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Oil Pressure gauge reads BACKWARDS???

Discussion in 'SN95 Cobras' started by badass98svt, Sep 10, 2010.

  1. badass98svt

    badass98svt YOUR HONDA SUCKS Established Member

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    Ok, so the new motor is in and running great. The shop owner calls today and tells me that the oem oil pressure gauge is working backwards.
    He said you turn the key to on and the needle shoots over to the "L" in normal, then when you start it, the needle shoots to the left and sits around the "O" in normal.
    Anyone have any idea?

    he hooked up a mechanical oil pressure gauge and said everything is reading just fine.


    I also feel I need to point out that my tach has been acting starnge for about a year now. if I goose it the tach needle will be working fine, then sometimes when I switch gears the needle with dart forward instead of dropping down. When it does that, the needle ends up going all the war around and getting stuck UNDER the pin for the tach needle. Are you guys following me? Once this happens, the next time I'm on the gas, the tach needle will actualy move backwards as well. If I really goose it and bang a couple gears, i can sometimes get the needle to go all the away around backwards so it will eventually sit on the pin.

    I'm clueless. is it just an electrical issue? Do i need my cluster worked on?
     
  2. Gallows

    Gallows Make My Day... N/A Premium Member

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    I'd check the plugins behind the cluster first to make sure they are tight and there aren't any frayed wires. IIRC you put LEDs in awhile ago. Did you have this problem before or after the LED's were put in?
     
  3. badass98svt

    badass98svt YOUR HONDA SUCKS Established Member

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    The tach was screwy long before the LEDs
     
  4. Gallows

    Gallows Make My Day... N/A Premium Member

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    It was just a thought. Maybe Mwolson will jump in with some insight. Congrats on getting the Cobra back on the road:burnout:
     
  5. badass98svt

    badass98svt YOUR HONDA SUCKS Established Member

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    I pick it up tomorrow!!! Thanks btw.

    Hoping mwolson chimes in!
     
  6. Mustang Matt

    Mustang Matt New Member Established Member

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    I think the oil pressure is a switch and not a sender. So maybe the wires are just backwards? Mark Olson would be the guy to talk to.
     
  7. cobraman1024

    cobraman1024 Waiting to be Blown.... Established Member

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    an oil pressure switch is usually just one wire; a single terminal plug and the switch itself is grounded to the engine block by its metal casing threads. when there is enough oil pressure (minimum 5psi I think) its enough to push the "normally open" switch contacts to a "closed" position to complete the circuit for the switch. you might just need a new oil pressure switch.

    On another note, have your guy check and make sure that he reconnected all of the drivers side ground straps.
     
  8. badass98svt

    badass98svt YOUR HONDA SUCKS Established Member

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    Any other ideas???
     
  9. na svt

    na svt say no to power adders Established Member

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    Mark is probably the one person who can troubleshoot this for ya.
     
  10. badass98svt

    badass98svt YOUR HONDA SUCKS Established Member

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    Well, where's he been? hahaha Haven't seen him post here in a while.
     
  11. na svt

    na svt say no to power adders Established Member

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    I'll email him
     
  12. mwolson

    mwolson Gray beard Established Member

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    Sorry guys, but I have had a super busy summer. Todd, thanks for the note. I am currently in South Dakota helping my mother who recently had a new valve put in her heart, so I haven't been on the forums much.

    Since I don't have access to my clusters at home til later in the week, all I can do is guess. But my guess is that your cluster is actually from a 94-95 Mustang and your car wiring is for your 98.

    If you look at this web page: 94 to 98 Cluster Pinouts, you can see that the oil pressure, volt gauge and tach wires are scrambled between 94-95 clusters and the newer wiring. You can see that the oil pressure gauge signal and ground pins were swapped.

    I'd need to look at the schematics (which are back in Cali) to try to predict the cluster behavior in such a situation. Some of the idiot lights may not work properly for example. I suspect swapping the OP ground and signal wires might cause the oil pressure gauge symptom you report.

    Is there any way you can pull the cluster to see if you can read the tag? If it isn't too faded, it will tell you what car it is from. Regardless, if you post a picture of the circuit board on the back of the cluster, I can tell you what it is.
     
