I wish I could figure out how to initialize the TPMS...

Discussion in 'Gen 4 Prius Technical Discussion' started by tomla, Dec 21, 2016.

  1. GT4Prius

    GT4Prius Active Member

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    I said I set the TPMS "at" 36 not "to" 36: that is I did the calibrate procedure when the tyre pressure was 36. No question that the car operated correctly. I didn't say otherwise. I said I was disappointed that the margin of error is 20%.

    The way to make this work in the way you suggest, is one that I already mentioned earlier in this thread. It is a work around, and I may try it next time, but fairly time consuming and not guaranteed to make one popular with the other drivers who are waiting to use the air line at the petrol station!

    But I disagree with you as follows:

    1. I very much doubt that the reason for having to calibrate the TPMS is in order to set your own warning levels in the way that you suggest. Surely it would be FAR better if that were the aim, to allow the customer to set their own warning alert thresholds on the TPMS screen. That is, to alert when pressure goes below a specific user specified PSI value.

    2. The sensors generally have a margin of error of +- 3psi don't they? So not entirely true to say that the car knows the pressures to one tenth of a psi.

    3. My reading suggests that the TPMS does not alert for pressures that are too high, as I said, and it sounds to me like @bwilson4web can confirm this from his own experience. Can you provide documentation to back up your view?

    Posted via the PriusChat mobile app.
     
    #21 GT4Prius, Oct 19, 2017
    Last edited: Oct 19, 2017
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  2. kithmo

    kithmo Couch Potato

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    I was under the impression that the TPMS only monitored pressure loss, not increase, so the tyre heating up should not set the warning off.
     
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  3. 2k1Toaster

    2k1Toaster Brand New Prius Batteries

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    And some vehicles allow that. Some vehicles allow you to change the TPMS warning level via a screen. Some of these are limited to only accept increments of 5psi. You can set your warning to 35PSI or 40PSI, not 36/37/38/39 PSI. This is a human interface problem and car manufacturers are dreadful at UX. The point that is is a variable you can change, means that it is something you can change, so do it.

    The car's precision on the TPMS sensor value is a tenth unit. It reads XX.X PSI. If the input to that system is a degraded sensor, or non-calibrated sensor, or standard sensor, the values will most likely be higher than that. However following the open TPMS standard at 315Mhz you can make a transmitter that broadcasts values that the car will pickup and will do so in 0.1 PSI increments. If you work at Denso, you probably have calibrated sensors in the tires that are accurate under normal conditions to that value. To everyone else who just has the discount tire shop do it, they're probably all out of whack. But the car doesn't know this.

    It is right in the manual, page 152. You will get an alarm when the pressure exceeds 500kPa, roughly 73PSI as I said.

    prius_page_152.jpg

    Something not in the manual but you can find in a Toyota tech datasheet is that they also alarm at 20PSI. Doesn't matter what you set your sensors to, if the car reads 20PSI you will get an alarm.

    The TPMS system used in all Toyota's with direct monitoring gets an actual numerical pressure value back from each sensor. It does not report anything other than XX.X PSI every so often. If the pressure goes up or down, doesn't matter. A numeric instantaneous value is reported to the ECU. The car's software then decides what to do. The tire heating up will not set the alarm off unless it heats up to past 73 PSI.

    My reference was if you set the system to alarm above your cold pressure but below your warm pressure, the light will turn on when you start driving and turn off after it heats up enough. The car doesn't care why or how the pressures are different. It just gets a number and compares it to the range of values that has been programmed to be good.

    Anything above 73PSI = BAD
    Anything below 20PSI = BAD
    Anything below 20% of your user programmable set value = BAD
    Less than 4 TPMS IDs programmed = BAD
    Less than 4 TPMS IDs reporting data every 1 minute = BAD

    It takes the logical OR of all of these to make the alarm light illuminate. Meaning if the light comes on one or more or all of these may be the cause and it is just letting you know something is wrong.
     
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  4. 2k1Toaster

    2k1Toaster Brand New Prius Batteries

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    And indirect TPMS (not using in-tire sensors, but using wheels speed sensors) is very manufacturer dependent on how they implement their ABS control module firmware. The NHTSA did a test not too long ago that tested some of the systems:

    indirect_tpms_warning_time.jpg

    You read that as it took System A 1.6 miles and 2.1 minutes to alarm for a single failure 50% under-inflated tire on the passenger front wheel.

    With 2 tires both on the same side of the vehicle, either left or right, both deflated to 50% values, neither system gave any alarm during the entire test course of 14-18 miles of driving. With 2 tires on opposite sides of the vehicle, System A also didn't report while the other 3 did. With 3 tires deflated they all kicked in, some faster than a single tire, some slower. And with all 4 tires under-inflated no system gave any warning during the entire length of the test.

    To me the most critical are the single tire and four tire tests. The single tire is you drove over something and it caused a slow leak, you will get a warning. Good. The four tires are the weather related ones. Your tires are perfectly fine, no leaks but the temperature has dropped from 100F to 20F and now they are all under-inflated. They are under-inflated simultaneously and equally and are keeping their pressure, just it is too low. This is what your average driver doesn't know about. PV=nRT. When the temperature T drops, your pressure P drops unless your tires also shrink at the same time. I see older cars around here all the time driving with tires that spread out on the bottom. I even told a lady once as I was walking into the store and she had parked, "hey your tires look pretty low, might want to fill them up". "Oh, they always look like that, they're fine"... No, they're not fine. But I do believe they always look like that having met the driver.
     
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  5. kithmo

    kithmo Couch Potato

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    Your quote from the manual says the TPMS will be disabled if the tyre inflation pressure exceeds 73 psi, meaning expect it not to work if you inflate your tyres to >73 psi.
    So you won't get an alarm, but the light will be on to tell you it's disabled.
     
    #25 kithmo, Oct 21, 2017
    Last edited: Oct 21, 2017
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  6. GT4Prius

    GT4Prius Active Member

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    2k1 toaster is confusing precision with accuracy.

    The sensors may report to a precision of one tenth of a psi, but they have an accuracy, i.e. margin of error between the actual and reported pressure, of 3 psi.

    As for "The point that is is a variable you can change, means that it is something you can change, so do it", I've already gone over that twice in this thread . . .



    Posted via the PriusChat mobile app.
     
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