4JHE Oil Pressure Gauge Bafflement

twalker H260 · Feb 26, 2013

Rick

I assume you have tried wiring the gauge directly to the sender in the engine compartment to see if it registers full pressure there? Could it be a wire either from sender to gauge or gauge to ground that has higher than specified resistance causing the gauge to read low?

RichH · Feb 26, 2013

The engine mounted gage sending units on Yanmars are grounded to 'work', the wires supplying voltage to the sending units are electrically 'hot' at full voltage from the engine 'panel'.

That your electrical sending unit is only reading '1/2 pressure', would indicate that you have high resistance somewhere in the primary wire supplying voltage to the sending unit (not receiving full voltage) ..... or a bad pressure switch.
... A. most probably a corroded wiring connector, etc. somewhere in the supply voltage circuit causing the high resistance in the hot/'supply' wiring.
... B. Could also be high resistance in the engine's 'ground strap' (to battery) connection and all the ground wiring back to Batt (-) ... (... and that would also affect your alternator charging circuit). If you have any 'charging problems' or other sending unit gage problems would make the engines 'ground' circuit highly suspicious for 'high resistance'.

Use a fused NEW wire directly from the Batt+ ..... and Start at the battery+ .... and 'in sequence' ..... check/compare **voltage across** each 'connector', keyswitch, all the way to the pressure sending unit ... on this 'alarm circuit'. Measure ohms from the pressure switch ground (engine block) all the way to the batt negative (-) on the engine *ground* circuit. Use a digital VOM.

anchorclanker · Feb 27, 2013

Yeah its a wiring problem. One mechanical gauge could be "off", but not two or three. Electrical gauges may not always be accurate either, but not all of them.

I once helped restore a 1961 Jag XK-E. In around 63' the original owner drove it into a barn and parked it with like 14K miles after the oil pressure gauge stopped reading, he figured it was the engine. Car rotted away for 24 years until we drug it into the shop. Yup, bad gauge. She ran like a top. Car was really rotted though, pretty sad waste of a car over a bad gauge.

Rick486 · Feb 27, 2013

Rick D said:
OK, so this is an issue that has been around for 13 years on my 1994 Hunter Legend 40.5. After determining I would quit fooling around with it in 2006, I took another unsatisfactory run at it this weekend. Here's the deal, and I'll reference two prior threads dealing in more detail the issue. The Yanmar panel gauge indicates about 50% of what a mechanical gauge indicates. 14 years ago, my Surveyor noted the oil pressure was not working (the OP gauge, not warning light & buzzer). I had a separate engine survey, and the mechanic reported he had put a manual gauge on it and the pressure was fine. Over time, I replaced the sending unit twice and the gauge twice and, while it did show pressure, it was always low. Later, I purchased a mechanical gauge. It showed perfect pressure (40+ lbs. hot at 1K RPM). Later, I decided to try a different sending unit that started a whole other adventure documented in the threads.

So, this last weekend, I picked up a VDO gauge, calibrated for US sending units. Result: the same. Low pressure indication. That includes testing with an external sense wire outside of the loom. The mechanical gauge indicated almost 50 pounds on a cold engine with straight 40 weight (1200 RPM). The VDO & Yanmar show mid-20's each with their own sender. There is 12.9 volts at the hot lead. The ground is common to the other instruments and they work fine.

I may just park this for another five years except it totally makes me nuts. I can't think of any logical reason this would happen. I'm baffled. Any ideas?

http://forums.hunter.sailboatowners.com/showthread.php?p=423292&highlight=oil pressure

http://forums.hunter.sailboatowners.com/showthread.php?p=817441&highlight=oil pressure

Some years back I went through a similar process with a Perkins 4-108. Everything was fine for years. I had replaced the original gauge with a Stewart Warner gauge, and here's the key, an OEM sender matched to the gauge. After a period of years the gauge began reading erroneously so as you did I replaced the sender. Twice. I never could get it to read correctly again and eventually I just gave up and accepted that the new normal was 20 psi instead of 40 something. The SW people told me that if I wanted to get correct readings I should buy a new gauge, complete with factory sender. Trying to match after market senders with an old gauge was unlikely to work. If it bugs you that much I suggest going out and spending the 100 bucks or so on a new good quality gauge with sender.

Chris Patterson · Feb 27, 2013

If this problem were mine, I would replace it altogether with a manual gauge. You can get those kits that has the complete hose that goes all the way to the engine. Secure the sending hose good, and it should be reliable. People have been using them on dragsters for 50 years. They tend to be very accurate, instant readings, and not contingent on any electrical anomalies.

timvg · Feb 28, 2013

Rick, does your gauge have the numbers "0, 4, 8", with a green area that start a little to the left of 4? I want to see if we have the same gauge for comparison.

And yes, I've replaced the sending unit a few times. Once it stopped working completely (That gets your attention.) The other times, the pressure just seemed lower than normal and with a new unit, the pressure on the gauge went back to what I consider normal.

anchorclanker · Mar 1, 2013

Chris Patterson said:
If this problem were mine, I would replace it altogether with a manual gauge.

Plus 1+

This is the wisest thing anyone can do to provide reliable and accurate instrumentation. Its the way its done on almost all piston engine aircraft as its the ONLY way to provide an instrument reading after a loss of electricity, as coudl happen in an emergency. You can also run a mechanical temperature gauge which also needs no electricity, and which is also much more accurate and reliable than an electric.

On the oil gauge, make sure to run aeroquip (or equivalent) stainless steel braided line with aeroquip type fittings. The commonly sold copper or plastic line will not hold up to the vibration and you'll end up with a broken line, a big mess, and it could ruin your engine.

Rick486 · Mar 2, 2013

tvangrove said:
Rick, does your gauge have the numbers "0, 4, 8", with a green area that start a little to the left of 4? I want to see if we have the same gauge for comparison.

And yes, I've replaced the sending unit a few times. Once it stopped working completely (That gets your attention.) The other times, the pressure just seemed lower than normal and with a new unit, the pressure on the gauge went back to what I consider normal.

Tim: I no longer own that boat but I don't recall any colors on the gauge front. Just white on black background. Looked like this and about 2 inches in diameter.

timvg · Mar 6, 2013

Re: tvangrove: that's the one.

OK, here's what I see on mine:

When I first get to crusing speed (2900 rpm's) the pressure is at about the 4 or slightly lower. After a while, as the engine has been running a while, the pressure usually shows up at the llower (left) side of the green (about 3, I guess), or just a little bit below the green.

It's always been this way and we've had the boat since 2000. When the sensor gets "tired", it will register somewhat lower. I then replace it and it goes back to my normal readings.

Charles Erwin · Mar 6, 2013

This is probably stupid but are you all sure that the units on the gauge face are PSI not BAR.

50 PSI would be about 3.5 BAR.

Charles

Bill Roosa · Mar 6, 2013

The sending unit is just a changing resistance. Less resistance is higher pressure. if you pull the wire off the sender it should read 0 bar.
Sooooo if you have corrosion in one of the 5 connectors between the sender and gauge......... and the current in the wires heats them causing even more resistance.... you get a low reading gauge that reads even lower after it runs for a while.
While fixing the starter switch problem I removed all the connectors and it fixed my low oil pressure reading.

4JHE Oil Pressure Gauge Bafflement

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