TPM accuracy, calibration. - K-Bikes.com - Excellence in Motion
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post #1 of 25 (permalink) Old Sep 2nd, 2008, 12:56 pm Thread Starter
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TPM accuracy, calibration.

The GT is in the shop today: 12K service, brake pad light on, battery power issues, and tire pressure monitor questions.

Last month when it was in for the 6K service the tech remarked that my rear tire was overinflated at 46 lbs, so he let some air out to 44 lbs. It had been showing 42 on the TPM before this, and showed 38 after this, so I thought maybe the tech's gauge was wrong. I later pumped it up to 44 at home using my bicycle pump which has a gauge, and the TPM read 42 again.

When I got the new tire put on mid-trip, they said they filled it to 44 lbs. As I rode home from the service, the TPM blinky thing came up showing 38 in the rear. This apparently didn't happen when the service tech did the test ride, but it's not too unusual for a newly mounted tire to lose some air as everything seats, so I stopped at a gas station and filled it back up. Again I relied on the TPM to get the cold-adjusted pressure right (since the tire was hot), and set it to 44 on the TPM. (At the high end because I was fully loaded.)

Now that I'm home and unloaded the pressure is back to 38/42 by the TPM. But I'm concerned that the TPM is reading low. ...explained all this to the techs, and they think there's a way to recalibrate the system.

Anyway, that's all just a long way to getting to my question:

Have you ever had problems with your TPM being inaccurate? How did you address the problem?

Last edited by bonafidebob; Sep 2nd, 2008 at 4:47 pm.
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post #2 of 25 (permalink) Old Sep 2nd, 2008, 3:15 pm
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Quote:
Originally Posted by bonafidebob
The GT is in the shop today: 12K service, brake pad light on, battery power issues, and tire pressure monitor questions.

Last month when it was in for the 6K service the tech remarked that my rear tire was overinflated at 46 lbs, so he let some air out to 44 lbs. It had been showing 42 on the TPM before this, and showed 38 after this, so I thought maybe the tech's gauge was wrong. I later pumped it up to 44 at home using my bicycle pump which has a gauge, and the TPM read 42 again.

When I got the new tire put on mid-trip, they said they filled it to 44 lbs. As I rode home from the service, the TPM blinky thing came up showing 38 in the rear. This apparently didn't happen when the service tech did the test ride, but it's not too unusual for a newly mounted tire to lose some air as everything seats, so I stopped at a gas station and filled it back up. Again I relied on the TPM to get the cold-adjusted pressure right (since the tire was hot), and set it to 44 on the TPM. (At the high because I was fully loaded.)

Now that I'm home and unloaded the pressure is back to 38/42 by the TPM. But I'm concerned that the TPM is reading low. ...explained all this to the techs, and they think there's a way to recalibrate the system.

Anyway, that's all just a long way to getting to my question:

Have you ever had problems with your TPM being inaccurate? How did you address the problem?
Simply put, yes.

I note that the TPM values increase 1-2 psi from cold to warmed-up tires, if that makes a difference. I compared the TPM with several different gauges at home. They all match.

In May, I took a nail in the rear tire. The TPM let me know. The warning was flashing and the TPM read the back tire around 28 psi. I went to the dealer and had the tire replaced. As soon as I left, I checked the pressures with the TPM, and found that the TPM was reading three psi low in each tire.

I rode back to the dealer, where the service manager showed me the large brass gauge that they have in the shop, and used it to measure the pressure in each tire. He claimed that their gauge is well calibrated, and that he gets the TPM question all the time. He says that the TPMs all read about three psi lower than actual tire pressure, and that it is "within spec". He says that BMW knows about the problem but hasn't done anything to fix it.

I asked him what people do about the readings. He shrugged and said that some people live with the correct tire pressure (incorrect TPM reading) and other people overinflate their tires so that the TPM reads correctly.

I included my TPM concern when I brought my GT in for recalls (ESM, steering damper), but all that came back was "within spec".

I have been looking for another independent, calibrated source to verify the accuracy of the TPM and my gauges, but no luck so far. I haven't tried very hard, either.

In the meantime, I am one of the "make the TPM read correctly" types, so I am probably overinflating my tires by 3 psi. I like slightly overinflated tires. Most of the time, I commute on ordinary flat streets and freeways. The slight overinflation make it a little easier to steer at low speeds and in parking lots. In addition, it gives me longer intervals between tire fill-ups, as I don't have a good compressor at home yet.

If it is true that the TPM is consistently low, it should be a simple fix to compensate for it in the OBC display. This is an exercise for student programmers, for goodness sake.
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post #3 of 25 (permalink) Old Sep 2nd, 2008, 3:50 pm
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Ditto. In search of correct TPM pressure readings I had the monitors replaced twice, w/ no improvement. IMHO it's not ready for prime time. Here in Colorado the swings in temp during a day are large, especially when you get rained on going over a pass....the TPM would light up until the temps warmed up again, which is a real pain. One service manager suggested putting duct tape over the yellow triangle...great response, eh? I too ended up overinflating the tires as an unsatisfactory work around.

I gave up and will not get this feature again.

Maybe there is an aftermarket solution that is consistently accurate. If so, I for one would like to hear about it.

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post #4 of 25 (permalink) Old Sep 2nd, 2008, 4:13 pm Thread Starter
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Quote:
Originally Posted by DavidS
In search of correct TPM pressure readings I had the monitors replaced twice, w/ no improvement. IMHO it's not ready for prime time.
Well, that's a bummer. I really do like the TPM display. I noticed recently that it updates in real time too -- e.g. if you leave the ignition on you can use it while inflating the tires to precisely adjust the pressure, no more relying on crappy gas station gauges or the little pencil things that you can carry.

