Oil pressure switch - 2 terminals vs. 1
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  1. #1
    Gone Fishin' Haakon's Avatar
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    Default Oil pressure switch - 2 terminals vs. 1

    The oil pressure switch on the R21 failed in a spetacular manner, dumping a LOT of oil out the side of the block... Am now in the process of finding a replacement...

    Have some R25 units, but they have an integral pressure gauge sender, so no good there. Fuego sensors only have one lead (ie normal).

    The R21 has two.

    Now the odd thing is that one appears to serve no function I can determine. It has no resistance to ground, has no voltage supplied ignition on or off, and does not illuminate the oil light when bridged to the other wire.

    The other seems to function as a single wire sensor connection - ie illuminate the oil light when connected to ground.

    Any ideas on what this is all about? An oil pressure signal to the ECU perhaps? The Haynes manual ties to tell me the R21 should have a single wire sensor, as do the parts websites when i plug my model in...

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    Last edited by Haakon; 27th December 2010 at 10:48 AM.

  2. #2
    1000+ Posts robmac's Avatar
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    It's possibly more technically correct than necessary.

    Because the voltage on a sensor input to an ECU are prone to input "noise" it's good practice to run an earth from ECU to each device (or to a designated low impedance earth point or junction) to minimise earth loop induced noise.

    If there is any resistance in the earth return, this causes voltage drop, which is effectively superimposed on the sensor line and can be injected into the ECU. Many ECUs use a 5volt pull up so even small induced noise voltages can cause problem.

    Of course an oil lamp in series with a lamp never had this issue.

    I assume the oil pressure switch triggers an ECU input, which in turn triggers an ECU output which switches the oil lamp ?

    You may be able to use engine ground as a reference instead of the ECU ground , but I'd be worried about possible noise injection and strange ECU behavior.

    You want to be sure that the "earthy" side of the pressure switch wiring is definitely ground and not some intermediate pull up or pull down voltage before going too far.

    You may find something here to suit, but they are quite expensive:
    http://www.tridon.com.au/Products/Pr...5&G=483&P=2011

  3. #3
    Gone Fishin' Haakon's Avatar
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    That triggered an idea - the second wire is not an earth feed, its something else thats receiving a pressure signal.

    Both the terminals on the switch are the same - same as TPS016 on that list you linked. The multimeter tells me they are both providing a ground connection with the engine off, and both severe that when its started. The switch is definitely using the engine ground, not a supplied one from the ECU.

    And I think the ECU is also not switching the lamp on from what I can tell - that illuminates with just one one lead connected.

    This means I can use any single wire sensor and just connect up the other. The question is of course - what else wants a pressure signal?

    What would the ECU want with it? Safety shut down?

  4. #4
    1000+ Posts robmac's Avatar
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    Quote Originally Posted by Haakon View Post
    That triggered an idea - the second wire is not an earth feed, its something else thats receiving a pressure signal.

    Both the terminals on the switch are the same - same as TPS016 on that list you linked. The multimeter tells me they are both providing a ground connection with the engine off, and both severe that when its started. The switch is definitely using the engine ground, not a supplied one from the ECU.

    And I think the ECU is also not switching the lamp on from what I can tell - that illuminates with just one one lead connected.

    This means I can use any single wire sensor and just connect up the other. The question is of course - what else wants a pressure signal?

    What would the ECU want with it? Safety shut down?
    Yes, probably a generic sensor would fine - hope you didn't read the the first version of this post. I've been distracted and not read what said properly.
    Last edited by robmac; 27th December 2010 at 01:06 PM. Reason: READ THE POST READ THE POST READ THE POST

  5. #5
    Gone Fishin' Haakon's Avatar
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    After all that, my pack rat tendencies paid off - found a spare switch

  6. #6
    Fellow Frogger!
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    One last thought - single terminal for a warning light, dual terminal for a gauge?

    Regards,

    Fento

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