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Are There Different Electric Oil Pressure Gauge Sending Units?

Discussion in 'The Hokey Ass Message Board' started by Jay Ess, Jun 3, 2011.

  1. Jay Ess
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    Jay Ess Member

    The rebuilt flathead in my 51 reads 30 lbs. of oil pressure when measured with a mechanical gauge. The car came with an electronic oil pressure gauge that did not work. I was told it was a bad sending unit. Changed the sending unit with a new one from Summit. Still did not work. Ordered an NOS "Vintage" brand gauge from a supplier and now at least the needle moves but it only registers 10lbs. at the identical RPM at which the mechanical gauge shows 30 lbs. Another mechanical gauge shows about 30lbs as well.

    Questions:
    Are there different oil pressure sending units that send different levels of electronic signals to the gauge?

    Is there a way to amplify or diminish (not sure which) the signal coming from the sending unit to get an increased reading at the gauge?


    Short of pulling apart the sealed gauge and trying to reposition the needle, any suggestions on how to get this gauge to read the same as the mechanical gauge?

    Thanks as always,
    js
  2. squirrel
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    squirrel Member

    The sender needs to match the gage. They're all different. Buy a gage with it's proper sender. Don't try to mix and match, it won't work.
  3. Jay Ess
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    Jay Ess Member

    Thanks for the response. The gauge is one of a matched set that was installed at least 15-20 years ago. They are no longer being manufactured and I felt lucky to find a NOS on a shelf in a suppliers warehouse as it would match the others. The seller was not sure about what type of sending unit was required. Time for me to do more research I guess...
  4. F&J
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    F&J Member

    Wonder if you can use an adjustable potentiometer to get the range of ohms? .set the Pot to show zero, half and max, then measure the ohms resistance on the Pot settings?

    if so, then you would need to know the ohm range of various brands of senders.

    In pre-internet days, there were always customers at Radio Shack buying parts; I'd ask for advice and they always knew how to do whatever you needed.
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  5. squirrel
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    squirrel Member

    That's a good suggestion, get a pot with perhaps 5k ohm resistance. linear taper.

    Now the folks at radio shack know a lot about cell phone plans...not so much about electronics.
  6. F&J
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    F&J Member


    I am not up on sizing pots...I guess I'd just pick out two or three and see which one worked :) They can't cost much at R.S. ?
  7. squirrel
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    squirrel Member

  8. Jay Ess
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    Jay Ess Member

    You guys are the best, even including a link! I am an electronics novice (and that is giving me more credit than I am due). Is the linear* potentiometer used to test the sending units or to fix the gauge? How would I use the potentiometer to help me?

    Thanks for your patience and guidance.
    *if graphed I presume it increases at straight 1 over 1 range vs. a 1 over 1, 1 over 2, etc. which would be a curve?

    js
  9. F&J
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    F&J Member

    A pot will be an "adjustable grounding source" or in other words, adjusts the resistance. A sending unit would be variable resistance as the sender changes from zero pressure to normal oil pressure.

    So, you are using the pot to take the place of the missing sender only to get the ohms readings from the pot. Adjust the pot to have the gauge read zero, then disconnect the gauge from the pot and measure the ohms at the pot. Now do the same at high gauge pressure and measure the new ohms setting on the pot. That gives you the range of ohmss needed for a proper sender.

    The problem might be in finding specs for a OEM sender, or SW, Autometer, etc. You can't find a sender unless you know what it's rating is.
  10. d2_willys
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    d2_willys Member

    There are ways of scaling the sender unit you have. But you say you are an electronics novice and this circuitry would probably boggle your mind. The suggestion to sub the pot for the sender and characterize the gauge readings to the sender is probably the best solution so far. I was going to suggest this, but I would use a smaller pot value, 500 ohms. The linear pot is correct as the sender is linear. There are two types of a typical pot, audio and linear, don't get the audio, as it is logrythimic in response.

    If u have a spare gas sender unit you might try this in place of a pot. They are about 90 ohms and might get you close to the right value.
    Last edited: Jun 4, 2011
  11. Jay Ess
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    Jay Ess Member

    Thanks for all the help! I am going to show this thread to a buddy who is knowledgeable in electronics and ask him to help me with this. I am hoping that the gauge manufacturers might know what ohms range is used by their senders. Again, thanks.
  12. Jay Ess
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    Jay Ess Member

    My mechanic informs me of the following:

    On My gauge:

    240 Ohms = 0 psi
    33 Ohms = 80 psi

    Now that I have these numbers, what am I to look for in a sender, or how do I modify the sender to have the gauge read 30lbs when it is only showing 10 lbs.?

    Thanks,
    js
    Last edited: Jun 6, 2011
  13. hotroddon
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    hotroddon Member

    I believe that a Stewart Warner 279A sender will do exactly what you want.
    240 ohm at 0, 3.5 ohm at 80 PSI and for an 80 lb gauge.
  14. Gman0046
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    Gman0046 Member

    FYI, Dakota Digital oil pressure sender is a stock GM unit. I thought it would be something exotic.
  15. Jay Ess
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    Jay Ess Member

    Thanks for the tip but I found one that was made for VDO gauges that was an exact replacement.

    Just wanted to thank all that contributed to the thread and helped me solve the problem. The power of the HAMB never ceases to amaze me.

    js

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