How exactly does this circuit work?

On my 1984 190 D there is no low oil warning light. I was going to rig one up since I put the engine from a 1987 in the car and the sensor is there.

I figured that what happens is that 12v go to the sensor and if the oil is low it would ground out and light up the light. When when I hooked it up that way the light was always on - with the right amount of oil in the system.
(engine running or not - the light was always on)

Is there some circuit in the gauge cluster that controls this light. That seems odd to me, but it could be possible.

Yes, you would need the cluster from the '87 to get the proper circuit to interpret the sensor for the light, my understanding is that the circuit smoothes out the signal so the light is either on or off, not constantly flashing as the oil level moves around while driving. It has a timer circuit of some sort that decides if the sensor has activated enough in a given time that the oil level is really low enough, and then it turns on the low oil light.

The sensor is a simple float switch, sounds like either your float is stuck or your dipstick is wrong (and your engine really is low on oil).

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

The sensor worked fine in the 1987 - my guess is that you are correct in your first statement. There is something more to the circuit in the cluster.

I have the 1987 cluster, but that is not a direct drop in for the 1984 - believe me I have looked into this.

I think that the switch is fine but, my guess is that there is a level of resistance that is reached and the light comes on.

I was hoping someone would have more information on this, I may just have to replace the switch with one that is simpler. Or I would have to take a crapload of resistance readings and calculate when the light should come on and build a circuit to do that......

Also just to reiterate this engine was working completely great in the 1987 - it has the correct dipstick, the oil level is fine, and every sensor worked when pulled. It was pulled working and put in within a two week period.

It is just that the difference between the sensors and gauges on the 1984 to a 1987 is crazy. I am having to install aftermarket oil pressure and temperature sensors. I really want a low oil warning light and I will find a way, but it is staggering that the dumbest warning light should be this difficult.

I'd guess that it's just a N.C. contact, opens with low oil pressure, makes it fail-safe.

I had a 300D sedan that would turn on the light with the oil just below the full mark, and it did flash on/off as I drove.

__________________

Gone to the dark side

- Jeff

No it is not.

It has nothing to do with oil pressure.

There is a reservoir in the crankcase with a float switch in it.
The reservoir is closed when the engine oil is cold.
The reservoir opens when the oil is up to operating temp.
When the reservoir is open to the crankcase it can read the oil level.
This prevents false "low" readings when the oil is cold and not flowing back to the crankcase fast enough.

I believe that the gauge cluster must be matched to the sending unit for it to work properly. There may be a range of models and years that are compatible.

__________________
Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO

Sorry, meant oil level, my bad.

I don't remember seeing a partition between the oil reservoir sections, at least not on the 603. You sure about that?

__________________

Gone to the dark side

- Jeff

Where is the connection for that "crankcase float switch"?

I assumed that the low oil sensor was the one that was in the drivers side of the oil pan. If that sensor is not the low oil sensor what is it?

Oh and FYI this would be a 5 cylinder OM602 - but I have an OM603 on an engine stand and I have the OM601 that came out of the car - but that has no low oil pressure sending unit as far as I can tell.

1987 Mercedes-Benz 190D 2.5 Sedan - Oil Circulation - Page 4

__________________
Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO

So I am pretty sure that is the one that is on the side of the oil pan.

I have a few gauge clusters laying around I will trace the circuit through one of them and see what it is. (I had not done this yet, because when I tried to do it before I had some trouble figuring it out)


I really thought it would be a much simpler system - I may just see if I can find a basic dummy version to replace it with......

The electrical diagrams should be here: Untitled Document

__________________

Gone to the dark side

- Jeff

I have the FSM for the w124 and the w201 and I have not been able to find one that shows what may or may not actually be happening. They just show the path to the Gauge Cluster and not exactly what may be happening in the cluster to control this signal.

I believe that they do not show what is really going on in the gauge cluster because it is not even a part the the dealer would service - they would just replace the whole cluster or the boards in the cluster. Dealerships are not really set up to do tiny electronics repair.

So I have traced the circuit back on a W124 gauge cluster. I have not traced every element, but yes there is a logic circuit there. The positive side of the bulb traces back to a small sub-board. I think I may be able to de-solder that board and use it in my car. There are three connections to that board.
I know for a fact that one is the signal path to the bulb, the second and third are what I have not entirely figured out yet.

My suspicion is that it may be a ground and a path to the signal from the sensor, but I got some odd readings on my volt meter and need to further test it.