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Old 09-26-2015, 02:19 PM   #1
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Mechanical oil pressure gauges go bad ?

1993 vintage Murphy, barely reading 5 lbs in the engine room but the VDO electric gauges for that engine reading 25 to 30 lbs.

And yet the direct reading one ought to be the most accurate, so it's a bit unnerving to see such low pressure all of a sudden on this gauge (this at the dock, 600 rpm, Detroit 12v71TA with 280 hours since overhaul)

I guess my question involves wondering how common is it for a mechanical oil pressure gauge to fail ?
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Old 09-26-2015, 03:43 PM   #2
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I assume you have two engines? If so swop the gauges and see what happens.
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Old 09-26-2015, 03:57 PM   #3
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I assume you have two engines? If so swop the gauges and see what happens.
Good advice, the same as I was just going to give. Had to do that just a few weeks ago myself, one gauge reading 40-50 lbs as always, the other reading 10 all of a sudden. I created a little wire jumper with a couple alligator clips, the low gauge read 40-50 once I jumped it over to the other engine. Turned out to be a bad sending unit, surprisingly cheap (for boats), about $40 to replace, but then I have pretty common 454's.
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Old 09-26-2015, 04:01 PM   #4
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Make sure to swap out the whole assembly from engine to gauge.
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Old 09-26-2015, 04:48 PM   #5
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Make sure to swap out the whole assembly from engine to gauge.
As a first step why not just do the gauge? It might tell you right up front that the gauge is the issue.
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Old 09-26-2015, 08:45 PM   #6
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Good advice, the same as I was just going to give
Good grief....I was looking for an answer to the question, not Captain Obvious observations. Yes, of course I could swap gauges but before I go to that trouble just wondering how common direct oil line gauge failures are.
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Old 09-26-2015, 09:09 PM   #7
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Good grief....I was looking for an answer to the question, not Captain Obvious observations. Yes, of course I could swap gauges but before I go to that trouble just wondering how common direct oil line gauge failures are.
I had one fail this year
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Old 09-26-2015, 09:14 PM   #8
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I have capillary tubes to my lower helm gages. They seem reliable and accurate but they're starting to weep.

Before swapping anything, take a look at it component by component. I'd consider removing the sending unit and looking for an obstruction in the oil line. Clean as much as possible and reinstall to retest.

Next, run a new capillary line temporarily by whatever route is easiest to test the other existing components. Run it from the existing sender to the gage. Fill it with oil by starting the engine before connecting to the gage. Any difference?

Then start swapping gage connections to see where the problem lies.
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Old 09-26-2015, 11:29 PM   #9
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Mechanical gauges do fail. pressure pulsations in the line wear them out. Replaced many over the years. Try another gauge.
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Old 09-27-2015, 03:55 AM   #10
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Good grief....I was looking for an answer to the question, not Captain Obvious observations. Yes, of course I could swap gauges but before I go to that trouble just wondering how common direct oil line gauge failures are.
Gee, now I'm sorry I bothered. Feel free to fumble along on your own.
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Old 09-27-2015, 07:26 AM   #11
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I guess my question involves wondering how common is it for a mechanical oil pressure gauge to fail ?

Everything can fail , but with mechanicals check the tubing has not been crushed.

BIGFOOT , lives in some bilges.
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Old 09-27-2015, 10:32 AM   #12
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Mechanical gauges do fail. pressure pulsations in the line wear them out. Replaced many over the years. Try another gauge.
Thanks. Based on your experience I was inspired to switch port and starboard gages and the port engine that was reading 5 lbs is now reading 32 lbs at idle. So, yep....failed gage.

The interesting part is once removed, I see the working gage is obviously newer (body and internal bracket look much cleaner compared to the other gage, and has pristine bright white model no. sticker), so apparently this isn't the first failure of one of those gages.

The worrisome part is now that I know the model number I can access the data sheet on the website, and see that the two electrical wires on each gage are suppose to activate a solenoid or whatever to automatically shut down the engine when the pressure is low enough. This didn't happen...if 5 lbs isn't low enough, what is ??

