oil pressure gauge not working on cummins 4bt

Discussion in 'Diesel Engines' started by Northeaster, Jun 21, 2016.

  1. Northeaster
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    Northeaster Senior Member

    Hi Folks,

    I am sure it worked a couple of years ago when engine ran for testing - but when I now turn key the oil gauge immediately "pins out" at highest reading, before engine starts. Once key is turned off it goes back to zero.

    Is this a common issue, sensor, wiring, ground, etc? have not changes wiring but something may have got bumped.....
     
  2. slow fred
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    slow fred Junior Member

    Screw a manual gauge in the block where the electric sender is located. If it is still extremely high the relief valve on the oil pump is stuck. If the pressure is normal check the electric sending unit.
     
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  3. Northeaster
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    Northeaster Senior Member

    Hi Fred, I can do that if needed, but I don't think the pressure is really high - I think the electrical connection is falsely making the gauge pin out - as it does this BEFORE the engine starts - it does it with just the turn of the key to energize, but not start engine.
    wondering if this is a common or known electrical problem, like short, ground, etc...
     
  4. PAR
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    PAR Yacht Designer/Builder

    If the sender or wiring is grounding out, it will peg the oil pressure gauge needle, as it works on percentage of ground.

    A quick check is to pull the sending unit wire off at the sending unit on the block. Next check the gauge to see if it pegs out (ignition on, wire off sending unit). If it does, the fault (short) is in the wiring. If it doesn't when you pull it off the sending unit, ground it to the block. If it pegs when you ground the wire to the block, it's the sending unit that's at fault (the usual case BTW).
     
  5. Northeaster
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    Northeaster Senior Member

    great info - thanks!! will try next time at boat, on weekend.
     
  6. FAST FRED
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    FAST FRED Senior Member

    Before bothering with electric gauges , take a look at Murphy Switch Gauges.

    These are mechanical gauges that have adjustable contacts .

    They can sound an alarm if the setting is met , low or too high oil or temp.

    When out boating the gauge watches for a problem , you dont need to.
     
  7. Northeaster
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    Northeaster Senior Member

    Update. When I pull the 2 wires off the sending unit, the gauge is still pegged - even when not running, with just the key on. When I pull the (double) violet wire from the back of the gauge, the needle goes to zero. If I leave them on and pull any other wires off, it stays pegged. I was surprised at the number of wires on each gauge (many are daisy chained from other gauges and alarm lights)
    Not sure what to do next. Any further help would be appreciated!
     
  8. Northeaster
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    Northeaster Senior Member

    ok - had a look at the wiring diagram (not my specialty...) and it looks like the violet / purple is the positive that is daisy chained to all of the gauges. So, if I remove it the gauge returns to zero - guess that figures as it no longer has power. If i remove any other wire it stays pegged..
    can someone advise me what to check next?

    PS a few other (possibly related) issues. tach works fine in gear and when reving in neutral below 2000rpm or so. In Neutral above 2000rpm, it goes back down to zero and then comes up again as you throttle down.
    have not checked to see if it uses a alternator pickup or flywheel pickup for tach yet.
    Also, the one-wire alternator does not seem to be putting out - voltmeter and guage just read 12 volts. I connect electric pump to put load on but with a 8D battery is there a chance this alternator does not see a load yet?
    Tried exiting alternator by connecting temp wire from hot post to one of three regulator slots per local electric repair shop but may not have touched correct spot. Hard to get at with cooling hoses and oil filter back there. Watched youtube video on procedure but mine does not have the 2 spade clip posts I see in videos where they connect to.
     
  9. PAR
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    PAR Yacht Designer/Builder

    The daisy chain wires are grounds and hot feeds for the lighting. Some gauges will need power, but an electric oil pressure gauge doesn't unless it's digital. It sounds like a wiring short and a continuity test is the next thing to check. If the gauge is pegged when the sender wires are pulled, the gauge is stuck (replace the gauge) or it's a short to ground in the two sender wires. The simple check is to remove the sender wires from the back of the gauge and attach new ones. One wire is likely run through an idiot light (or low oil shutdown module) first, while the other is a straight run from the sender. Simply ground the sender wire, to a known good ground under the dash and the gauge should peg. If it does, the gauge is probably fine and you have a wiring short. If the gauge doesn't peg, pull it out and replace it, as it's junk. This test takes longer to describe than to perform.
     
