Low Oil Pressure Light B500 engine

[gregbasso]

I feel somewhat at a loss for not owning a cool B5100 tractor this engine sits in. Alas, this particular B500 has been doing duty powering a sailboat for the last 30 years. That's a long time but I doubt it amounts to more than 700 hours of operation. This marine version of the B500 was sold by Universal as a 5411 and is raw (salt) water cooled. The last year or so the oil pressure light has been coming on at low rpm (<1000). Last season, the engine leaked a few quarts of that raw water into the crankcase. After replacing head gasket, milling head, glass beading the blocked water passages in the head, and fixing the source of the water leak, the oil pressure light was a little worse. It is never hard on, it just glows dimly, but gets brighter as the engine warms and/or rpm drop from about 1100 to 500. I changed the oil pressure switch but the problem persists. I checked the pressure by mounting a T with the oil switch and mechanical oil pressure gauge. At around 500 rpm (really slow) the pressure is a steady 10psi. When the engine is cold, it will hit 40 psi at about 1200 rpm until gradually dropping to about 32 psi when the engine warms to about 140 deg F. Backing off to around 1100 rpm will start to yield a faint glow from the oil pressure light. The pressure reading at this point is 28 psi. From 1100rpm/28psi down to 500rpm/10 psi the light gets brighter. WSM says that idle speed oil pressure is 10 psi. At rated speed (not sure what that is) WSM oil pressure should be about 30 to 60 psi. My question: is this oil pressure switch set to go off at 28 psi? Is the oil pressure too low from 28 to 10 psi or 1100 to 500 rpm when oil light glows steadier on? Or is the new oil switch not operating properly? Seems to me if the engine oil pressure spec is 10 psi at idle, the switch should stay hard off above 10 psi. I've gotten all sorts of advice from diesel guys about this – one suggesting I remove the bulb and forget about it. Not sure I feel good about that one, even with a mechanical gauge in place. Using 15W40 oil.

 

[bruceatlam]

Sounds to me like you ought to replace the low oil pressure switch. It seems to be activating too early. The low oil light should come on at 7 psi ---- according to what I know.

 

[gregbasso]

After removing and screwing back the oil pressure gauge while engine was running (oil pressure was reading a steady 30psi), oil pressure gauge began to bounce rapidly between 0 and 50. As engine continued to run, this bouncing subsided. I'm wondering if a trapped air bubble could cause this reading. If so, it would explain the behavior of the oil pressure light. I replace switch with generic auto switch set to open at 7 psi and oil pressure light now responds correctly. I checked the new Kubota oil switch using compressed air. It also opens at around 7 psi. So perhaps there is a technique to installing the pressure switch? Or maybe the Kubota switch is defective and responds too quickly to pulses from the oil pump.