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1989 300SE 240k miles
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Discussion Starter · #1 ·
Hello All,

This regards my 1989 300SE with 242k miles. The last 3 weeks, my low oil warning light has come on 3 times. In all 3 cases, it turned on within about 100 feet of same location. It is happening about 1/4 mile from our warehouse, on my drive home. All at about 45 mph.

The first time, I immediately pulled off into a parking lot and added 1/2 quart of oil. I did not check the level. It was late and I figured 1/2 quart would be fine no matter what. The next morning, the oil level was normal, but I didn’t pay attention to its exact level.

The second time was about a week later. I was just past that same parking lot, which meant I was on the road until the other side of the interstate. While driving to the other side, I was stopped at a red light. My oil light went out. So I figured I’d wait to see if it happened again.

The third time was tonight, another week later. This was the same scenario as the second. Same location. Same light. Oil light went out there. Except when I got home I checked the oil level. It is midway between high and low.

I did replace the o-ring for the oil level sensor with my last oil change. It was leaking previously. And my wiring harness didn’t feel as snug as I’d like. So I kind of was thinking it may be a loose connection there. Which is why I was willing to drive over the interstate.

The oil pressure gauge works as it always has. 3/3 with gas, 1/3 at idle.

The “tick” I’ve referenced in another thread is not present.

Anyone experience such a thing? Ideas of where I should look?

Thanks,
Bill
 

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W140 S320 ´95, blue black / black leather; MB 300SE '90, anthrazite grey, black leather
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Nothing to help your situation directly I'm afraid; I had the light coming on intermittently when the oil level was about at half or even a little more than a half. On and off it went a long(ish) while before the level got so low that it was on all the time (tested this one time, usually I just filled to the max right away to keep the annoying light off :) When turning right, the light went off for a while as one could expect). Never got around to change the level sensor though so I lived with that "feature"...
 
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Discussion Starter · #3 ·
Yeah. I appreciate that. And I read elsewhere where you had that happen. Or at least someone had that happen. 😉

I’m also concerned because prior to replacing my Bowden cable, my bell housing was damp with oil. Which I couldn’t track. Which has me concerned about an oil leak at the rear main seal. Which I understand rear main seals can just rupture, and only leak when moving. And I recently went under the car to reinstall the O2 sensor shield and the bell housing was bone dry.

Also when replacing the Bowden cable, I found drops of oil at my alternator, where it meets the engine. So I’m pretty sure at a minimum, my upper timing chain gasket didn’t set. Maybe the lower timing chain gasket is leaking at its top.

So right now I know I’m leaking oil. But not enough to hit the ground and make a mark.

Knowing the light can come on and go away helps.
 

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Discussion Starter · #6 ·
I’d say there’s less of a tilt there than in most Florida roads. There is, however, almost a bunch of speed bump waves in the lane. That cause the car to kind of slowly go up and down in rhythm. It’s much more pronounced in my miata due to its short wheel base. That could be it. Thanks!
 

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'17 GLS450, '14 GLK250 "Grandpa's Roadster" Project Car, 350SDL (Sold)
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As far as I know, mine never gave a false indication, so, If your oil level is not below the "Add" mark, I'd suspect either the sensor is bad, or you have an exposed sensor wire somewhere that's shorting to ground. I believe it has some circuitry magic to prevent the light from coming on due to a transient condition. (I suppose that could be faulty, too).
 

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Discussion Starter · #8 ·
The light came on this morning. I was driving in a totally different location, like 10 miles away front eh others. This time it was further from where the car was parked (1/4 mile vs 1/8 for the previous). This time I was already stopped at a red light, idling in D, when it came on. This time it came on and turned off in under 1 second.

I read where the sensor does not work until the engine reaches a certain temperature. I wonder if it’s the sensor glitching when it first activates. Because this morning it was a biting, full on winter’s 55° F when it happened. So it would take longer for the engine to warm, vs mid-70°’s F the other times.
 

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I read where the sensor does not work until the engine reaches a certain temperature.
I doubt there's any truth to that.
this morning it was a biting, full on winter’s 55° F
Queue the violins! It was 19° here.

I doubt this is your problem, but I had a friend who blew a VW Diesel engine back in the '80s because the oil could not drain back to the sump fast enough. He checked the oil and checked the pressure and all was well, but driving on the highway it simply ran out of oil.
 

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Discussion Starter · #10 ·
Hmmmm… appreciate the terrifying story.

It happened a second time today. Cold engine (71° F ambient). When I got 1/4 mile, engine about 60° C, it came on and turned off within one second, while driving 50 mph.

…Maybe I’ll do an oil change.
 

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it came on and turned off within one second, while driving 50 mph.
Was there anything else out of the ordinary going on at that instant, that may have somehow disrupted the power or ground to that indicator?

Can you unplug the sensor and see if you continue to get false indications?
 

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Discussion Starter · #12 ·
There has not been anything out of the ordinary at any of the times. Believe me, I’m looking for something! ..It came on for a second this morning on my drive to the warehouse.

I can unplug the sensor. It’s fairly easily accessible on the m103. Just remove the skid plate and it’s right there on the side of the oil pan. And just one wire to unplug.
 

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I just checked the schematic for the oil warning system. When the circuit is open the light comes on (The switch is closed when the oil level is normal.) So, first thing I'd check is to be sure the wire makes a good, tight connection to the sensor. Unplugging the wire should put the light on all the time, and won't tell us anything.
 

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Discussion Starter · #14 ·
Thank you. Yeah, my suspicion is the sensor is coming unplugged. I did not like its fit when I replaced the o-ring and re-installed it. ..I also couldn’t think of a way to better secure it. But, I didn’t apply much time to it
 
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