Obviously the pressure wasn't really that low, but the gage thought it was, so it should have shut down the engine. So now I need to figure out why it didn't.

If curious, click on "diagrams" at link below to see what I mean. Mine is Murphy A20P-150.

20P / 25P Series | Murphy by Enovation Controls

================

(on edit)

Further contemplation of the diagram sheet seems to indicate that perhaps it is only the PABS models that will actually set off an alarm and shut down the engine automatically at preset low pressure.

Then again it looks like the standard P model could be configured for auto engine shutdown, just not pre alarm. My A20P gages do have two wires on each gage....if not for auto shutdown then what would those wires be for ? What do you guys think ?




(I probably should mention, before anyone suggests looking at wiring diagrams, there aren't any except for select systems like the helm toggle wiring, bilge pump system and halon fire system. Pleas to Viking for more wiring schematics have fallen on deaf ears in the past)
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Old 09-27-2015, 11:21 AM   #13
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"This didn't happen...if 5 lbs isn't low enough, what is ??"

First you need to set the low pressure point at which you wish the alarm to operate.

Then you need to have operating an electric alarm , horn , bell whatever.

To secure the engine automatically requires either an electric stop solenoid , or if a mechanical cable pull stop the Murphy pull spring system installed and ARMED.

25 + years of operation doesnt sound bad , just check to see if the auto secure System is on the boat , the gauge alone wont do it.

Tho an alarm if the pressure at cruise drops is easy and cheap to install.

"Pleas to Viking for more wiring schematics have fallen on deaf ears in the past) "

There might be some Viking folks on a Viking boat board.

Otherwise offer to PURCHASE a set of schematics , you may have better results.
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Old 09-27-2015, 11:56 AM   #14
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I have seen gauges get plugged with something. If you can crank the engine with the gauge out to get a little oil to flush the passage. Have fuel off so it wont start. You don't need more oil in Detroit bilges.
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Old 09-27-2015, 12:03 PM   #15
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Often that is for setting off an alarm, not auto shutdown. It's the same alarm that sounds on most diesels when you turn on the ignition before you turn the engine over. So if wired for auto shutdown , no starta da engine. It varies, though, some boats have a separate oil pressure switch from the gauge system.
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Old 09-27-2015, 12:12 PM   #16
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I have seen gauges get plugged with something. If you can crank the engine with the gauge out to get a little oil to flush the passage. Have fuel off so it wont start. You don't need more oil in Detroit bilges.

Might even want to follow that with a brake cleaner flush (inside the sender port). I've flushed the offending particulate matter out successfully a couple of times through the years.


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Old 09-27-2015, 12:18 PM   #17
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Otherwise offer to PURCHASE a set of schematics , you may have better results.
I have done exactly that, including setting no limit on how much I would pay. I even have a contact that works at Viking (allbeit in relatively low office position) that has had no luck other than finding 2000 year 65C schematics (which are completely different).

1993 schematics would have been all on paper rather then electronic, and have heard excuses ranging from "they needed the room and just threw them away" to "lost in hurricane Sandy" !

Keeping in mind these excuses I'm hearing third hand so the grain of salt that remains, keeps my hopes up that something will turn up eventually.
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Old 09-27-2015, 02:09 PM   #18
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Good grief....I was looking for an answer to the question, not Captain Obvious observations.
LOL Thanks, Captain Obvious.

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Old 09-27-2015, 03:09 PM   #19
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Mechanical gauges do fail. pressure pulsations in the line wear them out. Replaced many over the years. Try another gauge.
X2. take the bad one apart and you will see why. Gears and springs. It may also have a snubber (restricting orifice) that may be clogged.
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Old 09-27-2015, 07:52 PM   #20
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X2. take the bad one apart and you will see why. Gears and springs. It may also have a snubber (restricting orifice) that may be clogged.
http://www.fwmurphy.com/uploaded/doc...pdfs/94116.pdf

Just noticed the "pulsation dampener" which is removeable for cleaning...maybe I won't need a new one after all.
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