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Hi, all --

We're having some challenges with our oil temps, so it makes sense to check the sensor (especially since I can't

  https://www.rotax-owner.com/en/912-914-technical-questions/9283-new-or-old-cht-sensors-hot-oil-checking

just swap it *sigh*).  I've reviewed the IM 76-00-00 Fig 3 on pg 7 (2015-01) to understand the resistance vs temperature graph, and on its face it makes sense (predictable resistance X at temperature Y), but then there's this little notice above on pg 6 that the graph is only valid at 20C.  Um ... what?  How can I measure any other temp if it only works there?  In fact, why draw a curve at all??  So obviously I don't understand this after all :-)

Anyone have any guidance on testing this sensor thoroughly enough to have some idea whether or not it's in spec?  [Oh, and how the 10% is applied, like 80C = ~110O == 99-121, or like 80C is ~110O == 72-88?]  And, for that matter, when they go do they degrade linearly or are we about to see a total failure?

Thanks again! 


  • Re: oil temp sensor resistance

    by » 2 weeks ago


    Why note place it in the steam, just above the surface of boing water - consistent 100C, Sea Level and see what the reading is.


  • Re: oil temp sensor resistance

    by » 2 weeks ago


    Sean Griffin wrote:

    Why note place it in the steam, just above the surface of boing water - consistent 100C, Sea Level and see what the reading is.

    Not least because I don't know what I'm doing :-)  That certainly isn't an ambient temp of 20C.  Does that matter, since the notice in the book says the graph is only valid at that point?

    Of course, I can't get to sea level from where I am, either, but that seems the minor challenge.


  • Re: oil temp sensor resistance

    by » 2 weeks ago


    Unless you live in one of the Worlds very high mountain ranges , don't worry too much about your height above sea level. The lower boiling point of water with increasing altitude, is unlikely to make a big difference to an engine coolant/oil sensor.

    You will need

    At least one helper, two better

    To remove your sensor from the engine, reattach sensor lead (to your cockpit readout), firmly wrap a length, of bared, 12 V wire around sensor barrel - connect back to engine (this is your earth/ground return).

    Switch on aircraft engine monitoring system

    Use a pair of long nose pliers or tongs to hold the sensor (keep hands away from steam)

    Bring a vigorously boiling kettle (best) or saucepan, close enough to enable sensor to be put into steam at kettle spout/just above saucepan surface of water.

    Read instrument in cockpit - should indicate around 100C 

    Note:

    This is not about precise (100C) temperature on the gauge, only that the sensor/gauge is able to give an approximate/acceptable read out  at this temperature. 

    An inaccurate reading could be either the sensor or the gauge - try a second sensor - same result? suggests gauge failure.


  • Re: oil temp sensor resistance

    by » 2 weeks ago


    The VDO oil temp and CHT sensors are extremely resilient. In 20 years I have only replaced two. On one someone broke off the electrical connector and the other had the plastic melt from a broken exhaust pipe gas. All poor sensing issues I have run across have been from poor grounds, wire rubbed through on a metal surface, poor wire connections and the connection on the sensor was too loose. 


    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


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