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  1. #1
    Bob Chadwick's Avatar
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    Default Help Troubleshoot an Electric Oil Pressure Gauge

    I've been having problems with rapid oil pressure fluctuations. I put in a mechanical gauge and have confirmed that the problem is the gauge and not the motor. While the electrical gauge would flutter between 60 and 80 psi intermittently, the mechanical gauge would stay rock steady at 50 psi. So it would appear the motor is not the problem.

    The sending unit is new, the gauge was out and checked, the gauge circuit board was replaced and the plug that connects to the sending unit is new. None of this changed anything. The oil pump was also replaced.

    The way I understand the system to work is that power is supplied to the gauge from a switched fuse and that a wire then runs from the gauge to the sending unit where it grounds through the sending unit. I don't understand how the sending unit uses the ground to create the pressure reading.

    Checking a gauge that is not working is supposed to be as simple as grounding the wire attached to it. If it pegs out, these means a bad sending unit. Because the pressure always seems to rapidly fluctuate up and back to the original reading it seems to me that either the wire is grounding or the sending unit has an issue with the resistor that is allowing full ground.

    Any thoughts? Is it possible to check a sending unit. I read somewhere that it should have a range of resistance. Of course, when it is intermittent it is always harder to figure out.

  2. #2

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    The sending unit contains a variable resistor operated by a diaphragm which senses pressure and moves the wiper on the resistor. One end of the internal resistor gets 12 volts, and the wiper which moves along its windings is grounded to the sender body (which is grounded to the block); as pressure changes, the wiper connected to the diaphragm moves, and changes the resistance in the circuit between the gauge sender terminal and ground, which changes the gauge reading.

    The only reference I have at the moment shows that GM electric oil pressure senders have one ohm resistance at zero pressure, and 80-88 ohms resistance at max pressure reading. Using the same test logic used with temp and fuel senders (which also use variable resistance to ground to operate the gauge), if you disconnect the wire at the sender (infinite resistance), the oil pressure gauge should peg at max, and if you ground the sender wire to the block (zero resistance), it should peg at zero. That will check the gauge and the circuit to the sender.

    You can do a gross check on the sender by disconnecting the wire and connecting a digital VOM set on ohms between the terminal on the sender and ground; with the engine off, it should show one ohm; with the engine running, it should vary up to 88 ohms as rpm increases.

  3. #3
    Bob Chadwick's Avatar
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    Default

    Thanks John.

    I've got one of those multi-meter's that was given to me. I guess I'll just need to go figure out how to use it.

  4. #4
    Member jdp6000's Avatar
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    I bought a new oil pressure sensor.They are cheap. Anyone know how you get a wrench on the one on the car...its real tight?

    Jim

  5. #5
    Bob Chadwick's Avatar
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    Got one already. Mine is down under the car. Yours sounds like it is beside the distributor.

  6. #6
    Member jdp6000's Avatar
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    Correct, its right beside the distributor. any idea on how to remove it?

    Jim

  7. #7
    vette-dude
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    Quote Originally Posted by jdp6000
    Correct, its right beside the distributor. any idea on how to remove it?

    Jim
    They make a special deep socket to remove/install these oil senders. Should be available at any discount Auto parts store. Personally, I have use a 7/8 deep socket or (in some cases where room will permit) a small off set plier to remove. Installation is what you need to worry about. You don't want to crack the plastic seal. The sockets are cheap and I finally bought one and now never have a need for it!! And no, I won't sell it as you never know when it will be needed (just like half the tools in my garage)!!

  8. #8
    Member jdp6000's Avatar
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    Your only 8 hours away from me. Can ya drive up an get it off for me? Eh? Just kidding. I'll see if the the local garage has one of these sockets. They must have built the engine around this sensor.

    Jim

  9. #9
    Bob Chadwick's Avatar
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    When I put the mechanical gauge in temporarily I looked at the spot on the front of the engine and the one near the distributor. Guess where I tapped in.

  10. #10
    vette-dude
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    Quote Originally Posted by Bob Chadwick
    When I put the mechanical gauge in temporarily I looked at the spot on the front of the engine and the one near the distributor. Guess where I tapped in.
    Taking the easy way out, Huh!!! Did you get the gage problem fixed??

    Randy

  11. #11
    Bob Chadwick's Avatar
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    Quote Originally Posted by vette-dude
    Taking the easy way out, Huh!!! Did you get the gage problem fixed??

    Randy
    Certainly.

    The car is in the shop now for trailing arms and bearings. I'll take a look at it when I get it back.

  12. #12
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    Quote Originally Posted by Bob Chadwick
    Certainly.

    The car is in the shop now for trailing arms and bearings. I'll take a look at it when I get it back.
    Bob,
    So what was the fix? New sensor? I have the same prob on mine.
    thanks
    .
    .
    .

    lone73
    '73 4 spd
    '04 6 spd

  13. #13
    gec
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    Mines pegged at max...just one other electrical problem I have'nt got around to yet, does that mean the sender is bad or the ground?

  14. #14
    nyernga
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    Quote Originally Posted by Bob Chadwick
    Thanks John.

    I've got one of those multi-meter's that was given to me. I guess I'll just need to go figure out how to use it.
    C'mon Bob....I used that thing for 2 years. Call me if you need help deciphering it...

    Len

  15. #15
    Bob Chadwick's Avatar
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    Quote Originally Posted by lone73
    Bob,
    So what was the fix? New sensor? I have the same prob on mine.
    thanks
    .
    .
    .
    At this point I'm not sure. I replaced or rebuilt everything and need to do some exploring. When I get to it, I'll make sure I post what I find out.

    Quote Originally Posted by nyernga
    C'mon Bob....I used that thing for 2 years. Call me if you need help deciphering it...

    Len
    I'll take you up on that Len.

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