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.