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Ambient temperature reading

Discussion in 'Prime Main Forum (2017-2022)' started by MikeDee, Sep 6, 2022.

  1. ChapmanF

    ChapmanF Senior Member

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    Have you tried putting one of the thermometers you trust behind the bumper of either Prius, next to where the sensor is, waiting for readings to stabilize, then comparing?
     
  2. MikeDee

    MikeDee Senior Member

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    I think that's already addressed by post # 4 and 17 above.
     
  3. ChapmanF

    ChapmanF Senior Member

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    Except maybe for temps over 100, the ones where you think the sensor may be inaccurate.
     
  4. MikeDee

    MikeDee Senior Member

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    Maybe I'll do that. It's supposed to get up to 109° F today (ugh). I can use one of my remote temperature sensors for my digital clock and put the clock in the car. Not sure it's worth the trouble though as I've already confirmed to my satisfaction that it's not reading right at high temperatures. Question is, is it normal or is the sensor bad or maybe something else like wiring, electronics or firmware? Seems like it's normal behavior to me since I have two Priuses doing the same thing.
     
  5. MikeDee

    MikeDee Senior Member

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    Maybe I'll do that. It's supposed to get up to 109° F today (ugh). I can use one of my remote temperature sensors for my digital clock and put the clock in the car. Not sure it's worth the trouble though as I've already confirmed to my satisfaction that it's not reading right at high temperatures. Question is, is it normal or is the sensor bad or maybe something else like wiring, electronics or firmware? Seems like it's normal behavior to me since I have two Priuses doing the same thing.
     
  6. ChapmanF

    ChapmanF Senior Member

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    The more people interested in the thread, the better the supply of potential answers to the question. Confirming the sensor isn't reading right to your own satisfaction is good as far as it goes, but by going a little beyond that you might hold the attention of more readers.
     
  7. MikeDee

    MikeDee Senior Member

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    Moved car (was in garage all day) from garage to outside in shade and left for 30 minutes. Remote sensor from digital clock in grill next to sensor:

    6:20 PM outside temp: 101 Prime 89
    6:50 PM outside temp: 101.8 Prime 89
     
  8. ChapmanF

    ChapmanF Senior Member

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    Interesting. What I might do next would be to replace the sensor with a variable resistor, and watch the car's indicated temperature while varying the resistance. From what I've seen, Toyota generally uses negative temperature coefficient thermistors, so as you reduce the resistance, the temperature display should read higher.

    Plotting the resistance against displayed temperature may show you a plot that somewhere has a knee in it, and stops following the curve it was following at lower "temperatures" (higher resistances).
     
  9. lech auto air conditionin

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    Remember when you’re taking your temperature when you’re driving down the road on the blacktop there’s literally several degree difference added a few inches above the blacktop because of the heat radiation heating up the blacktop the air is hotter


    So if you have 103° day you may find 9 inches above the blacktop it could be 109° 106° maybe even 115°

    If you seen some of my videos guys and automotive should be getting like the Fieldpiece temperature probes I use they could stick 12 inches down the duct of the dash reading the temperature down there instead of temperature sensors that are just placed right at the surface of the vents

    And they lose Bluetooth right to your phone so you can drive down the street with them attached in the front of the grill or behind the bumper of course add some duct tape or wire to hold them in place do not rely on a magnet going down a bumpy road you will lose your hundred dollar Temperature humidity Bluetooth sensor

    And when you come to a stop sometime Sarah circumstances were the fans are on blows out hot air across your condenser and your radiator your exhaust manifold and it rolls back around underneath the engine compartment up in front underneath the bumper and you’ll see a really high engine temperature sometimes
     
  10. lech auto air conditionin

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    If you’re a professional auto technician in this trade and you do any quantity of HVAC jobs at all seriously think about getting a quality tool for measurements and you can actually datalogge and record humidity and temperature

    Fieldpiece JL3RH Flex Psychrometer Probe
     
  11. MikeDee

    MikeDee Senior Member

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    I did a search and found a number of complaints in this forum about this temperature reading being inaccurate, yet no one says they fixed the problem nor how they did it.

    I think I'm going to buy a new sensor. I'm thinking if it work OK at low temperatures and not high, the rest of the system is good. Any tips on getting the plastic cover off that's under the bumper so I can gain access to the sensor?
     
    #30 MikeDee, Sep 8, 2022
    Last edited: Sep 9, 2022
  12. ETC(SS)

    ETC(SS) The OTHER One Percenter.....

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    The ambient temperature sensor doesn't affect engine or efficiency.
    In some cars they do affect the HVAC.
    Knowing what I know about Priuses, it's a good bet that the car uses the OAT value for SOMETHING.

    Remember, your temp sensor isn't a thermometer, it's a thermistor, and it's located behind all of the (usually black) plastic in the nose of the car so it might take a while for it to read outside air temps if you park somewhere unusually warm, or cold.
    They're relatively easy to replace but I'm having a hard time figuring out why you would want to do so unless the unit has failed completely.
    Oh wait. That was a stupid thing for me to say. I forgot I was in PRIUSchat.

    If you look at your phone or smart watch and it has a temperature readout, it isn't telling you what the temperature is where your watch or phone is actually at.
    You're probably getting ASOS data from the closest airport.

