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

Yeah... but you do that on your Chevy like I do my Chevy right? Never on a BENZ!!! :rolleyes:

The beating of the dash is a pure frustration outlet. The fact it works occasionally is just icing on the cake!

On the W126 you can get it from the top, easy as can be...:D;)

D'oh, I forgot this wasn't a 124. Near impossible to change that thing from the top on a 124... the cooler lines, steering box, and washer reservoir are all in the way. Just imagine what a 602 in a 201 must be like!

:stuart:

If I hook my manual pressure gauge up to this... Does anyone know what the PSI should be?

Bill

3 bar works out to roughly 45 psi, so it should be well above that whenever the car is not idling.

Now starting ten minute countdown by the end of which someone will have proved that it's in fact 43.5113213psi instead of 45.

However, as you're staring at your gauge, you'll see the needle hovering somewhere over "45" and be happy :D

Well a new sending unit arrived before I could find a metric machine thread fitting to hook a manual gauge into...

Results with the new sending unit are the same. When I started it up she was pegged at 3 right away... as it warmed she dropped to 2 and just slowly kept dropping over the next few minutes to almost 1. I turned off the engine at this point. I've never seen the pressure so low.

I'm still going to see if I can hook up a manual tester if I can find a fitting that is M12 x 1.5 into something I can hook a hose to.

Going back to something Sixto said... I will admit I have thought my engine to be knocking loud for quite some time. You had stated that it could be a crankshaft bearing or a rod bearing... Probably this stuff is over my head... because I don't think you can replace them simply by taking off the oil pan... How would these bearings affect oil pressure?

As Ipicture it there's a gear on the end of the crank with a chain around it that turns the oil pump. Shouldn't I test my oil pump first some how?

Also on another thread on this topic it talks about a check valve being open or even the O rings in the oil filter housing... Now I'm a thousand or so miles since my last change and this happened fast so it wasn't like I was just in the housing... I took off the lift pump, cleaned it up, put it back... and now low oil pressure.

I appreciate additional input on this.

Thanks,
Bill

Just getting the oil pan off is a lot of work. The engine doesn't have to come out of the car but it has to be lifted off its mounts for enough room to remove the pan. With the pan off, you can remove the main and rod caps to check the bearings. I think you can get to all of them.

Something to consider is oil analysis. There might be enough clues to determine what's causing low oil pressure. How long has the current fill been in the engine?

Folks on this forum have used Blackstone Laboratories and Performance Analysis (run by George Murphy).

Sixto
87 300D

I have about 1,000 miles on the oil right now. But considering this is supposed to be my daily driver I just might take it into the shop to get it done faster. Inside of the engine would be new territory for me.

She's got over 300,000 miles so wear on these bearings wouldn't surprise me, but it just happened so fast! I'm still not convinced I haven't over looked something simple...

I think I'll call the shop on Monday.

Bill

I know someone with an 86-87 SDL. Records show a failed vacuum pump was repaired before he got the car. After years of second ownership the oil pump somehow sucked a ball bearing. At least that's what a post-mortem suggests - ball bearing size hole in the mesh, deformed ball bearing in the impellers, oil pump sprocket sheared off the pump (that's some chain!). Oil pump seized causing a main bearing to seize so the engine seized. Everything else looked fine.

For kicks, does the vacuum pump on your engine have 4 visible screw heads?

Should it turn out to be a lower end problem, consider a good used engine or short block vs having your engine rebuilt. I don't necessarily advocate it, just providing food for thought.

Sixto
87 300D

Throw in a new oil filter and see if that helps. Cut open the old filter to see if there's anything in it besides bugs and twigs :)

Sixto
87 300D

The '86 performs in an identical manner.

I've got a suspicion that the oil cooler plays some part in the illumination of the light. The light won't illuminate when the engine is cold...........it's got to be fully warm after a decent run on the highway. Once illuminated, it cycle depending on grade, engine temperature, or curves.

The only way to keep the light off is to keep the oil level at, or near, the full mark.

OK... I finally found a M12x1.5 to NPT male adapter so that I can hookup a cheap water pressure guage that I had lying around. Here's the scoop...

My first question is why when I turn on the ignition was the car's oil pressure gauge pegged at 3 without the engine even running. Seems short sited to me to have the gauge default to 3. So I started up and got 70psi, almost 5 bar. It took a while to warm up and at moderate revs (2000rpm) I could still hit 70 psi, but once warm it idled around 20psi or just under 2 bar.

