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Thread: Highly inaccurate TPMS readings - dealer says no fix

  1. #1
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    Highly inaccurate TPMS readings - dealer says no fix

    So I have a 2019 F750GS I bought new. Manual says tires should be 31.9 front / 36.3 rear for one up load. So with tires cold I air to 32 / 36. I have 3 gauges and they vary by about 1psi so I feel Iím reasonably accurate. Within 10 min of riding the warning pops up saying my pressures are 28.6 / 32.2 with minor variations. If I check them after they have cooled down, they are exactly where I set them. Dealer acts like this is a common issue and claims there is no fix or adjustment. They say just inflate them more to avoid the alarm going off! What? That seems crazy! This from a company that gives you inflation specs to the tenth of a pound and positions itself as cutting edge technology? Has anyone experienced this?
    I donít know if itís relevant but Iím riding at 8000 - 9000 ft elevation. All thoughts welcome.

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    Fortis Fortuna Adiuvat Omega Man's Avatar
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    On my Motorhome, my add on tire pressure monitoring devices show 90 PSI coldÖ..(cold is usually described as ďnot drivenĒ). Out on the highway, in this heat, the temperature will rise to 107*.
    Once the bike is headed down the road and the system wakes up, things will vary.
    How about in the winter?
    It sounds to me like you are ok.
    OM
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    Debbie's Servant Lee's Avatar
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    Quote Originally Posted by sea2summit View Post
    So I have a 2019 F750GS I bought new. Manual says tires should be 31.9 front / 36.3 rear for one up load. So with tires cold I air to 32 / 36. I have 3 gauges and they vary by about 1psi so I feel I’m reasonably accurate. Within 10 min of riding the warning pops up saying my pressures are 28.6 / 32.2 with minor variations.
    That's strange to have the reading drop 3 to 4 psi.
    The TPMs are temperature compensated to 68 degrees F and normally do not change much as the tire heats up.
    If I check my tires when the air temp is near 68 degrees or use a table to adjust to 68 degrees my TPM reading are usually within 1 psi of my gauge when I first leave the house.
    Same thing for my wife's bike.

    If it's really hot or cold when checking pressure with my gauge I use this chart.
    Tire pressure for the R1200RS is 36 and 42.
    Tire Pressure.jpg
    Lee
    2016 R1200RS
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    Past BMW Bikes: 2011 K1300S, 2003 K1200RS, 1991 K75S, 1987 K75T, 1984 R100RT

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    Registered User wbrownell9's Avatar
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    The OP's issue is that the tires appear to be losing air, then reinflating themselves after they've cooled down. How nice, to have self-inflating tires!

    Yes, BMW corrects the pressure sensed by the TPMS to 68 degrees, but I'm not convinced that does anything useful, I'd prefer they correct it to whatever the ambient temperature is. Like I was told once, "It's not the tires that hold the bike up, it's the air in the tires", by which he meant the difference between the air pressure inside the tire and the pressure outside. Fortunately this is what tire gages actually measure so there's no need to compensate for altitude, or for temperature either - the whole point of "cold" temperature measurements is to have the air inside the tire the same temperature as the air outside. (An exception would be if your garage is a wildly different temperature than outside, then use a temperature compensation chart). Tire pressure goes up as the tire warms, this is considered when recommending inflation pressures and is the reason one doesn't bleed air from warm tires, because they'll be underinflated when they cool down!

    My own practice is to check the pressure with a gage, note what the TPMS says (generally close, although not in the OP's situation), then monitor the TPMS for changes. The actual number doesn't really concern me, it's a change that I worry about.
    2020 R1250 GSA Low

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    Debbie's Servant Lee's Avatar
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    Quote Originally Posted by wbrownell9 View Post

    My own practice is to check the pressure with a gage, note what the TPMS says (generally close, although not in the OP's situation), then monitor the TPMS for changes. The actual number doesn't really concern me, it's a change that I worry about.
    This is what we do when heading out on a trip. Check the TPM readings when leaving the house and check each day on the trip to make sure the readings are close to that first day.
    We can go a couple weeks with very little change.

    As for the OP, I think the module under the seat is faulty and should be replaced under warranty.
    Lee
    2016 R1200RS
    MOA # 30878
    Past BMW Bikes: 2011 K1300S, 2003 K1200RS, 1991 K75S, 1987 K75T, 1984 R100RT

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    Quote Originally Posted by sea2summit View Post
    I don’t know if it’s relevant but I’m riding at 8000 - 9000 ft elevation.
    It is not only relavant, it is the entire reason that your TPM and your gauge don’t match.

    Most TPMs, since they are enclosed in the pressure that they are reading, read in PSIA (or Absolute). Their reading does not change much with change in atmospheric pressure.

    Virtually all tire gauges are differential gauges that measure pressure differential above an assumed 14.7 psi (atmospheric pressure at sea level. They will vary depending on what altitude you are testing the tire pressure at.


    Atmospheric pressure at sea level is 14.7psi.

    Atmospheric pressure at 8000’ is 10.9psi.

