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Thread: OPG - real gauge vs dummy gauge

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    Forum Sponsor revlimiter's Avatar
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    OPG - real gauge vs dummy gauge

    ONE of these is the gauge you're looking for.


    I just put this on my blog. I thought the forums might appreciate it as well.

    Here’s a quick post showing the differences between the two NA Miata oil gauges. As you may know, the 89-94 cars got working oil gauges that actually report oil pressure. The 95-97s got dummy gauges that only show “some” pressure when the sensor reads anything above 7 psi.

    This is not new information, but I was hard pressed to actually find a detailed write-up of this a couple weeks ago. I found some forum posts where some “expert” replied that a photo of the back of a gauge was “definitely the real one” but couldn’t find anything concrete.

    This post will produce concrete.



    The above shot is good news. Take a look. ALL of the faces minus one came from real oil pressure gauges (called OPGs for the rest of this post). This is from my collection in the shop. As you may know, I make gauge faces and often do installs.

    I was actually surprised at this photo. Just one out of… apparently 8, was a fake face. And that holds up to my data with orders – there are a lot more NA6 Miatas out there (that’s 89-93) than NA8s. Mazda just produced more.

    That said, the face doesn’t guarantee that you have a real, working OPG.


    Which one is the fake? Three are real, one is a dummy.


    In the above photo, there’s ONE fake oil pressure gauge. The others are real. And, aside from the one with the face in the photo, they’re all identical. You can’t look at a photo of the back of one of these gauges and determine which is which. The board layout is the same and the color of the board is meaningless.

    Let me say that again – the back view of the gauge doesn’t tell you which gauge you have.

    As for the face, I once purchased a “real” oil pressure gauge with the correct OEM face. It turned out to be a dummy gauge. This was about a decade ago, but I still remember and get angry thinking about it.


    Real on the left, fake on the right.


    The best way to tell the difference is by reading the resistors. They’re different on the real and dummy gauges.

    Real – as viewed from the front.
    Left: blue, red, black (62 ohm, I believe)
    Right: green, brown, black (51 ohm, I think).
    And both sides have the gold band on the end which means 5% tolerance.

    Dummy – as viewed from the front.
    Left: brown, black, brown (100 ohm, possibly)
    Right: grey, red, black (82 ohm, maybe)
    And the dummy also has the 5% (gold band) resistors.

    SO. That’s what you need to know. You need to see the resistor values to determine if the gauge is a working OPG or a dummy OPG.

    But we can take it a step further.



    All you need is a desktop power supply and you can see the difference in the gauge function for yourself. Just wire power to the “+” terminal, ground to the “-” terminal, and the “U” terminal is signal. Grounding it will show the gauge’s behavior.

    When the real gauge receives a ground signal, it rockets up to 90 psi.
    When the fake gauge receives ground, it drops – very slowly – to 0 psi.

    And there you have it. Everything you ever wanted to know about Miata oil pressure gauges.

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    Super Moderator atank's Avatar
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    Very informative post Adam!!!!!!!!!

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    5,000 rpm - there be torque here! WASABI's Avatar
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    Thanks for putting this up. Good information.

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    Admin / Pit Boss / Miataholic Phatmiata's Avatar
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    Get info!! I was looking for this info a few weeks ago and had to start looking in my old MCA Magazines, very time consuming. Great info and pictures as usual !!

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    Forum Sponsor revlimiter's Avatar
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    Thanks guys! Hope it helps someone out in the future.

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    2,000 rpm - light wheelspin, no bog here! jnshk's Avatar
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    Adam, the circuit layouts are the same, but the resistors are different. Are the "windings" the same? What I'm getting at is, could a dummy gauge be converted to real gauge by swapping the resistors (similar to the method for converting the temp gauge to a linear gauge)?

    Also, thanks for the technical info!
    [ Project Moneypit: 1958 Volvo 444 Sport w/ Ford 2.9L EFI and drivetrain || Project Fail v.2: 1997 Mazda Miata (PEP) ]

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    2,000 rpm - light wheelspin, no bog here! Satisaii's Avatar
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    What are the differences in the sending units, and if so, how do you tell them apart?

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    Forum Sponsor revlimiter's Avatar
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    Quote Originally Posted by Satisaii View Post
    What are the differences in the sending units, and if so, how do you tell them apart?
    The NA6 sending unit is HUGE. It sends out varying resistance throughout the pressure range.
    The NA8 is just a switch that's set at 8 psi (I think) and is comparatively tiny. When it reads below the set pressure, the switch closes and grounds the gauge, making it drop to zero.


    Quote Originally Posted by jnshk View Post
    Adam, the circuit layouts are the same, but the resistors are different. Are the "windings" the same? What I'm getting at is, could a dummy gauge be converted to real gauge by swapping the resistors (similar to the method for converting the temp gauge to a linear gauge)?

    Also, thanks for the technical info!
    As far as I know, the windings are backwards. Watch the video and see the behavior of the gauges when power is applied. They seem to be backwards from each other.

    The NA8 also has damping grease.

    So... in theory, you could convert one gauge to the other by rewinding it, removing the grease, and changing resistors. OR you could take advantage of the cheap and plentiful NA6 gauges (they're usually around $20 each) and save yourself a ton of time.

    The expensive item is the NA6 sensor. You can still buy them new, but they're rather not-cheap and I'd personally not trust a used one.

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