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

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?
 
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
5,935
548
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
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
3,082
1,342
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
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.
 

JPP

Active Member
Feb 4, 2013
3,082
1,342
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).
 
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.
 

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