another state with hotter weather. Think snow slope to start and dry desert sunny day down below to finish.

Rule of thumb is 10 F = 1 PSI (not mathematically accurate just easy to remember and still useful, more so than remembering PI as 3.1)

Set your tires at 0F and drive them to the desert at 100F and that alone is worth about 10 PSI. Add on top of that the friction of driving and you'll get more than 10 PSI swing.

Plenty of other factors for changing PSI (altitude, road temps, speed, duration of travel, alignment issues, etcetera) but it's doable.

Personally I don't drive long enough or fast enough to see that big of a swing due to my driving but just driving a 15 mile commute at low speeds I see a better than 5 PSI swing.
 

Drumheller

Member
Jan 20, 2016
715
4,995
Arizona
I ran into the same scenario with a flat time and the TPMS. I'm very surprised that this expensive of a car (P85+) does not display the actual PSI values of the tires somewhere in the controls. My 2008 Pontiac displayed that information.
 

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