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Thread: Oil pressure switch - ways to fail

  1. 8th April 2009, 01:21 PM #1
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    vnx205
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    Oil pressure switch - ways to fail

    How is it possible for an oil pressure switch to fail so that the light comes on at idle even though the pressure is good?

    I have this mental picture of a switch consisting of a spring that holds a contact closed until the pressure on some sort of little piston is high enough to push against the spring and open the contacts so that the light goes out.

    I can visualise a situation where the light stays off even when the engine isn't running because of some sort of dirt on the contacts.

    I can even imagine that over time the spring might weaken so that the light goes out at lower pressure.

    I can't work out what would cause it to need higher pressure to open the contacts.

    I'm curious because I have seen them fail that way.

    1973 Series III LWB 1983 - 2006
    1998 300 Tdi Defender Trayback 2006 - often fitted with a Trayon slide-on camper.
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  2. 8th April 2009, 02:40 PM #2
    2stroke Guest
    Is the oil pressure really ok? I agree it would be unlikely to fail that way.
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  3. 8th April 2009, 03:11 PM #3
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    Not having opened one up to check exactly how they work so I'm just theorizing.

    Using your "small piston" theory, if the pressure was to get on both sides of the piston (ie leaks past the piston and can't escape the housing) then the spring would still hold the piston in the "low pressure" position.
    Same would happen if it was some sort of metal diaphragm and it developed a crack.

    Steve
    Last edited by steveG; 8th April 2009 at 03:12 PM. Reason: clarity
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  4. 8th April 2009, 03:12 PM #4
    PhilipA
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    It is the most likely failure . I believe that the seal around the diaphragm leaks and some oil is able to go behind it.
    I have had 2 pressure switches fail this way.
    They often leak out the joint between the brass body and end cap also.
    Regards Philip A
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  5. 8th April 2009, 04:54 PM #5
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    That seems like the explanation. There were signs of oil having made it past some sort of seal in the switch.

    I can see now why it would be easy for them to fail that way.

    In my case, it definitely was just a problem with the switch. A new one behaves exactly as it should.

    1973 Series III LWB 1983 - 2006
    1998 300 Tdi Defender Trayback 2006 - often fitted with a Trayon slide-on camper.
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  6. 8th April 2009, 04:59 PM #6
    2stroke Guest
    Hmmm... makes a lot of sense about the switch, my 300 tdi took ages to turn off the light so I replaced it with a pressure gauge but it agreed with it. This thread must have struck a nerve I guess.
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  7. 8th April 2009, 05:43 PM #7
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    Pedro_The_Swift
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    Believe it or not--

    I had one blow its centre out,,
    covered the engine bay in oil,




    luckily I checked under the bonnet before the next race
    "How long since you've visited The Good Oil?"

    '93 V8 Rossi
    '97 to '07. sold.
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  8. 8th April 2009, 07:17 PM #8
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    Quote Originally Posted by PhilipA View Post
    It is the most likely failure . I believe that the seal around the diaphragm leaks and some oil is able to go behind it.
    I have had 2 pressure switches fail this way.
    They often leak out the joint between the brass body and end cap also.
    Regards Philip A
    yes had one like that.
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