Even when I release from 2000 rpm oil pressure did not drop below 1.5 bar while it rebounded to 1.8 bar.

So far so good... The interesting news is that my Benz's oil pressure gauge gradually dropped the whole time... It ended just under 2 bar by the time I decided I tested enough. I'm betting it would have kept dropping just like when I was test driving. So it appears to me I have a faulty dash gauge... or maybe something is shorting it out.

What I find as odd is that when I had the stock gauge hooked up it would fall and rise with the rpms... but always gradually falling overall. So maybe it did respond somewhat to what the sender was putting out, but has some other problem as well.

I have a spot open that I could put in an aftermarket oil pressure gauge. I might feel more comfortable with that then replacing the stock and possibly still have a wiring problem.

The only real downer is I'm now burning oil for some reason. I think this goes back to my original project where I took off the lift pump and cleaned it out. I guess something's not right there and perhaps oil is seeping in thru the fuel? But I'm not sure that would give me the same smokey results for why most people see a lot of smoke.

Thoughts overall anyone?

Bill

I'm not sure exactly where it is but check the ground.

If your dash gauge pegs at 3, with engine off, something is amiss, however your mechanical gauge should level off when the engine is hot at idle, and should be a min of say 15 psi, and if it does keep dropping, then you have a problem.

70 psi cold is ok, no problems there.

It sure sounds like the oil pressure "problem" is just a faulty gauge. Try a different one, they're relatively cheap used. Also make sure the wiring at the cluster is all intact & well connected.

For the oil burning, what kind of amounts are we talking about here? Is there any evidence (smoking, etc) or just the level dropping in the sump over time? The lift pump would definitely not affect this... nothing you do to the IP will let oil into the fuel supply. The reverse is possible though (fuel can get into the oil).

:sick3:

The burning oil thought is due solely to the proliferation of smoke coming out my tailpipe that I did not have prior to taking off the lift pump (not IP) and the subsequent and apparent gauge problem.

This car has never smoked so much. It could be from sitting for a week, but somehow I doubt it... I'm going to get a better hookup for a mechanical gauge and then I'm going to start driving her again and see if the smoke clears up or what. That's about all I can think of for now.

Bill

If the wire is not connected at the sensor, the gauge pegs at 3. I assumed he forgot to connect it, or it's just more proof the gauge is flaky (or, there is a broken wire somewhere).

Also, I thought the mechanical gauge always stayed above 20psi, and was not dropping. It was the electric gauge that was dropping, correct...?

:confused:

It seems that pegging at 3 before starting the engine is normal. With the wire/sender disconnected.

I'm very interested in this thread because I just started experiencing this same problem. Runs fine when cold or revving, always peggs at 3. But idle when hot and it goes down to 1.5 or so. Sometimes 1 bar and even down to zero when hot and at idle. Brand new oil just put in this last weekend.

What is the normal pressure at idle?

Danny

Danny,

Normally, when hot and idling, the pressure should be around 1.5 bar or so. Maybe a bit less, but I've rarely seen anything as low as 1.2-1.0 bar. However, I believe the official spec in the FSM is something silly like 0.5 bar, which IMO is too low. You may just have an electric gauge problem, though; assuming all W126 chassis have electric oil pressure gauges. (?)

:batman:

That's what I thought. Just wanted to make sure.
It was probably my own fault. I used the cheap Coastal oil. I went tothe airport Sunday and saw it at zero when parked at idle. :eek:

When I got back I went to Walmart and got some Rotella T and put that in. Actually just put that in this morning. So far looks better but I'll know better when it's run hot later. Both were 15W-40, I'll post what I get during the drive home.

Danny

Well it started out good. Pegged at 3 as soon as I started it up and stayed there all the way home.
Stopped at a light as soon as I got off the highway and it stayed at 3 the whole time.
Stopped at the next light and it went down to 2.5.
Stopped at the next light and it went down to .5. Went up to 1.5 in neutral.
when I pulled into my parking space it was down at zero again.

Can I pull out the oil filter without getting oil all over? Does the filter housing drain out when the engine is off?

This sucks. I got so much going on right now I don't need this crap!

Danny

Should be no different from when you change the filter during an oil change. Stick your hand into a plastic bag, roll the bag over the filter as you pull it out of the housing, invert the bag quickly once the filter is out for little to no drip.

Sixto
87 300D