Murphy switch. How does it work?
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Thread: Murphy switch. How does it work?

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    Murphy switch. How does it work?

    So lots of unmanned engines have murphys switches incase of a failure when no ones around. Engine over heat or low oil pressure....ect. This is my question, how do these monitor engine temperature and oil pressure? Do they get feedback from the gauges or do they have separate sensors that monitor electrical resistance and such to find out these values? Mainly I'm curious because if one of thoes gauges were to get stuck or fail, like my oil pressure gauge sits in the same spot all the time, would this make that safety switch not work? Mainly just a curiosity sort of thing as I have a bad oil pressure guage on an engine with a murphys switch and I'm like.... does this disable my murphys switch in the cause of a failure in the oil system? The gauge sits at 30 no matter if the engine Is on or off.

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    Meant to have a ? At the end of that...not sure how to change it

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    jgayman's Avatar
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    Looks like they have a conventional sensor and the gauge hand acts like a mechanical electrical switch.



    https://www.fwmurphy.com/




    The gage mechanism is enclosed in a steel case coated to resist corrosion. A polycarbonate, break- resistant lens and a polished, stainless steel bezel help protect this rugged, built-to-last instrument.
    These vapor-actuated gages feature a sealed capillary tube and a sensing bulb. When subjected to heat, the liquid in the sensing bulb changes to vapor creating pressure against the diaphragm mechanism. The diaphragm translates this vapor pressure into a mechanical gage reading.
    For series 20T and 25T, the gage pointer acts as a temperature indicator and as one switch pole which completes a circuit when it touches the adjustable limit contact. Contact(s) are grounded through the Swichgage case. They have a self-cleaning motion to enhance electrical continuity.
    Models 20TE and 25TE have internal snap-acting SPDT switches. Gage-only models without contacts (MurphyGage® instrument) are also available.
    These instruments are used on industrial engines and equipment in oil field, marine, irrigation, construction and trucking industries and for monitoring engine coolant, crankcase oil and transmission oil.
    Attached Thumbnails Attached Thumbnails murphy.jpg  
    Last edited by jgayman; 08-13-2019 at 06:20 PM.
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    So essentially then my engine has no protection from oil pressure issues until I fix the oil guage. I wonder if an oil guage can get plugged? It's stuck at 30kpa

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    jgayman's Avatar
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    Quote Originally Posted by Shadow_storm56 View Post
    So essentially then my engine has no protection from oil pressure issues until I fix the oil guage. I wonder if an oil guage can get plugged? It's stuck at 30kpa
    Is your gauge stuck at 30 kpa with no movement at all?

    I have no experience with gauges of this type but assuming you have a mechanical oil pressure gauge, they can get dirt in the little rack & pinion mechanism that couples the pressure tube to the needle. Sometimes you can take them apart and very carefully clean them. In some cases the rack & pinion can skip a tooth and cause the gauge to not return to zero.

    Below is a diagram of how most mechanical pressure gauges are constructed.
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    It never moves off that spot, not even a tiny bit

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    Quote Originally Posted by Shadow_storm56 View Post
    It never moves off that spot, not even a tiny bit
    If you can, pop the back off and see if the rack & pinion mechanism is jammed. That will also allow you to gently flex the pressure tube to verify the linkage works.
    2012 2720 -- 200CX Loader -- 54" Quick Attach Snow Blower -- Frontier LR5060 Rake -- Land Pride RB1660 Blade (Hydraulic Angle) -- Artillian 42" Forks -- Ken's Bolt on Grab Hooks -- Fit Rite Hydraulic top-link -- 2013 X500 for mowing duties

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    It's very old and rusty, chances are it would not go back together so I likely will get a new guage.

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