I sometimes see questions here about what should temperature and oil pressure gauges show, and what causes the oil light to flicker. Maybe this longwinded narrative will answer some of these questions.
I have owned my YM240 several years now with no issues at all, after an initial month 'renovating' it. (Mostly finding and putting Yanmar parts back on it where the previous owner had butchered something, and an extensive servicing). Then after a recent oil change, and after the engine was warm, the oil light would flicker at 1300
rpm and brighten to solid red at lower speeds. I first suspected the oil or filter. But this didn't seem likely while the oil had only 5 hours use.
I shut it down and got my infrared remote temperature sensor. Temperatures were normal. I found 140 degrees on the bottom of the oil pan, 165 at the lower block, and 195 near the injectors, at the upper radiator hose fitting, and at the radiator upper tank. The radiator lower tank was below 100. Ok, this is what I normally measure so it wasn't overheated.
So I installed a HF oil pressure test gauge (like
this except fewer adapters, under $10 on sale). I found its hose fit in place of Yanmar's oil pressure sender without any adapter.
For the technically curious: Yanmar used 1/8" BSTP, British Standard Tapered Pipe, while this gauge is labelled 1/8" NPT [US] National Pipe Thread. The diameter and threads per inch match up but I've read that the depth of the threads is quite different between the two standards. At any rate it threaded easily by hand for a few turns then sealed properly with wrench tightening. In my opinion these standards seem interchangeable.
Since the pressure looked good I went back to work and got it warm again. Yanmar specifies 35-50 psi 'at working
RPM'. I assume that means 1800 to 2400. I found 40-42 at
PTO rpm after warmup. And 35 psi at 1500
rpm, down to 22 at 1100
rpm, where the idiot light had been red.
Conclusion: No problem with the engine's oil pressure, but the oil pressure sender was failing. From what I've read, and after consulting a Japanese-specialist auto mechanic, that sensor should keep the light off down to under 5 psi.
I went to Autozone and looked at oil pressure sensors used on Datsun and Toyota in the 70's, since I have noticed that Yanmar often used the same third party 'soft parts' as 70's Japanese autos. Those senders had the right thread but none had the screw terminal I needed, they were all blade or pin cable connectors. So I opened boxes until I found a match. I see it is listed for 80's Corolla and 70-90's Subaru but it really is a 'universal' connector with a blade added on top of a screw terminal. I could have used any of the blade-connector sensors and hacked the Yanmar's wiring, but this one has the screw terminal like the one I removed. It was only $5.99. Problem solved.
And a tip of the hat to rScotty, if you're reading this, whose wisdom found elsewhere was a guiding light through this maze.
Low oil pressure? No, bad sender! 
It is cast and I believe it is iron so it would be very difficult to repair it. I am kind of bummed about it. I think that tomorrow I will start a thread about it.
Linear Mode
