No offense, but does this guy do major maintaninace, too? Any mechanic who would diagnose main bearings from an idot light and non-descript noise needs to go back to school. It is probably the wiring harness. After checking the wire to the sending unit, making sure it isn't grounding to anything, doesn't have any chaffed spots, and is properly connected, then preasure test the lubrication system. Jst remove the sending unit and connect a mechaniical oil peasur guage. The preasure should go up when you accelerate the engine. If it doesn't, the oil pump or sump may be at fault. If you have at least 15 lbs of oil preasure at hot idle, the light should stay off. Same for running or under load, but 15 lbs is not enough to lubricate any engine over about 12oo rpm.

John --

It could be a problem with worn bearings, or possibly sludge buildup in the sump (which can block the pick-up screen).

Or might be a failing oil pressure sender (which is basically a pressure sensitive switch).

When your engine is under load, it tips slightly. The late '70s to mid/late '80s 240s were famous for crumbling engine harnesses. Furthermore, the wire to the oil sender is right behind the alternator and routinely gets oil soaked during oil changes, and gets baked from the exhaust pipe heat. This causes even worse degradation of the insulation.

If the wire is intermittently touching ground as the engine tips, it can cause the light to flicker.

Your mechanic should remove this switch and install a genuine (mechanical) oil pressure gauge on a hose. You and he could watch the true oil pressure under varying engine conditions, and this would narrow his focus while solving the problem.

I'd first eyeball the wires behind the alternator—it's free, easy, and quick.

Don Foster (near Cape Cod, MA)