4JH4 erratic oil pressure

[hpeer]

4JH4-TE, about 2,600 hours

Factory gauge set normally showed oil pressure around 40-45 PSI
About 2,400 hours I changed the factory panel to a new panel (old panel was broken) with VDO gauges. New panel came with new senders.

New panel showed oil pressure at about 65PSI. OK off from before but still shows I have pressure, gets the job done.

Yesterday I had to make a 10 hour passage. About 2 hours into the passage the oil pressure became erratic, it would jump up to 80, peg for a while, fall back down, etc. Temperature remained rock solid. Where I was there was no hope of service assistance, so I kept on going. The problem remained for a couple of hours but then the reading seemed to settle back to normal, 65PSI, for the last 4 hours or so.

I am thinking this sounds much like a poor wiring connection. I can’t find anything that tells me the resistance range and how that correlates with the reading. Say 0 to1,000 ohms and 650 ohms is 65 PSI, or something like that.

Currently in Antigua, Jolly Harbor.

Thoughts appreciated.

[Wotname]

Probable intermittent faulty sender.
Possible poor wiring connection.
Unlikely to be gauge.
Very unlikely to be a engine problem.

[hpeer]

OK, I finally found this which I need to cogitate upon for a bit.

US and EU gages have different resistance ranges, I have VDO.

US (e.g. Farina)
0 PSI is 240 ohms
40 is 103
80 is. 33.5
Non linear, decreasing
A high resistance connection, or open would show LOW pressure. Opposite of my symptoms.

EU (e.g. VDO)
0 is. 10
40 is 95
80 is. 180
Linear increasing

A high resistance connection, or open would show HIGH pressure. These are my symptoms. This is encouraging.

https://www.tadiesels.com/assets/doc...ance_Value.pdf

[pcmm]

Originally Posted by hpeer

OK, I finally found this which I need to cogitate upon for a bit.

US and EU gages have different resistance ranges, I have VDO.

US (e.g. Farina)
0 PSI is 240 ohms
40 is 103
80 is. 33.5
Non linear, decreasing
A high resistance connection, or open would show LOW pressure. Opposite of my symptoms.

EU (e.g. VDO)
0 is. 10
40 is 95
80 is. 180
Linear increasing

A high resistance connection, or open would show HIGH pressure. These are my symptoms. This is encouraging.

https://www.tadiesels.com/assets/doc...ance_Value.pdf

Add to that the threads on a Yanmar are NOT NPT, they are bspt which are almost the same but technically incompatible. getting the corect sender both electrically and mechanically ( fitting) is a challenge. The only place I have found so far has been Catalina Direct who make a standard NA water and oil pressure sender with the required BSPT thread!

[sailorboy1]

I found it less trouble last time I replaced to just a matched sender+ gauge

[Wotname]

Originally Posted by hpeer

OK, I finally found this which I need to cogitate upon for a bit.

US and EU gages have different resistance ranges, I have VDO.

US (e.g. Farina)
0 PSI is 240 ohms
40 is 103
80 is. 33.5
Non linear, decreasing
A high resistance connection, or open would show LOW pressure. Opposite of my symptoms.

EU (e.g. VDO)
0 is. 10
40 is 95
80 is. 180
Linear increasing

A high resistance connection, or open would show HIGH pressure. These are my symptoms. This is encouraging.

https://www.tadiesels.com/assets/doc...ance_Value.pdf

Your analysis is correct in so far as the problem will very likely to be due to a high (or infinite) resistance. However it does not tell you where the high resistance is occurring. It could be inside the sender or in the wire (or connectors) connectors between the sender and gauge.

Most senders are mechanical in design and vibration does cause them to fail, often becoming intermittently high resistance or even open circuit.

The normal quick test regime for the wiring of an analogue oil pressure gauge is -

1. Key ON & Engine OFF, gauge should read 0.
2. Remove wire from sender, needle should go full scale deflection (FSD) for EU systems or zero scale deflection (i.e. drop just below 0) for US systems.
3. Short wire to engine block and the opposite should occur.
4. Start engine and normal (expected) oil pressure should be indicated.

To determine if it is the sender or the wiring, carry out the above test next time the fault presents itself (while the engine is running). Of course this needs a careful approach as removing wiring from the sender from a running engine has obvious dangers.

[hpeer]

Tonight I will have the Wife stand in the cockpit with key ON, engine NOT running while I dork with the wires to see if I can get the needle to wiggle.

The constant low oil pressure alarm will be special and assure I work quickly before I loose my helper.

[Wotname]

Are you getting a low oil pressure alarm as well as the gauge problem?

The gauge and the alarm are usually two totally independent circuits.

[Wotname]

If you don't have audile mute switch, disconnect the low oil pressure alarm while testing the low oil pressure gauge (just to silence the alarm).

[hpeer]

Well that was simple, maybe.

Wotname, I get the normal ALARM when the switch is ON and the engine is OFF.

First thing was to look at the sender. Two terminals, the nuts are brass, tall, with knurled black plastic around them. Difficult to get to, no flats for a wrench. Seem to be some metric size, a wee bit bigger than SAE #6.

One of the two was touching but came loose with a mild push. I could not duplicate the problem but I am going with this being it. Sure looked like it might be the culprit. I out a lock washer on each wire and tightened them as much as I could with my finger tips.

Started engine and watched oil pressure come up to 60PSI at cold idle.

Still don’t know why the old and new units disagree so much on absolute pressure reading, 45 (old)vs 65 (new) but will live with the ignorance.

I did find this bit of info for a 5 bar sender.
45 is roughly 3 bar. (111 to 121 ohms)
65 is roughly 4.5 bar (140 to 186 ohms)
Given the tolerance +/-, one sender reading low, and another reading high, almost overlap. Given my flaky memory, or a wonky meter and yeah, maybe it all works out OK.

https://www.dynamicelements.co.za/ve...e-sensors.html

[Wotname]

You may have combined sender with one wire for the alarm and one wire for the gauge OR you may have a 2 terminal gauge sender where one wire goes to the gauge and the other wire is the negative return i.e. should go to the engine block or other negative bus.

If removing both wires silence the alarm (key ON, engine OFF), then you have a combined sender. If the alarm continues with both wires removed, then you have a 2 terminal gauge sender and one of these wires must be grounded for normal operation.

[Scubaseas]

From over 50 years of auto wrenching as a master mechanic I can say 99% of the time with your symptoms, the issue is the sender or the wiring to the sender. Best way to confirm your oil pressure is to put a mechanical gauge on the engine to verify what the sender says. Wotname has it spot on.