Model 3 TPMS sensors are junk

[voip-ninja]

My cars TPMS reads about 2 lbs lower than my tire guage.

The kind of variation I am seeing is not due to a mis-match between a tire pressure gauge and what the TPMS sensors read.

I have six tire pressure gauges and they all pretty much agree with each other.

The TPMS sensors on the Model 3 are consistently under reporting tire pressure levels by an amount that directly matches with the TPMS sensors thinking they are at sea level when they are in fact at 6,000 feet.

Now if I ran the tires at Tesla's recommended 45-49 PSI for maximum range that wouldn't be an issue. Instead I run the tires at Elon Musk's recommended 39 PSI (I actually have to run at 41 even when "warm" to avoid alarms) for maximum comfort since the roads where I am suck.

Tesla needs to fix this.

 

[CarlK]

fwiw: I picked up my M3 in April, but the first cold spell (in SoCal) when temps dropped into the 70's,

I really feel for you guys.

BTW a good analog racing tire gauge like this is a must in every garage. The bleed valve also allows you to easily set the tire pressure accurately. Those cheapo digital gauges can be as bad as if not worse than the TPMS. Use a good gauge to set and monitor the pressure and TPMS just as reference.
https://www.amazon.com/gp/product/B00404WDUC/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

 

[rlb4]

The kind of variation I am seeing is not due to a mis-match between a tire pressure gauge and what the TPMS sensors read.

I have six tire pressure gauges and they all pretty much agree with each other.

The TPMS sensors on the Model 3 are consistently under reporting tire pressure levels by an amount that directly matches with the TPMS sensors thinking they are at sea level when they are in fact at 6,000 feet.

Now if I ran the tires at Tesla's recommended 45-49 PSI for maximum range that wouldn't be an issue. Instead I run the tires at Elon Musk's recommended 39 PSI (I actually have to run at 41 even when "warm" to avoid alarms) for maximum comfort since the roads where I am suck.

Tesla needs to fix this.

I'm in NJ. About 85 ft above sea level.

 

[voip-ninja]

I really feel for you guys.

BTW a good analog racing tire gauge like this is a must in every garage. The bleed valve also allows you to easily set the tire pressure accurately. Those cheapo digital gauges can be as bad as if not worse than the TPMS. Use a good gauge to set and monitor the pressure and TPMS just as reference.
https://www.amazon.com/gp/product/B00404WDUC/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

I have a high quality analog gauge and it agrees with my $60 digital gauge.

The issue is not setting tire pressures using crappy pressure gauges. The issue is Tesla doing a crappy job of accounting for (low temps) + (comfortable tire pressure) + (their crappy TPMS implementation)

 

[CarlK]

I have a high quality analog gauge and it agrees with my $60 digital gauge.

The issue is not setting tire pressures using crappy pressure gauges. The issue is Tesla doing a crappy job of accounting for (low temps) + (comfortable tire pressure) + (their crappy TPMS implementation)

You could just ignore the "warning" when you want the comfort pressure setting. It will still give you the alarm when pressure is to low too drive safely or is dropping too rapidly. I got the warning in the X a months ago when pressure of one tire only dropped to 35 psi verses spec of 42 psi. I did not have a gauge in the car to double check so I went to a gas station and spend $1.5 to get it pumped up. It was ok for a couple days but then I found out I'm losing pressure at one psi every day or two. Eventually tire store found a very small nail. If the warning came out late I could have been driving with a nail in there for a long time. One can play it safe and wanting an earlier warning or want not be annoyed by false alarms. Either way it is always a compromise and not going to please everyone.

 

[voip-ninja]

You could just ignore the "warning" when you want the comfort pressure setting. It will still give you the alarm when pressure is to low too drive safely or is dropping too rapidly. I got the warning in the X a months ago when pressure of one tire only dropped to 35 psi verses spec of 42 psi. I did not have a gauge in the car to double check so I went to a gas station and spend $1.5 to get it pumped up. It was ok for a couple days but then I found out I'm losing pressure at one psi every day or two. Eventually tire store found a very small nail. One can play it safe and wanting an earlier warning or want not be annoyed by false alarms. Either way it is always a compromise and not going to please everyone.

Or Tesla could just fix it, or is that not an option?

 

[CarlK]

Or Tesla could just fix it, or is that not an option?

What is the "fix" in your mind? It could be set at lower pressure but with higher risk that you're not getting the warning soon enough. Someone else who run tires at high pressure would complain that it is useless.

