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  1. #1
    YOUR HONDA SUCKS badass98svt's Avatar
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    Oil Pressure gauge reads BACKWARDS???

    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?

    1998 SVT Cobra #4941 of 5174 built on June 19,1998 (1 of 674 with same options) 263rwhp/258rwtq (OEM engine,not Aluminator)
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  2. #2
    Make My Day... N/A Gallows's Avatar
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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?


    1997 Cobra - 336rwhp/298tq, 03 Aviator long block with a few mods and a built IRS with 4:56's My build thread - http://www.svtperformance.com/forums...-97-cobra.html

  3. #3
    YOUR HONDA SUCKS badass98svt's Avatar
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    The tach was screwy long before the LEDs

  4. #4
    Make My Day... N/A Gallows's Avatar
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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

  5. #5
    YOUR HONDA SUCKS badass98svt's Avatar
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    I pick it up tomorrow!!! Thanks btw.

    Hoping mwolson chimes in!

  6. #6
    Insane SVT Poster
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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.

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  7. #7
    Waiting to be Blown.... cobraman1024's Avatar
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    Quote Originally Posted by Mustang Matt View Post
    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.
    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.

    1995 Rio Red Cobra #182/4005, 283/293, 221k miles, Roush Stage III intake, export mirrors and taillights, ported heads, E303, authentic Saleens, tuned by Dez
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    1993 Mustang LX 2.3L, 267k miles, T5 conversion, 8.8/3.27 gears, 03 Cobra steering and brakes, black interior

  8. #8
    YOUR HONDA SUCKS badass98svt's Avatar
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    Any other ideas???

  9. #9
    say no to power adders na svt's Avatar
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    Mark is probably the one person who can troubleshoot this for ya.

  10. #10
    YOUR HONDA SUCKS badass98svt's Avatar
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    Well, where's he been? hahaha Haven't seen him post here in a while.

  11. #11
    say no to power adders na svt's Avatar
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    Quote Originally Posted by badass98svt View Post
    Well, where's he been? hahaha Haven't seen him post here in a while.
    I'll email him

  12. #12
    Gray beard
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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 by mwolson; 09-26-2010 at 11:19 PM.
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  13. #13
    Bust'n ass since 1996 NVENEM8U's Avatar
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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 by NVENEM8U; 09-27-2010 at 06:24 AM.
    09 gt auto, bolt-on/spray

  14. #14
    Gray beard
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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. #15
    Insane SVT Poster
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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.
    Triple-black 1997 Mustang Cobra convertible, lowered 1-1/2", Vortech-supercharged, 18" Saleen wheels mounting Michelin SP2's : 245/45's front/295/35's rear, 2-core radiator, 4.10 rearend w/SVO girdle, 31-spline axles, parallel ECT fan mod.

  16. #16
    Gray beard
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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. #17
    YOUR HONDA SUCKS badass98svt's Avatar
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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. #18
    Gray beard
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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. #19
    YOUR HONDA SUCKS badass98svt's Avatar
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    Email sent Mark. Thank you

  20. #20
    Gray beard
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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 by mwolson; 10-01-2010 at 03:47 PM.

  21. #21
    YOUR HONDA SUCKS badass98svt's Avatar
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    Wow Mark, that's some interesting stuff right there. Unfortunately, I don't have an ohm meter to perform the tests.
    I definitely plan on sending you my gauge cluster over the winter though!

  22. #22
    Bust'n ass since 1996 NVENEM8U's Avatar
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    the 9 dollar sending unit I listed earlier will fix ur issues with the oil press gauge .....Mark good job with the gauge cluster stuff u've figured out.

  23. #23
    Gray beard
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    I was able to duplicate your tach problem with the speedometer last night. I am now pretty sure that your tach signal is intermittant.

    But first, a little background on VSS signals. A couple of weeks ago, I helped a guy set up a Speedcal on an OSS 02 Sable with a 96 Taurus VSS tranny. I put an O-scope on the VSS signal and we spun it with a variable speed drill. I noticed that the amplitude of the VSS sine wave went up with frequency to well over 1V p-p.

    Here is what I set up on my bench.