    14.7 - 10.9 = 3.8psi. Does that number ring a bell?

    Your differential tire pressure gauge is reading 3.8psi low because it is assuming 14.7psi atmospheric pressure when you only have 10.9.

    Generally TPMs aren’t intended to be accurate enough to SET your tire pressures with. They are intended to warn you when things get out of whack while you’re riding.

    If your spending all your time at 8000’, set your pressures 3.8psi higher than recommended with your gauge, see what your TPM reads, and go have fun.


    Or set your pressures with your gauge to where your bike feels best and realize the numbers on your TPS might not match your gauge and use the TPS monitor to warn of notable change (lower).


    Last edited by 98lee; 07-04-2021 at 07:47 PM.
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    Debbie's Servant Lee's Avatar
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    I read the first post wrong and thought he was checking the TPM when he first took off and again 10 miles later.
    Lee
    2016 R1200RS
    MOA # 30878
    Past BMW Bikes: 2011 K1300S, 2003 K1200RS, 1991 K75S, 1987 K75T, 1984 R100RT

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    Debbie's Servant Lee's Avatar
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    Our house sits at 1340 feet in elevation. I wonder why we didn't get a low pressure warning when were over 12,000 feet several times last year on trips.
    Lee
    2016 R1200RS
    MOA # 30878
    Past BMW Bikes: 2011 K1300S, 2003 K1200RS, 1991 K75S, 1987 K75T, 1984 R100RT

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    Quote Originally Posted by Lee View Post
    Our house sits at 1340 feet in elevation. I wonder why we didn't get a low pressure warning when were over 12,000 feet several times last year on trips.
    As you went up in altitude, your TPM would stay relatively stable (other than any rise or fall caused by tire temperature). But, if you were to use the same tire pressure gauge that you used to check your tire at home, the reading would be 4.9 psi higher at 12,000 ft. If the tire temp was the same as when you checked it at home.

    https://www.coyoteents.com/tire-pressure-gauges-work/




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  10. #10
    Debbie's Servant Lee's Avatar
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    Quote Originally Posted by 98lee View Post
    As you went up in altitude, your TPM would stay relatively stable (other than any rise or fall caused by tire temperature). But, if you were to use the same tire pressure gauge that you used to check your tire at home, the reading would be 4.9 psi higher at 12,000 ft. If the tire temp was the same as when you checked it at home.

    https://www.coyoteents.com/tire-pressure-gauges-work/




    Makes sense.
    One thing with the BMW TPM system there's not much change in the reading due to temperature like I see in our cars.

    Next time we go to Colorado I'll adjust our tire pressure when we get to the mountains.
    I don't think I will bother with every 4,000 to 5,000 foot elevation change while we're there.
    Lee
    2016 R1200RS
    MOA # 30878
    Past BMW Bikes: 2011 K1300S, 2003 K1200RS, 1991 K75S, 1987 K75T, 1984 R100RT

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    Quote Originally Posted by Lee View Post
    Makes sense.
    One thing with the BMW TPM system there's not much change in the reading due to temperature like I see in our cars.

    Next time we go to Colorado I'll adjust our tire pressure when we get to the mountains.
    I don't think I will bother with every 4,000 to 5,000 foot elevation change while we're there.
    I'm thinking that if you set your tire pressures at home and if your TPM reads the same at altitude that it did at home, accounting for any temp variance, I'd leave it alone,

    If you check with your gauge, just be aware that your tire gauge will show higher by about 1/2 psi per 1000' than what it really is because of the lower atmospheric pressure.




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    Lee Fulton Forum Moderator
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    98lee, fantastic explanation!
    I will roll with your advice. Much appreciated.

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    How do the tires themselves feel about pressures that are over their recommended maximums once they're at high elevations? If the front is set at 42 at sea level the gauge will exceed the manufacturer's recommended 44 max at 12000 elevation, won't it?
    '20 R1250RT,
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    Fortis Fortuna Adiuvat Omega Man's Avatar
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    Quote Originally Posted by sailorlite View Post
    How do the tires themselves feel about pressures that are over their recommended maximums once they're at high elevations? If the front is set at 42 at sea level the gauge will exceed the manufacturer's recommended 44 max at 12000 elevation, won't it?
    Itís engineered into the tires.
    OM
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    Quote Originally Posted by sailorlite View Post
    How do the tires themselves feel about pressures that are over their recommended maximums once they're at high elevations? If the front is set at 42 at sea level the gauge will exceed the manufacturer's recommended 44 max at 12000 elevation, won't it?
    It's the gauge that is READING high (because it is reading differential pressure ie. pressure above atmospheric). The pressure inside the tire is still correct.

    It is the gauge reading that is effected by the change in atmospheric pressure, not the air in the tire or the TPM. The gauge is effected because it in the atmosphere. The tire air and the TPM are in a sealed container (the tire).

    Even without all this, the couple of psi change is not worth worring above every time you go over a mountain pass.



    Last edited by 98lee; 07-04-2021 at 04:34 PM.
    LONG MAY YOUR BRICK FLY!

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    Lee Fulton Forum Moderator
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