I think if there's no way to recalibrate then I'll just do the adjustment in my head. I also like generally higher pressure in my tires, especially when loaded. As long as the damn blinky lights stay off, I can handle adding three in my head.
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post #5 of 25 (permalink) Old Sep 2nd, 2008, 4:44 pm
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Bob,

I have something to add to this. I am kind of a tire pressure fanatic. I like to check pressure (using a high-end gauge) every couple of rides or so. So I checked my GT that is equipped with TPM; I just wanted to be sure TPM was accurate. It wasn't.

My TPM was registering about 3psi lower than actual. Then, just before I left on a long trip, out of the blue it began registering 6-7psi low! That triggered the yellow and sometimes, the flashing red esclamation mark indicator. For the entire trip I had to run my tires 3psi overinflated to just keep the flashing red light off (of course, the yellow light remained on for 2000 miles).

When I got back, I went to the dealer and they reloaded a new software disc. Now I am back to the TPM registering 3psi low! They told me that it was considered acceptable within 10% accuracy. At least I can run the correct pressures without an idiot light coming on.

Finally, due to the TPM inaccuracy, I figure it is only useful for indicating a sudden loss of pressure (nail, screw or similar) by flashing the yellow or red esclamation. Otherwise, I rely on my trusty air pressure gauge!

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post #6 of 25 (permalink) Old Sep 2nd, 2008, 6:32 pm
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P.S. I still like the TPM. It warned me when I picked up a nail, and it is consistent in its behavior, so it is still useable for me. I would like it more if it were accurate with a 0.5 psi precision.

Criticism: What good is the TPM's advertised temperature compensation capability, if the darned TPM is inaccurate by more psi than temperature variations induce?
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post #7 of 25 (permalink) Old Sep 2nd, 2008, 7:00 pm Thread Starter
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Quote:
Originally Posted by XMagnaRider
Criticism: What good is the TPM's advertised temperature compensation capability, if the darned TPM is inaccurate by more psi than temperature variations induce?
This is a good illustration of the difference between accuracy and precision. Hopefully the TPM is inaccurate but precise -- that is, if it's 3lbs low when cold, it's also 3lbs low when hot. If so, then it's still useful as long as you know to add 3, or add in whatever error correction your own TPM setup needs. As for temp variations, I think it's 1psi per 10 degrees F ambient temp for a cold tire, and then about +5 psi for a "hot" tire from riding. So even if it reads 10% low, the pressure still goes up by more than than when riding.

...still haven't heard from the dealer. [Update: called the dealer, they're working on the bike now, won't be ready 'till tomorrow, no status to report.]

Last edited by bonafidebob; Sep 2nd, 2008 at 7:06 pm.
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post #8 of 25 (permalink) Old Sep 2nd, 2008, 7:02 pm
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The guy in the shop with the big brass mechanical guage has the best accurate pressure meter!

Just because a display is digital and it's really measuring voltages converting it into numbers, doesn't mean thats the accuracy or repeatability of what's being measured.

That's down to the device measuring actual pressure. Generally you have to have a significant distance or strain movement to get good repeatable accuracy and I doubt if the pressure monitor is anything like that. Sometimes they screw up at the display meter end and readings can change with the state of battery charge.

If I was getting within 3psi of absolute pressure with +/- 1psi variation due to temperature, that's all I would expect from an automotive product. The main thing is what you get is repeatable and stable. If you know your TPM is reading high/low and stays stable, then you can allow for it.



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post #9 of 25 (permalink) Old Sep 2nd, 2008, 7:10 pm
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Quote:
Originally Posted by voxmagna
The guy in the shop with the big brass mechanical guage has the best accurate pressure meter!

Just because a display is digital and it's really measuring voltages converting it into numbers, doesn't mean thats the accuracy or repeatability of what's being measured.

That's down to the device measuring actual pressure. Generally you have to have a significant distance or strain movement to get good repeatable accuracy and I doubt if the pressure monitor is anything like that. Sometimes they screw up at the display meter end and readings can change with the state of battery charge.

If I was getting within 3psi of absolute pressure with +/- 1psi variation due to temperature, that's all I would expect from an automotive product. The main thing is what you get is repeatable and stable. If you know your TPM is reading high/low and stays stable, then you can allow for it.
But the problem for me is that when the temperature drops significantly, like rain or elevation, the yellow or even red triangle show up. Don't know about you but I do not like visual distractions at speed, especially when it is an indication that something is not nominal. I have had this happen dozens of times in the 33K miles I put on the '07.

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post #10 of 25 (permalink) Old Sep 3rd, 2008, 2:48 pm Thread Starter
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Heard from the dealer, bike is ready.
  • TPM readout is 3lbs low, front and back both, considered "within spec". ...guess I'll practice adding 3.
  • Brake pad wear indicator due to broken wire (again.) Fixed.
  • Battery considered good enough after load test. ...time for an aftermarket battery I guess.

I've also had the TPM warning come on yellow due to weather conditions a couple of times. Once after a short rest stop at altitude on a cool day, and later after a trip to the car wash. My theory is that the air in the tire cooled off faster than the TPM sensor did, so the temperature correction overcompensated. But it sure is annoying, both times I stopped and added a bit of air to get the light to go off.

Last edited by bonafidebob; Sep 3rd, 2008 at 2:55 pm.
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