    I installed a freeware tone generator package from cnet.com called Expression Tone generator. I set it up to generate a fixed frequency sine wave by using the equation: sin([frequency]*t). I maxed out the volume and hooked one side of a stereo mini jack to the speedometer signal on a 120 MPH V6 cluster with a powered up speedometer. I was able to drive the speedometer up to about 100 MPH with this setup by entering the correct frequency into the formula. The max voltage the PC was able to put out was about 0.1V. My theory is that the voltage must go higher for the speedometer to work above 100 MPH. I will test that with a different signal generator sometime this weekend.

    Here is what I learned. Below about 90 MPH, if you remove the signal, the speedometer needle will drop to zero. But if you suddenly remove the VSS signal above about 90 MPH, the needle will sweep all the way clockwise to under the needle stop pin. If I try to get the needle to move to a speed lower than about 90 MPH, it looks like the needle just pushes on the pin. It I feed it a signal that sets it to 100 MPH, it goes there, but then rotates clockwise back to under the pin when I remove the signal. But if I feed it a signal at about 90 MPH, the needle will rotate counter-clockwise to the 90 MPH mark. If I then remove the signal, it will drop to zero counter clockwise as it should.

    The speedometer and tachometer must use current through two coils to create the magnetic fields needed to move the needle correctly. One coil creates a reference magnetic field that sets the reference for the needle rotation. Then the meter uses the frequency of the input signal to figure out what deflection field should be generated to move the needle to a certain angle. It seems that, with an absence of an input signal, the needle aligns to the reference field so it goes back to the pin. But, depending on which half circle the needle is on, it will rotate to the pin in a clockwise or counter-clock wise direction.

    With normal operation, the signal field varies gradually up and down, so the needle will move up and down relative to the reference field, and we never see it go above the maximum speed that the needle goes to. The only time it can slam all the way to the back of the pin is if the signal is lost after the needle has rotated past 180 degrees from the pin.The only way to get the needle back is to run it up above 180 degrees from the pin and then back it down to less than 180 degrees from the pin before losing the signal.

    The tach input needs a DC signal, so I was not yet able to try to reproduce this behavior with the tach, but I bet it will work the same way. I need to set up my fancy tach calibrator setup before I can try this experiment with the tach. I will try this with the tachometer this weekend.

    But the data is pointing towards an intermittent tach signal somewhere between the PCM and the tach circuit in the cluster.

    The oil pressure gauge issue appears to be completely different from the tach issue.

  24. #24
    Waiting to be Blown.... cobraman1024's Avatar
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    Seriously, its been suggested twice that you change out the oil pressure switch for a new one. Its not such an outlandish idea that your pressure switch may have gone bad and its a whole lot simpler to change that out than it is to bench-troubleshoot your gauge cluster online in a forum.
    Last edited by cobraman1024; 10-02-2010 at 06:01 PM.

  25. #25
    Gray beard
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    I pulled out the signal generator I use for calibrating tachometers tonight to see if I could get the tach to fail the same way I was able to get the speedometer to fail.

    I learned that the tach input is a DC square wave that needs to be about 6V or more. An AC signal will not work to drive the tach.

    I was able to drive the tach up to the 7 grand limit of the V6 cluster with my calibrator, no problem. I was even able to go all the way up to 9000 RPM which pointed the tach needle straight down. But when I removed the tach signal from the tach input, the tach immediately dropped to zero RPMs, regardless of where the needle was pointing, even straight down. So my guess was wrong about the failure of your tach.

    FYI, I was able to AC couple the tach calibrator signal by using a 0.1uF cap in series with the signal to drive the speedometer. With this calibrator, I was able to make the speedometer needle wrap around the back to the needle, so this is a legitimate failure mode for a speedometer with an intermittent VSS signal. But it does not explain the failure of your tachometer.

    Looks like I will need to get your specific cluster on my bench (when it is ready) to get a better chance of figuring out your tach issue.

    I ordered a sweep function generator today in order to get more control over the signals I can generate in my lab. Should be here within two weeks.
    Last edited by mwolson; 10-02-2010 at 11:19 PM.

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