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Thread: Oil Pressure Gauge Issues

  1. #1
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    Oil Pressure Gauge Issues

    I'm trying to troubleshoot the oil pressure gauge in a 91 Spirit R/T dash. When I start the car, the gauge pins to as low as the needle can possibly go (9 o'clock position). I've done everything I can think of to try to figure out the problem and am stumped. Here's what I have tried so far:

    - Installed a mechanical gauge and determined oil pressure is well within spec
    - Swapped in cluster from another Spirit R/T: the cluster from the other R/T did the same thing and this cluster worked fine in the other R/T
    - Tested the oil pressure sending unit/switch: tested OK, but bought new one anyway and no change (also swapped known good sending unit from other R/T and that did not work either)
    - Ohmed out the gray/yellow wire and it tests clean from sending unit all the way to instrument cluster

    It looks like the circuit is a simple one wire from the sending unit to the gauge and all the gauge does is measure the resistance that the sending unit puts out. So I'm stumped as to what could be causing the gauge to do what its doing. Does anyone have any ideas on what I might be missing?

    And no, I don't really want to permanently install the mechanical gauge, so that suggestion is off the table. LOL!
    Last edited by jonnyb; 07-18-2017 at 04:48 PM.

  2. #2
    Garrett booster
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    Re: Oil Pressure Gauge Issues

    Does your car have a single sender with 2 terminals or separate senders for the idiot light and the gauge? Is it possible that the wires are switched around?

    Measure the resistance between the terminal on the sending unit and ground with the wire(s) disconnected. Re-connect the wire(s) and measure the resistance between the cluster plug and ground. You should get about the same reading. With the wire(s) disconnected from the sender there should be no continuity to ground(or anything else) at the cluster connector.

    Does the gauge peg when you turn the ignition to the on position or literally only once the engine starts?

  3. #3
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    Re: Oil Pressure Gauge Issues

    It has the single sender with 2 terminals.

    I never did try continuity to ground like you suggest. Basically, even though the wire Ohm's out OK, you're thinking that maybe it's grounding out somewhere and that's what is messing up the gauge?

    It pegs the gauge as soon as I turn the ignition to the on position.

  4. #4
    Rhymes with tortoise. Turbo Mopar Staff cordes's Avatar
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    Re: Oil Pressure Gauge Issues

    Quote Originally Posted by jonnyb View Post
    It has the single sender with 2 terminals.

    I never did try continuity to ground like you suggest. Basically, even though the wire Ohm's out OK, you're thinking that maybe it's grounding out somewhere and that's what is messing up the gauge?

    It pegs the gauge as soon as I turn the ignition to the on position.
    There is most likely a short to ground somewhere. I'll try to check my wiring diagrams to confirm.

  5. #5
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    Re: Oil Pressure Gauge Issues

    Quote Originally Posted by jonnyb View Post
    It has the single sender with 2 terminals.

    I never did try continuity to ground like you suggest. Basically, even though the wire Ohm's out OK, you're thinking that maybe it's grounding out somewhere and that's what is messing up the gauge?

    It pegs the gauge as soon as I turn the ignition to the on position.
    That's what I was getting at. I don't know if the gauge would peg high or low if the wire was grounded. I was also thinking maybe you had the gray and gray/yellow wires switched somehow because that would cause the gauge to peg also but It would un-peg once oil pressure rose enough to trip the oil pressure switch. I do know that the sender is just a variable resistor connected between the gauge wire and ground so checking the resistance to ground at the gauge end should give some insight. Check for voltage too in case it's shorted to 12v or something strange like that.

    I checked rockauto after I posted yesterday and there are two different part numbers for that switch. One says w/gauge and one says w/light. Not sure what the difference is though. http://www.rockauto.com/en/catalog/d...+/+switch,4588

  6. #6
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    Re: Oil Pressure Gauge Issues

    Thanks for the replies.

    I had some time today to do more diagnostics, but still no joy. I'm really stumped.

    With the ignition on/car running, the wire shows no continuity to ground all the way up to the cluster plug and I see no voltage on it either. The wire also shows continuity all the way up to cluster plug, and with the car running, on the sensor pin I get 25-30 ohms cold and it slowly goes to around 50ish ohms warm.

    My next step is going to try to pull pins from a spare harness and run a wire from the sensor plug to the cluster plug and see what happens. Unbelievable!

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