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Electronic dipstick sensor

Discussion in 'Mondial' started by westyguy, May 14, 2013.

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  1. westyguy

    westyguy Formula Junior

    Apr 27, 2009
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    Steve
    #1 westyguy, May 14, 2013
    The low oil light quit working on my '84 QV. If I unplug it the light illuminates and if I then short the connector the light extinguishes. I cleaned the sensor area with contact cleaner, and still measured a short at the dipstick connector. I stuck a toothpick in the dipstick and the short went to an open. I assume the sensor is just two copper leads set a small distance apart and there is enough conductivity in the oil to short across those contacts, allowing the light to operate. Can someone verify this? If that is the case, how is the thing calibrated to work properly? I've attempted to slightly move those copper contact pieces to no avail.
    Steve
     
  2. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    #2 Steve Magnusson, May 14, 2013
    Did you take apart the sensor (to access the internal switch contacts)? Can you post a photo of it? If the sensor has a couple copper tabs stick out of the end, these might be used for thermal reasons (not electrical reasons).

    My take on the symbol shown for this low oil level sensor on the 1984 US Mondial QV schematic is that it is a self-heating thermal-mechanical switch -- i.e., if the sensor end is submerged in oil (even hot oil) = enough heat is withdrawn from the internal heater so that the bimetallic element stays cool enough to keep the internal switch contacts closed (= light "off"); if the sensor end is in air, less heat is withdrawn, the internal bimetallic element flexes, and the internal switch contacts open (= light "on"). However, (based on the symbol) when the internal switch contacts open, this also turns off the internal heater -- so if the oil level was low (sensor in air), the light would come "on" (after things heat up), then go "off" (as things cool down), then go "on" again (after things heat up again), etc.. Can anyone confirm/deny if this is the light's behavior when the sensor is held in air? Not sure that I trust that the symbol shown on the schematic ;)
     
  3. James_Woods

    James_Woods F1 World Champ

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    #3 James_Woods, May 14, 2013
    Oil has practically zero conductivity, unlike water which has salts or other ions dissolved in it. I would assume that the sensor is not electrical conductivity.
     
  4. westyguy

    westyguy Formula Junior

    Apr 27, 2009
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    #4 westyguy, May 14, 2013
    I didn't disassemble it, and my schematics got taken out of the car by the body shop and not given back...I have to pick them up this weekend and take a look. As pointed out, the bare contacts just didn't make sense as oil is normally a pretty good insulator. We use oil-filled capacitors in high voltage power supplies in the broadcast industry, and oil (though not motor oil) helps insulate and cool the plates of the capacitors. Some transformers were wound open-frame and operated in a lead-lined concrete bunker, open at the top, filled with oil. In those days, the oil contained high levels of PCB's, but those have all been replaced and disposed of now, as the removal campaign was about 20 years ago. I never thought about a thermal switch, though. Only thing possible that might make me wonder is the fact that normally the light will still operate depending on the oil level with the engine stone cold and the oil at ambient temperature. If the oil is the same temp as ambient air, how would the thermal setup distinguish oil presence or absence?

    One scenario that crossed my mind was similar to that of a moisture/ice sensor that we use on big towers to operate the antenna heaters. It has a vibrating element and when ice or moisture is on it, the oscillation frequency is altered. Oil could do the same thing compared to a free-air resonance.
    Steve
     
  5. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    #5 Steve Magnusson, May 14, 2013
    Last edited: May 14, 2013
    It's not detecting the instantaneous temperature of the surrounding outside medium -- it (the bimetallic switch) detects how hot the switch gets when the internal heater is on:

    A. When the internal heater is "on", and the sensor is in air = the internal temperature of the switch goes high (because the thermal conductivity of the air is low) = the bimetallic mechanism moves a lot, and the switch contacts open.

    B. When the internal heater is "on", and the sensor is in oil = the internal temperature of the switch stays lower (because the thermal conductivity of engine oil is ~6X higher than air) = the bimetallic mechanism moves little, and the switch contacts stay closed.
     
  6. fgcfire8

    fgcfire8 Formula Junior

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    #6 fgcfire8, May 15, 2013
    Sept 2011 I got one from T Rutlands for $158 they may still have one
     
  7. Dr Tommy Cosgrove

    Dr Tommy Cosgrove Three Time F1 World Champ
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    #7 Dr Tommy Cosgrove, May 15, 2013
    Why is this a part that one of us needs to worry about replacing? I mean I am all about getting everything to work correctly on these cars but THAT thing??

    Who actually drives around not checking the oil regularly and just watching for that light to come on?
     
  8. afterburner

    afterburner F1 Rookie
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    #8 afterburner, May 16, 2013
    I do. I never manually checked the oil level inbetween services (6'000mls). As long as there is no oil on the ground, good oil pressure, correct oil temp, and no low level light on, why check? After the servicing, the oil level is at the upper end of the range. After 6K, it's at the lower end. I never needed to top up.

    PS: And yes, the light does work.
     
  9. Dr Tommy Cosgrove

    Dr Tommy Cosgrove Three Time F1 World Champ
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    #9 Dr Tommy Cosgrove, May 16, 2013
    To each his own, I guess.
     
  10. ricksb

    ricksb F1 Veteran

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    #10 ricksb, May 16, 2013
    +1. Whether or not the sensor is working, I check the oil. Regularly.
     
  11. bigeasy

    bigeasy Formula Junior

    Dec 31, 2012
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    #11 bigeasy, May 18, 2013
    "I ain't never been nothin but a winner"
    -Coach Paul "Bear" Bryant 1982

    except when he played N.D.
     
  12. bigeasy

    bigeasy Formula Junior

    Dec 31, 2012
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    #12 bigeasy, May 19, 2013
    I agree 100% Check the dip stick, why do you think they call them idiot lights.
     
  13. afterburner

    afterburner F1 Rookie
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    #13 afterburner, May 19, 2013
    You guys also check the oil level on your new cars by dip stick?
     
  14. fgcfire8

    fgcfire8 Formula Junior

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    #14 fgcfire8, May 20, 2013
    All except the MB E500 they have done away with dipsticks only the electronic oil level sensors are available--i miss being able to sniff and look at the oil on the stick
     

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