 

[mongo]

What is the "fix" in your mind? It could be set at lower pressure but with higher risk that you're not getting the warning soon enough. Someone else who run tires at high pressure would complain that it is useless.

Tesla need to correct the absolute pressure needed in the tire based on the ambient pressure where the car is.
voip is at a higher altitude, thus 40 psi differential/ gauge is fewer psi absolute.

 

[voip-ninja]

Pressure with Height: pressure decreases with increasing altitude

 

[GregD60]

I've seen a few people mention that the warnings aren't happening or aren't as sensitive in newer software releases but that hasn't been my experience.

I recently bought a new, pretty high end digital tire gauge and inflator. Set all four tires to 41 PSI. A week later temps dipped into the teens and the car showed tires at 37-38 PSI and low tire pressure alarms were active on at least one tire.

I hooked the tire gauge back up and the tires are at 40.5 PSI.

So the TPMS thinks tires at 37 and the expensive gauge shows they are at 40.5.

Tesla needs to fix this. The car has GPS and knows what altitude it is at all the time, there is no reason it can't calibrate the TPMS sensors properly for altitude, it's trivial.

The TPMS on my car also shows 3 PSI lower than what my pressure gauges show including one very accurate calibrated one. I agree with you that the TPMS is not compensating for the altitude. My question is should it compensate for the altitude? The weight of the car doesn't change to any appreciable amount with a few thousand feet of altitude, so the tires still have to support the vehicle at altitude the same as they do at sea level. This being the case, shouldn't we set the tire pressure to the sea level equivalent, i.e., add 3 PSI at 6,000 feet?

 

[voip-ninja]

The TPMS on my car also shows 3 PSI lower than what my pressure gauges show including one very accurate calibrated one. I agree with you that the TPMS is not compensating for the altitude. My question is should it compensate for the altitude? The weight of the car doesn't change to any appreciable amount with a few thousand feet of altitude, so the tires still have to support the vehicle at altitude the same as they do at sea level. This being the case, shouldn't we set the tire pressure to the sea level equivalent, i.e., add 3 PSI at 6,000 feet?

No, I don't think so.

At altitude there is less outside air pressure resisting the pressure that is inside the tire. You need less air inside the tire to exert the same amount of force to provide support for the structure of the tire and to hold up the weight of the vehicle at a higher altitude than you do at a lower altitude.

The pressure gauge is showing you the pressure between the inside air and the external (outside) air pressure. If Elon Musk says that "for comfort" this should be 39 PSI then this reading should be the same at sea level and at 6,000 or 8,000 feet.

The problem is if you try to set the tires for 39 actual PSI at altitude the TPMS sensors believe that the tires are set at 36 or so PSI and will put them into alarm.

I don't believe this is right and I believe other manufacturers have solved for this problem by some means such as mass air flow sensor or something else adjusting an offset for the TPMS reading.

Over-inflated tires are as big of a problem as under-inflated tires. If you set your tires at an abornally high PSI level at altitude to make the Tesla TPMS sensors happy (say 52 PSI to read 49 PSI on the car) and you then go down to sea level the actual PSI will be much higher and could result in rough handling and in some rare cases (hitting a bad pot hole) could increase the chance the tire blows out.

Tesla needs to fix this. The car has a GPS unit that knows what altitude it is. This could be used in the computer to offset the readings on the sensors and compute a "real" reading.

This is just laziness, lack of acknowledgement or lack of caring on Tesla's part.

 

[mongo]

I don't believe this is right and I believe other manufacturers have solved for this problem by some means such as mass air flow sensor or something else adjusting an offset for the TPMS reading.

Great write up. TPMS reads absolute, tire inflation is relative.
Other OEMs use the Manifold Absolute Pressure (MAP sensor) reading from before the engine is started.

 

[Skipdd]

I really feel for you guys.

BTW a good analog racing tire gauge like this is a must in every garage. The bleed valve also allows you to easily set the tire pressure accurately. Those cheapo digital gauges can be as bad as if not worse than the TPMS. Use a good gauge to set and monitor the pressure and TPMS just as reference.
https://www.amazon.com/gp/product/B00404WDUC/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

I have learned a lot from reading this thread. I don't have OP's issue but do experience drops w temperature, and now realize I can't trust the car's sensors. I have been going to places that have air machines with which you set the air level you want and it automatically inflates to that level. I like this gauge - probably going to order one. Thanks!

 

[GregD60]

No, I don't think so.

