Lower than usual oil pressure reading

[loco]

Hi all,
I'm finding that the oil pressure reading on my old Penta gauges (1990's analogue gauges from an AQ205a) is reading lower than usual. It would previously be at the top of the green, and would then creep to be a little higher than max level at higher RPMs. The gauge now tends to sit vertically, still midway in the green, but clearly lower than it has been historically. The gauge does move as the revs fluctuate, so I'm sure there is still signal being sent.

That said, some of the wiring in the boat is coming up to 30 years old, so I would expect some voltage loss along the way.

It's got very fresh oil in the engine, and the engine only has ~150 hours on it, so is still quite young. However, I've had some issues this year when I overtightened the inlet manifold, resulting in water getting into the engine, but was all pumped out and has had two oil changes since replacing the gaskets.

So, I need to do some checks.

First up, the sender could be faulty. It's one of the old manual units like this one:
http://www.iboats.com/Sierra-18-5899...view_id.172098
What should the voltage from it be? does it just go from 0V for no pressure up to ~13V max pressure?

Secondly, I need to check voltage loss along the line, and maybe run a new cable to the gauge, just to test.

Alternatively, I think you can buy a manual pressure gauge to plug straight into where the sender is, so I can directly test the pressure. Does anyone know the gauge of the thread on the sender, so I can pick up an appropriate gauge?

Thanks in advance.

 

[Scott Danforth]

first, you screw in a mechanical fuel gauge and directly read the oil pressure. you will want a 100 or 150 psi gauge. I prefer the glycerine filled gauges because it dampens the needle movement.

then sort out your sender and/or gauge issues

your oil pressure gauge works at 240 ohms at zero pressure and 33 ohms at full gauge pressure. you must match your gauge and sender 60 psi, or 100 psi, etc. its not a voltage thing, its a resistance thing. you measure the sender and its resistance to ground

then for the gauge, you use a similar resistance from the signal wire to ground on the gauge side to see if the gauge is reading right. 240 ohms will give you 0 psi, 33 ohms will give you full gauge reading.

most of the time, its simply a dirty connection which adds resistance to your sender signal. if this is the case, your connections need to be cleaned. use 3m scotchbrite, clean the connections so they are clean and shiny, and seal with marine electrical varnish.

 

[loco]

Thanks!
From reading around, it looks like the thread in there is a 1/8 NPT - does that sound right? I'll obviously need to buy the appropriately threaded gauge.

From reading around, 30-70 psi would be a healthy pressure reading, so I'll get a 0-120 PSI gauge.