  10. Northeaster
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    Northeaster Senior Member

    PAR. Thanks very much for the detailed instructions.
    Not at the boat today but should be there to have a look tomorrow.

    I did just read today that some alternators get their exitement.. from the oil pressure switch / circuit / dummy light.
    As my one wire alternator is NOT charging, and I have not been successful yet in exiting it (after it not running for about 2 years) by jumping the Hot/battery lead to the #1 spade terminal, it seems likely that one fault could be causing both issues.
    Also, my tach off flywheel sensor works up to 2200rpms and then goes to zero when I keep increasing throttle - not sure if that could be related...

    One question if you don't mind - my oil pressure sender had two wires. Would they likely both be sending wires, one to light and one to gauge? Or would only one be a sending wire and the other a ground or other function?
    Mine looks like this one.
     

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  11. PAR
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    PAR Yacht Designer/Builder

    It depends on how the sender is wired, but usually it's a two pole setup and both wires are signal. One wire is the primary and gives the pressure across the full range. The second pole is the low level warning light, only sending signal when pressure drops below a set point (5 PSI is common).

    There's no voltage sent through these wires, just a "percentage of ground", which sounds weird, but is how they work. Full ground (100%) will give you a full gauge reading (peg the needle), which is why you test it by shorting the wires to known good ground (the engine block). If the gauge pegs full on (max pressure) when the wire is grounded, the sender may be bad, as the gauge and wiring appear good. If the gauge shows less than max pressure with the wire grounded, you have a gauge (or other) problem.

    With a two pole setup, it may be wired (internally) so the low pressure alert is only provided, if both the regular signal and low pressure signal wire are in agreement, which prevents a false reading from the low pressure side of the sending unit.

    I'd be surprised if the "exciter" circuit is actually inside the sender. Typically, this is in the gauge cluster circuitry and you'd probably have a 3 wire sender, instead of a two. The third wire would be a reference wire to the gauge cluster (or module) with a dropping resistor that matches the PCM or gauge cluster (5 VDC is common) requirements.

    Since you have other issues, like the tach cutting out. my first guess is you have a wiring issue and something is chaffed, sorting out, etc., within the harness.
     
  12. Northeaster
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    Northeaster Senior Member

    Hi PAR (and others..) - hope you will bear with me a bit longer.

    As stated earlier, when the 12V lead to gauge is connected, the gauge pegs and therefore I assumed I had an issue, and it should not do this. So, I tried grounding sender wire with 12V wire off but had no activity on gauge.
    Today, I left the 12V wire on (and gauge pegged with key on) and tried grounding out the sending wire...low and behold the gauge dropped from pegged to about 60 psi when fully grounded. This was using a small voltage light as a conductor to ground. When I used a real wire to ground it went right to zero.
    So, as it is not pegged / reading full when grounded, does this still lead to a bad gauge?

    and, I tried checking continuity to ground on the 2 sending unit points, with an ohmmeter, and engine running, but I only showed open no continuity to ground whether running or not... does this indicate a bad sender or is there a better test?

    PS I did see on the wiring diagram that one sensor lead (to black wire) is a ground ( goes to most other sensors as well) and only the lt blue wire is a sending wire - I traced it out and it goes to the gauge first, and then to a "oil pressure in" plug on the small circuit board, and then another short lt blue wire goes from the "oil pressure out" plug of the board to the dummy light.

    I don't know which of the two sending unit outputs is supposed to be the black/ground and which is supposed to be the lt blue/ sending wire, but when testing continuity, I tried both ways and had no effect.
     
  13. Northeaster
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    Northeaster Senior Member

    update -after frigging with it - working now.... not sure what was wrong...
     
  14. PAR
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    PAR Yacht Designer/Builder

    Sounds like the sender wire and ground were switched. There's no voltage to the gauge other than lighting. There's also no voltage to or from the sender, just signal, sometimes one, or in your case two (one for a low pressure light, the other the gauge signal).

    Glad yours is working again.
     

  15. Northeaster
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    Northeaster Senior Member

    Well, they were switched a couple of times as I tried different things.
    My gauge does in deed have 12V power and ground leads connected (which are also daisy chained to other gauges) in addition to the sender wire and the light bulb and it's daisy chained separate power wire.

    Thanks for all of the help. Still have to see why alternator isn't charging. I do know more about the system and wiring than before thanks to help here.
     
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