At altitude there is less outside air pressure resisting the pressure that is inside the tire. You need less air inside the tire to exert the same amount of force to provide support for the structure of the tire and to hold up the weight of the vehicle at a higher altitude than you do at a lower altitude.

Why do you think that less air would be needed to exert the same amount of force? It seems to me that the amount of force that a tire can support should be based on the absolute pressure of the air inside of it; the outside air pressure doesn't make any difference in this regard.

 

[voip-ninja]

Why do you think that less air would be needed to exert the same amount of force? It seems to me that the amount of force that a tire can support should be based on the absolute pressure of the air inside of it; the outside air pressure doesn't make any difference in this regard.

Does a submarine care about how much outside water pressure there is for determining if the hull will remain intact if the air pressure in the hull is constant?

 

[mongo]

Why do you think that less air would be needed to exert the same amount of force? It seems to me that the amount of force that a tire can support should be based on the absolute pressure of the air inside of it; the outside air pressure doesn't make any difference in this regard.

It is net force on the rubber. The tire pressure pushes out, the ambient pressure pushes in. YouTube a balloon in a vacuum bell:

A normal gauge reads the difference between these two pressures. An in tire TPMS can only measure absolute pressure so the vehicle needs to adjust for local conditions.

 

[GregD60]

Does a submarine care about how much outside water pressure there is for determining if the hull will remain intact if the air pressure in the hull is constant?

Yes, but we're dealing with pretty radically different pressure changes on a submarine compared to a tire at altitude. As has been noted previously, air pressure changes about .5 PSI per 1000 feet. Pressure on a submarine increases about 14.5 PSI per 33 feet or about 440 PSI for every 1000 feet; about an 880 times greater rate of change.

With tires, how much force is required to support the structure of the tire versus supporting the weight of the vehicle? I would have thought that the necessary force to support the structure of the tire would be dwarfed by the weight of the vehicle.

Maybe I'm missing something here since I'm not a tire engineer and it's been way too many years since my physics classes, but I'm just not seeing why a reduced outside air pressure would reduce the absolute amount of air pressure needed in a tire to function as designed.

 

[GregD60]

It is net force on the rubber. The tire pressure pushes out, the ambient pressure pushes in. YouTube a balloon in a vacuum bell:

Tires aren't particularly flexible, and their volume changes very little with changes in pressure. I don't see how the balloon analogy works here.

 

[voip-ninja]

Yes, but we're dealing with pretty radically different pressure changes on a submarine compared to a tire at altitude. As has been noted previously, air pressure changes about .5 PSI per 1000 feet. Pressure on a submarine increases about 14.5 PSI per 33 feet or about 440 PSI for every 1000 feet; about an 880 times greater rate of change.

With tires, how much force is required to support the structure of the tire versus supporting the weight of the vehicle? I would have thought that the necessary force to support the structure of the tire would be dwarfed by the weight of the vehicle.

Maybe I'm missing something here since I'm not a tire engineer and it's been way too many years since my physics classes, but I'm just not seeing why a reduced outside air pressure would reduce the absolute amount of air pressure needed in a tire to function as designed.

The difference between the inside and outside air pressure absolutely matters. If it didn't then the tire would work the same with sea level outside air pressure as it works in a vacuum.

Now if you want to argue that the difference is small enough that we can ignore it, I'd say that there are a lot of factors that come into play. Even relatively small differences in tire pressure affect handling, tire wear, etc.

 

[GregD60]

The difference between the inside and outside air pressure absolutely matters. If it didn't then the tire would work the same with sea level outside air pressure as it works in a vacuum.

Maybe this is why the lunar rover didn't use tires with air in them. It would be nice if someone knew how to calculate this, but it's probably a pretty specialized area of knowledge.

Now if you want to argue that the difference is small enough that we can ignore it, I'd say that there are a lot of factors that come into play. Even relatively small differences in tire pressure affect handling, tire wear, etc.

I totally agree about small tire pressure changes making a significant difference. When it comes to setting the pressures on my street cars, I normally start at the manufacturer's recommendation and then adjust from there for best wear. In the last 20 years or so, that pretty much always has meant running a pressure a bit lower than that recommended by the manufacturer. For track or autocross use, I go with the pressure that gives best performance. For street cars being used on the track, the old chalk method usually works well to determine this, although a tire pyrometer is much better if you're also making suspension adjustments.

In the case of the Model 3, I'm just trying to figure out a good starting point for the pressure. Also, I think you are absolutely correct that the warning shouldn't kick in so close to the recommended pressure. It makes any adjustments for wear, tire size, tire construction, or use, likely to set off the warning.