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450 outside temperature sensor

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  • 450 outside temperature sensor

    I have two knackered outside temperature gauge sensors. Has anyone replaced the thermistor on these? There is a small NTC bead thermistor inside sensor housing. Looking for thermistor spec. One can of course record temperature resistance readings by measuring on a good sensor, then use the Steinhart-Hart equation to work out the thermistor's characteristics. Not that easy when both my sensors have expired.

    The sensor is opened out simply by immersion in piping hot water and pulling apart.

  • #2
    Would the temperature data be available via OBD? If it is then you could try a range of thermistor's plugged into the car and see how the values tally with the actual temperature.

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    • #3
      Originally posted by dervsmart View Post
      Would the temperature data be available via OBD? If it is then you could try a range of thermistor's plugged into the car and see how the values tally with the actual temperature.
      That way I may be spending more on thermistors than the cost of a new outside temperature sensor.

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      • #4
        Greetings Tolsen, what are you doing to these sensors to kill them??? The following info I glened from a working sensor. The meter I used is my trusty old analogue meter and is not calibrated, but someone else may do the same thing and you'll be able to compare figures.
        These measurements were carried out with the meter connected all the time.
        Put the sensor in the freezer -15 degC - 40,000 ohms. Room temp. 30 degC - 3,750 ohms and hot water, 48 degC - 1,100 ohms.
        I figured, not many places in the world get lower than -15 or higher the 48 and if it does, so be it. I'd suspect you'd be there because you liked being either hot or cold? Or being paid lots of dosh for the pleasure.,
        Cheers, Ian.

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        • #5
          Many thanks Mad smart person. I'll check those figures later when sobered up. Just back from the watering hole having consumed too many pints of good ale.

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          • #6
            Wouldn't it just be easier to buy a small range of thermistors and compare the value at room temperature to the original. I'm guessing the original doesn't have any markings on it?

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            • #7
              It looks to be the same part for most Mercs. A quick look online says it should either be a 10k or 3.3k.

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              • #8
                I have now a good outside temperature sensor on loan. Looks like these have a 3k0 thermistor with 2.4 mm bead head. Have to run a few additional readings to confirm for sure.

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                • #9
                  Greetings, for those that can still read, Merry Xmas. For those that are too plastered, I'm sure the Pub owner will be rubbing his/her hands for your donations and given another couple of days you'll back for more of the dog that bit you. I guess that's why there's a day or two between Xmas & New year, so you can recover to have another go.
                  I'd suspect the 3,000 Ohm resistance would be at what WHO, suggest would be a 'healthy' room temp. 20 degC?
                  I've not as yet seen it in any of the Smarts I / I've owned, but does an ice crystal appear on the speedo when the OAT goes below 4 degC?
                  I've noticed it other cars. It has tickled me also to see (-)0 and (+)0. Surely zero is zero, in either direction? Tucker for thinking.
                  Or should that be food for thort?
                  Cheers, Ian.

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                  • #10
                    Have now confirmed thermistor. R25 = 3k0. Beta 3988. Panasonic NTC thermistor B57861S0302J40 will do the job. Price £1.16 each from Radiospares.

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                    • #11
                      VAT is to be added to above price so works out slightly dearer at £1.39 per thermistor if collected from a Radiospares depot. Have just placed an order for two thermistors and should get both my knackered outside temperature sensors operational early next year.

                      Beware that thermistors come in all sorts of shapes and sizes. Resistance at 25 C, Beta value and physical size must all be right.

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                      • #12
                        Greetings, just out of interest, what's the price that Merc what for a new OAT sensor??? This could really rip their nighties.
                        Cheers, Ian.

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                        • #13
                          It'll surely be more than £1.38. Perhaps a fiver?

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                          • #14
                            Smart Stuff is charging an exuberant fee of almost thirty pounds.

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                            • #15
                              Smart Stuff's price of £25.84 each works out as £12920 per kilo, twice the cost of my Cabrio when bought nearly new many years ago.

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