Oil Dummy Light Trying to tell me something...
 

87' 300SDL

While driving on the interstate a few times my oil dummy light has come on. I wasn't driving excessively fast or accelerating. Surely less then 65mph. My pressure gauge showed full pressure and my oil shows half way between the marks on the dipstick.

I'm trying to figure out if this is some strange problem or a faulty indicator.

Thoughts?
Thanks,
Bill

The oil level sender can go bad. It's on the driver side of the sump aft of and above the drain plug. Start by ensuring the electrical connector is clean and properly seated.

Sixto
87 300D
97 C2500 'burb TD

I have always thought that a guage was more reliable than a light and this goes double for mechanical ones. I would suspect a faulty sensor, but it could be an intermittent short to ground. When oil pressure drops, a switch is closed and a full circuit is made lighting up the bulb. You should be able to take the sender wire and short it to ground and get the bulb to light, theoretically. Most system idiot lights are 12 volt, but some may not be and if yours is one of those, you could damage something shorting it to ground without using a 25 ohm resistor in the test circuit.

I think the light on the SDL is a simple level light not a pressure light.
I agress with the post above that says that the level sender could be going bad. See if you can remove it and make sure that if moves freely.

Sometimes throwing the car into a curve when it's not right up to the 'max' line can cause this symptom, if one of the other circuits that's supposed to provide a time delay has quit providing that delay.

Can you connect your problem to maneuvers that might be sloshing oil around? You say you were not driving quickly, but don't state if you were turning or going perfectly level.

I can attest to the finicky nature of the oil light. When my SDL gets about 1/4 quart from the full mark the light will start going off and on. I pick up my GF after work and most days I park on an incline down toward the passenger side and it will always turn on when I'm a little low on oil.
I would say make sure the oil is topped off and see if the problem persists.

On the 87 300D I used to own, I couldn't even trust the oil pressure gauge. It would suddenly drop to 0 without any warning. The first time it happened I almost had a heart attack. I was driving on the freeway going about 70. I was very lucky that it was only the gauge as I bet it would have probably seized faster that I could have pulled it over to the shoulder.
A good smack to the top of the dash would get it working again. I never did find the loose connection.

My OP gauge is implicitly trusted. My tachometer is the one that needs the dash beaten :D

Update!
 

OK... While working on a fuel pressure problem which I won't get into here except to say that I took off the lift pump. Sure some engine oil dripped out from this, but I put it back on and all was happy.

Or so I thought.

I've had the oil dummy light pop on for seemingly no reason... when driving evenly on a level straight away at highway speeds, then go out; when sitting at a light, then a short time later go out...

When I put the lift pump back on I took her out for a test drive... to my utter dismay I was losing oil pressure. It started at "2" then dropped after time I let up on the accelerator. It would never gain back it's previous place even after accelerating again.... slowly the high point would drop to 1 & 3/4, 1 & 1/2, 1 and 1/4... this wasn't even with high way acceleration... this was just around the block.

I added some oil and tried again. It started out at 2.5 but before I went around the block the high point was 1&1/3. HELP!!!

I've not diagnosed this before. Oil level is not topped off, but is more than halfway in the safe zone.

I'm afraid to drive it and not sure how to test on this one... hell I don't even know where the oil pump is!

Any advice and religious inspiration appreciated.

Bill

The oil pump is under the #1 cylinder driven by a chain off the crankshaft.

Since you show no fear in continuing to drive the car with the oil pressure gauge less than pegged with revs :) replace the oil pressure sensor and see if that's the problem.

Sixto
87 300D

Well I wouldn't say I have no fear... perhaps just lots of stupidity... but I didn't drive far...kinda like being only a little bit pregnant.

Thanks I've ordered a pressure switch.

I assume after that the next logical thing would be the oil pump?

Bill

If it is low oil pressure, it might be loose crank or rod bearings. There's usually knocking sounds that accompany loose bearings.

Sixto
87 300D

Hi everyone,

1) It's most likely to be a faulty oil pressure sensor. Replace that first (about $60, IIRC.) You access it from underneath the car. Simple to replace. Don't forget to re-attach the wire before turning the car back on, with the wire off, the gauge pegs at 3 before you even start the engine. ;)

2) The oil level light is supposed to have a 60-second delay before it illuminates. If it comes on during a sharp left turn, or comes on at all within 60 seconds after the previous time it illuminated... the delay circuit is bad. The circuit is located on the fuel/oil/temp gauge pod. It's pretty common to have this fail after 20 years.

3) If the delay circuit IS working properly, and the low oil level idiot light still turns on some of the time... it could be you have one of those "special" cars like mine, where the idiot light triggers with the oil level right in the middle between MIN and MAX on the dipstick. I need to keep the level between the middle and MAX to keep the stupid light off. I've only had this problem on my '87 300D (even with two new sensors), never on any other 124 that I've owned. Go figure. Could have something to do with the revised oil fill quantity... was originally 8.5 quarts, but MB later reduced this to 8.0.

:stuart:

No, the next logical thing would be to attach a manual pressure gauge, to verify the actual oil pressure present, and to rule out the possibility of a faulty electric gauge. If it's really low oil pressure, I'd at least replace the filter first (and check/replace the O-rings at the bottom of that shaft in the center of the filter). If nothing looks wrong, then *maybe* the oil pump could be suspect. But I can't recall ever hearing of a failed OM60x oil pump...

:nuke:

i'm glad to know i'm not the only one that beats the dash as a cure for various problems