    Last edited: Sep 26, 2010
  13. NVENEM8U

    NVENEM8U Bust'n ass since 1996 Established Member

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    You have the wrong type of oil pressure switch installed. I will bet money that the switch ur using is designed for a vehicle that has an oil pressure light instead of a gauge.
    I am a tech at my local ford dealer and I just went through the same thing on my cobra because for some reason the m/c part number listed for our cars is wrong. The proper part is motorcraft e9sz-9278-a OR sw-2220
     
    Last edited: Sep 27, 2010
  14. mwolson

    mwolson Gray beard Established Member

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    The 94-98 oil pressure gauges use a 6PSI pressure switch that is open below 6PSI and grounded above 6 PSI. In the 94-96 clusters, there is a 20 ohm resistor in series with the gauge signal circuit. That resistor limits the current so the gauge reads in the high-normal range when the pressure switch is closed to ground. Without that resistor, the gauge would slam above the high mark when the engine is running. The 97-98 oil pressure gauges have had the gauge windings changed so that the gauge reads high normal when the signal is grounded without the need for the resistor.

    You can test the sender with an ohm meter. Unhook the sender wire and but the ohm meter across the sender signal pin and ground. With the engine off, it should read infinite resistance and with the engine running, it should read close to zero ohms. If this is not true, you have the wrong oil pressure sender or it is bad.

    Then, to test the gauge, with the key on, the gauge should read 0 oil pressure with the oil pressure signal wire unhooked. With the key still on, you should ground the oil pressure sender wire and the gauge should go to high-normal. If this is not true, then there is something wrong with the wiring and/or the gauge. It is the wacky, backwards function of the oil pressure gauge combined with the wacky tach action that makes me suspect that you have a 94-95 cluster in the car.

    This is an interesting one. When I get home later this week, I will try to see if I can reproduce the oil pressure symptom on the bench.
     
  15. SNAKEYE

    SNAKEYE New Member Established Member

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    Based on the erratic tach and LEDs I'd suspect a bad dashboard ground connection.
    Find the ground wire connection(s) at the front of the console on the passengerside of the dash-to-transmission tunnel brace. They are green-colored screws. Remove them, clean the lugs and the surface of the brace steel, apply an electrical anti-oxidant if you have it (a dab of wheel bearing grease will work), and re-assemble the connections.
     
  16. mwolson

    mwolson Gray beard Established Member

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    That could do it too, especially if the oil pressure gauge and tach grounds run to the same lug. I was planning on looking at that in the EVTM after I get home. It is good to know where that ground lug is.
     
  17. badass98svt

    badass98svt YOUR HONDA SUCKS Established Member

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    It's definitely not a 94-95 cluster. I think nvenom8u may be right. That makes sense if itks labeled wrong in Fords system. I think the tach is a totally diff issue. Sometimes I just barely tap the plexiglass, and the needle will read correctly. I think it's an eternal issue inside the cluster. Mwolson, I don't suppose you'd be interested in taking a look at it for me, is there? You can play with it all winter long. I would love for my tach to finally work correctly. It's been screwy for 2 years now.
     
  18. mwolson

    mwolson Gray beard Established Member

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    Sure I'll be happy to put it on the bench for you. I'll pm the details to you.

    But just for fun, run the oil pressure sender and gauge tests I suggested first.
     
  19. badass98svt

    badass98svt YOUR HONDA SUCKS Established Member

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    Email sent Mark. Thank you
     
  20. mwolson

    mwolson Gray beard Established Member

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    I finally got back to my bench and set up a couple of tests. The first test was to set up a 2K RPM input to the tach, and then unhook the tach's ground wire. The tach instantly went to zero RPM. Then I rapidly grounded and un-grounded the tach ground wire to simulate an intermittent ground wire in the car. I found that the tach signal would bounce around a bit, but always between 2K RPM and zero. It never went above 2K RPM and it never went backwards.

    I then put 12V to the oil pressure gauge power and grounded the ground wire. The oil pressure needle went to the L mark. When I grounded the oil pressure signal, the needle went to high-normal as it should. I then opened the signal again and removed the ground wire to the gauge. The needle went a bit above the L mark. When I grounded the signal with the gauge ground disconnected, the needle went way, way above the H mark. So with the oil pressure gauge ungrounded, the needle indicated some oil pressure with the switch open and very high pressure with the switch closed. It did not go backwards.

    As a result of these tests, I no longer think that a bad ground could be causing your symptoms.

    I'd be very curious to hear the results of the tests I suggested...
     
    Last edited: Oct 1, 2010

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