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Old 29-12-2023, 11:12   #1
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fluctuating oil pressure gauge

Hi All. For some time, the cockpit oil gauge for our Perkins diesel has fluctuated, bouncing from zero to 70 or so psi, sometimes holding steady for a while at 70, sometimes reading zero for hours at a time, usually fluctuating quickly (the needle sort of bounces around). Naturally I thought I might have a bad connection somewhere, but checked that; even running a new wire from the pressure sender to the gauge made no difference. I also tried replacing the oil pressure sending unit (twice, both times with new units), also to no avail. It seems to me that the problem must be in the gauge itself, but a mechanic told me that gauges almost never fail. (I'm a little skeptical of statements about anything hardly ever failing, but that is what he said.) So, it seems to me that my next step is to replace the gauge itself. Ah--I should add that I also have a mechanical pressure gauge in the engine room, tee'd off from the same fitting with the oil pressure sending unit, and the mechanical pressure gauge gives a steady reading as expected, so the problem is not with the oil pressure itself. Questions: 1. Does replacing the gauge make sense, or am I missing something obvious? 2. Are the gauges fairly standard, or do I need to make an effort to find an exact match for the guage that is in place? All advice welcome. Thanks for your thoughts. -bill
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Old 29-12-2023, 11:32   #2
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Re: fluctuating oil pressure gauge

I have the same issue with mine but typically it'll read 0 until I get above a certain RPM.

You will need to find a match as gauges use resistance (ohms) to tell the oil PSI. Pull the part number off the gauge and see. It's also important the sensor ohm range be matched to the gauge.
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Old 29-12-2023, 11:39   #3
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Re: fluctuating oil pressure gauge

Make sure the gauge is solidly grounded to the engine block ground.
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Old 29-12-2023, 11:50   #4
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Re: fluctuating oil pressure gauge

Many thanks for your replies. Increasing the RPMs does not have a consistent effect on the needle in the cockpit gauge. Regarding checking the ground to the block: It seems to ground through the fitting to the sending unit--there is no separate wire as far as I can see. Does that seem right? Is there any way to check a ground connection between a part that is mounted to the engine? Not sure how it could not have a good connection, but I am often surprised by my ignorance.

Many thanks to both of you! Very helpful.
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Old 29-12-2023, 14:05   #5
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Re: fluctuating oil pressure gauge

Right. The sender is grounded through it's threads to the engine block. Don't use any sealant or tape there, it can add resistance.
But the gauge head itself is another matter. Check the ground there and run a temporary test wire back to the block if necessary. The intermittent behavior does sound like a bad ground.
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Old 29-12-2023, 14:17   #6
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Re: fluctuating oil pressure gauge

I should have thought of that. I visually and by touch checked the ground wire from the gauge and the connection felt firm on both ends, but I will run a separate wire all the way from the gauge to the ground. I definitely agree--the symptoms scream out bad connection. Thanks!
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Old 29-12-2023, 14:40   #7
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Re: fluctuating oil pressure gauge

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Originally Posted by Bill Streever View Post
I should have thought of that. I visually and by touch checked the ground wire from the gauge and the connection felt firm on both ends, but I will run a separate wire all the way from the gauge to the ground. I definitely agree--the symptoms scream out bad connection. Thanks!
If you happen to have a ground wire from the sender as well as the gauge it might mean that your engine is insulated earth, the Perkins Prima used this system and it made trouble shooting a little more challenging in that the earths needed to be traced back to the -ve bus rather than the engine block which is NOT usually grounded.
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Old 29-12-2023, 14:44   #8
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Re: fluctuating oil pressure gauge

Of course the bad connection could be inside the gauge itself, but like your mechanic said, that's unlikely.
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Old 29-12-2023, 15:15   #9
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Re: fluctuating oil pressure gauge

It is probably a bad connection somewhere. But, sending units fail, gauges fail, oil pumps fail, and bearings wear out.

Tests to try.
You can use an ohm meter on the sending unit. Disconnect the gauge, and measure from the terminal on the sending unit to the engine block, or between both terminals if there are 2.

You can measure voltage at the gauge. If voltages are fluctuating but the sending unit tested ok, then there is a wire problem.

You can test the gauge with a small power supply and a 1k potentiometer.

And, you can test for engine problems with a mechanical oil test set. A regular tick can indicate a spun bearing. If it goes to zero and comes back, the pressure release valve in the oil pump might be sticking.

https://www.amazon.com/dp/B0BBGG72JN...c2cea0908&th=1

I would spend a fair amount of time checking every possible connection for a problem before i got too deep in other tests though.
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Old 30-12-2023, 18:14   #10
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Re: fluctuating oil pressure gauge

Although I retired from involvement in marine repairs many years ago, I recall that many electrical issues were cured by running a decent size ground wire (depending on the size boat and the load, usually a #2 or 4) from the battery to a bus bar at the helm. The ends were crimped then soldered. After they were attached, they were coated with waterproof grease.


Try a good ground between the battery and the gauge even if you have to make a temporary connection with a spool of wire. If that doesn't cure the issue, get a new gauge.
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