The oil light in my '82 240 came on yesterday while I was driving. I stopped and checked the oil, which was full and clean. It has a Mann oil filter which is less than 3,000 miles old. Car has over 200,000 miles on it. When key is switched off and engine restarted, oil light goes off for 5 seconds or so, then back on. The light stays on strady and does not flicker or go off with increased RPMs. There is no rattling from the bottom end of the engine (which there certainly would be if an engine with that many miles had zero oil pressure), and I drove the car over 100 miles with the light on. No problems, no knocking. So I assume the oil pressure is OK. But what would make the light go off upon startup, then on after 5-15 seconds of running? Could the filter have an internal defect? Could the sending unit be bad? I've never heard of a sending unit failing and causing the light to stay on; usually when they fail, the light doesn't go on at all. What do y'all think?

TIA
Colin

I would immediately confirm the integrity of the oil pressure sending unit. The sending unit is activated by the oil pressure opening a pressure switch and interupting a complete circut of the oil pressure warning light. With no oil pressure (as when engine is not running), the circut pressure switch is closed and electrical power completes the circut to illumunate the oil pressure warning lite.

You can simulate this procedure by (with engine off, but key on and illuminating the warning light)disconnecting the electrical leads to the sending unit and interupting the circut to see if the light extinguishes or stays illuminated.

I agree with Fuel Pump and Arno P, but when I had a similar problem on an 84 240, it turned out to be the the motor mount on the right side was failing. (I had a new filter on and the oil was topped up too.) this caused the engine to tilt slightly to the right and the oil filter slowly began to contact the "mount arm". Not realizing the problem, I drove it after checking everything that could relate to the light for maybe 500 miles. Eventually oil began to seep from the seal at the filter and engine block. The sender wiring was being pinched too. I don't know if the light came on because of a drop of pressure or the wiring. Anyway, installing new engine mounts totally corrected the problem. The car had about 150000 on it at the time. It's near 200k now, and no indication of any oil problem. You might check your mounts if they haven't been changed. Also I found out that allowing oil to drip down on the engine mount when changing oil filters, doesn't help the life of the mount. The right seems to fail earlier the the left. I'd be interested in hearing how it goes. jim g

The oil pressure switch is next to the oil filter. Try unplugging the wire from the switch and see if the light goes off. If it does then you may have a bad sender. If it stays on you may have most likely a wiring issue.

Check the wiring around the back of the alternator. Also check to make sure that the wiring is not trapped under the oil filter. I have see this were a wire gets under the filter and eventually wears throught the insulation on the wire causing a short.

Good Luck

Brian
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86 240GL, 86 740GLE & others

Hi,

On my P130 it was the oil sensor. Maybe you can check if it also leaks a bit of oil. Otherwise it could be an electrical problem of the sensor.