    The temp sensor in your car isn't a precisely calibrated platinum wire Resistive Temperature Device (RTD) sensor mounted in a special cylindrical enclosure six feet above the ground, with a dedicated aspiration fan, it's a $0.99 (probably 130 Yen) thermistor that's connectorized and clipped to the front of your car.

    GIGO.

    If the value of your temp readout is consistently off from what you know outside air temps to be you can try to use a biasing resistor or just replace the sensor.
    They're actually not that expensive ($10-20) unless you pay the mother ship to have it replaced.
     
  13. ChapmanF

    ChapmanF Senior Member

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    The OP seems to have already shown that the displayed reading seems to change, from tracking well with other measurements, to not tracking well with other measurements (including right next to the sensor, in post #26), and believes the change is happening in the neighborhood of 100 ℉.

    I do not have a Prime repair manual ready at hand to look at. If the sensor used is similar to that in Gen 3, it should have a resistance-to-display curve like this pasted from a Gen 3 repair manual:

    vvv Gen 3 Gen 3 Gen 3 vvv

    amb.png

    ^^^ Gen 3 Gen 3 Gen 3 ^^^

    It would be good for the OP to look in a Prime repair manual to see if the specs are different there, but the above should be a good indication of what to expect.

    The OP has wondered:

    Judging at least by the Gen 3 curve above, Toyota does not seem to intend for anything weird to happen around the 100 ℉ point.

    The repair manual will suggest steps for testing the sensor, by removing it, exposing it to different temperatures, measuring its resistance at those points, and seeing whether the plotted points all fall in the allowable range. If the sensor has somehow gone weird in the way the OP suspects, the points so plotted would be likely to fall in the allowable range below 100 ℉ or so, but to depart off to the right above the curve to the right of that temperature.

    Thermistors are pretty simple physical devices, though, so I'd be surprised to see it do anything that interesting.

    The OP could also do a test that's the dual of that one, by unplugging the sensor, plugging in different known resistances instead, and seeing what temperature the car displays. That would reveal if something interesting is happening in "wiring, electronics or firmware". The behavior the OP suspects would be seen there as points that lie within the allowable range above 1 kΩ or so, but depart downward below the curve below that resistance.

    Again, I'm getting the 1 kΩ from the Gen 3 specs above; it's possible they're different in a Prime.
     
    #32 ChapmanF, Sep 9, 2022
    Last edited: Sep 9, 2022
  14. MikeDee

    MikeDee Senior Member

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    Well, I give up. As I pulled into the garage from a long drive the Prius read 108° and the house thermometer read 98.2°
     
  15. Elektroingenieur

    Elektroingenieur Senior Member

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    It does, but in the last thread about this, I posted only the table, not the graph, which is more or less the same.
     
  16. Trollbait

    Trollbait It's a D&D thing

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    :confused: Did you touch it at some point, or hit a bump?
    Starting to sound like a loose connect or intermittent short.
     
  17. ETC(SS)

    ETC(SS) The OTHER One Percenter.....

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    You can pull the sensor and inspect the connector but the symptoms lean away from a “problem” with the electrical connection.
    Exercising it might help if it’s a little loose, especially if the car has ever been to a body shop.

    If you really want to have some fun, use some dielectric grease.
    No.
    Not on the connector - in this forum. :p
     
  18. ChapmanF

    ChapmanF Senior Member

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    I also see the table matches the one for Gen 3, so they seem to be using a sensor with the same specs, and the graph would match too. The tables also match, unsurprisingly, for the cabin temp sensor and the aux battery temp sensor; they seem to have one set of NTC thermistor specs they like for these things.

    The graph makes things kind of visible: even though the absolute height of the "allowable range" decreases as the temperature increases, the slope of the curve decreases too, so the allowable ranges overlap more for the same temperature difference at higher temperatures than lower ones.

    Because it's an NTC thermistor, increases in wiring resistance also will tend to make the reading low. The sensor resistance is still close to a kΩ even around 100 ℉, so normal wiring resistance should be negligible in comparison. But if there's a bad connection with resistance in the ballpark of a hundred ohms, say, that would tend to skew the readings low, and by larger amounts at higher temperatures.
     
  19. MikeDee

    MikeDee Senior Member

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    Well, I did touch the sensor to see if the connector was pushed all the way on (it was) and jiggle it, but don't know when in this whole process I did that. Yesterday, it worked well, if not reading high instead of low. To remove and clean the connections at the sensor, I've got to jack up the car and remove the plastic cover under the car. Even though I can see and touch the sensor through the grill, it's mostly inaccessible. The heat wave is over now and not sure when I'll be seeing 100°+ temperatures again.
     
  20. rjparker

    rjparker Tu Humilde Sirviente

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    Thermistors react quickly to changes in temperature unless the object or fluid they are sensing is not changing quickly. If it contacts an object with a large thermal mass or it is isolated from the ambient airstream, it will faithfully report the temperature of the slowly changing object or fluid. Thermistors rarely drift; they typically work well but occasionally open or short if they fail.

    I find the sensor to be dead on well over 100f when the vehicle has been parked in a 110f garage all day. This is based on a separate Honeywell wifi sensor in the same space. In those cases, it may take ten minutes of 60 mph driving to accurately report outside ambient.