Chedman13

Correct, capillary action of the oil on the inside of the harness covers. It slowly - very slowly - makes it way throughout the entire harness.

cij911

Sorry, can you be more explicit - is the oil wicking on the wire itself or just the cloth-like fabric harness cover?

Chedman13

No worries. It travels on the inside. So you can't even visually see it until it comes out a connector. So if you picture a straw and a wire inside. The oil is inside the straw.

That's why I bet there are much more cases of this issue, but it doesn't always cause problems or a check-engine light. So you can run like this for a while, but not great for the ECU/connectors/sensors.

How many people open up their camshaft sensors and ECU to check for oil=)

cij911

Thank you - super helpful. I wonder if adding a dielectric grease would help? Also, it would be interesting to see how the resistance of the wires are affected by the oil (i.e. could we check resistance to see if we have an issue in the wire loom) ?

Chedman13

I've seen some models use a piggy back connector so something they added between the sensor and the harness which stops the oil creep. This was a MB fix, but not for all models which is weird.

Zero idea of that grease helping or not. But if it doesn't hurt, why not try it.

chassis

Yes, capillary action. My expectation is that MB uses multi-strand wire in the harnesses. The space between the two conductors is the capillary path. The conductors are surrounded by plastic insulation which is accomplished via plastic extrusion. No different than any other wire.

The oil insulates pin-vs-pin. It creates a film barrier, for example, between cam position sensor pins and the corresponding harness pins. Or, for example, between pins on the harness and the corresponding pins at the ECU.

The film doesn't need to be much. Intermittent loss of contact is enough to cause a problem. A test that I haven't seen people do here is to measure continuity and resistance between the ECU end of the harness and the sensor (cam position sensor, cam magnet). While interesting, it wouldn't rule out intermittent loss of electrical contact, and it would not indicate whether or not contact was being made when everything is connected. Star/Xentry/DAS could help with this.

Dielectric grease might help, or might make things worse. It has been demonstrated that MB harnesses are not wick-proof. The question is will grease wick as readily as oil. Likely not, but hard to know for sure.

Replacement of the offending sensors and harness, and ECU, is the highest confidence solution, and most expensive. The hardest DIY step is coding the ECU. A good relationship with an Indy shop could help with ECU coding.

Elric99

I think there was a class action lawsuit or something for a similar problem to this on a different MB engine model. Can't recall all the details. But part of the resolution was MB had to install pigtail cables between the engine and wiring harness to halt the oil seep. Really terrible they are giving us M278 owners absolutely no help on this since it appears to be the exact same design and issue, perhaps they need another lawsuit to get their attention.

chassis

Another option I haven’t seen people explore is to try the pigtail with another engine. Affected engines candidates are:

M278
M157
M272
M276

And probably others. The root causes are leaking cam magnets and cam position sensors, approximately around the early 2010s or so.

arsupisemnet

So many interesting hypothesises I can read here...
All contacts in connectors are "spring loaded"... Which makes it indifferent to oil film... As a matter of fact it used be recommended to put some of electrician dielectric grease into connectors some time ago to prevent water damage...
The secret is quite simple - plastic used for wiring insulation... "Biodegradable" crap or whatever they still use... Oil + vibrations accelerate wear and damage leading to shortcuts....
Been rebuilding xenon headlights on other MB project from 2009... All insulation crumbles by itself...

BlownV8

Yup, it was some type of German mandate that vehicles had to be biodegradable. The problem is that it biodegrades too soon. I replaced the headlights in my 164 GL due to this issue.

chassis

Would degraded insulation cause problems without the presence of oil at any point in the electrical system?

arsupisemnet

It will once wires touch each other....

OnlyGerman

Since OP has a 2015, shouldn’t he of already had the updated sensors from factory,# 276-905-10-00?

Chedman13

What updated sensors and what year - I assume 2015?

OnlyGerman

The cam sensors updated # 276-905-10-00 and cam magnet updated # 276-156-07-90

My 2013 ML 550 (03/2012) had the old part numbers but my 2015 S550 (11/2014) has the newer cam magnets, still need to confirm whether it has the old or newer cam sensors - I assume the newer.

Maybe there was a production cutoff for these sometime in 2014?

Chedman13

thanks I’ll make sure those get changed out on mine!

Hakon74

Is there any explanation as to why the engine requires removal other than the say so of Mercedes in order to replace the loom / harness.
Two cam position sensors leaking, oil in harness and has reached the ecu.
2015 car

marcd7tx

I have heard from a reliable source the engine does not need to be removed to replace the harness. Especially if someone has enough experience doing this job.

chassis

Just an idea - the reason for engine removal could be because access to the rear of the engine is difficult when removing the harness.


MB / WIS repair procedures seek the shortest time solution, generally. One example of this the is the M276 NA spark plug change procedure, which WIS calls for removing the intake manifold but this in fact is not required. I can see how intake manifold removal would be a faster spark plug change, than not removing it.

Study harness and engine diagrams, and look at your engine to see how you would release all of the harness connectors with the engine in place. At the rear of the entire the dash bulkhead likely needs to be removed and possibly connectors accessed from below the vehicle at the transmission bell housing. This might be for items mounted to the rear of the cylinder head and block such as crank position sensor, vacuum pump, fuel pump, PCV, etc. There is quite a bit of stuff back there.

arsupisemnet

On m278 v8 biturbo there is a piece of the harness running underneath passenger side exhaust manifold-turbo which will be hard to pull out pull in. That is the reason for engine removal.

arsupisemnet

In case of "oil reached ecu" what are symptoms/errors?

Hakon74

Thanks for the replies...
No codes or lights as yet but been experiencing the sensation of a sudden thump or jolt throughout the car on acceleration on partial throttle that is not associated with a gear change.
Only problem is that it doesn't do it all the time.

marcd7tx

Hakon74, your ECU will try to compensate which is what happened to mine before I checked the connectors, realized there was oil seeping into the harness, then had the harness changed. I didn't read all the way up so I don't know if you checked your connectors yet. If not you really need to. My GL would not idle well 90% of the time, now it purrs. It will take a LOT for oil in the harness to throw a CEL. So just because it's not throwing one doesn't mean you don't have this issue...

Hakon74

Both exhaust cam position sensors are covered in oil. The electrical block connectors to the ECU are covered and it's made it's way to one of the oxygen sensors.

chassis

@Hakon74 would you be willing to post closeup clear and well lit photos of the cam sensors, if you decide to replace them? They need to be 100% completely cleaned of oil. The goal is to find the precise failure point.

My thought is that the injection molded plastic housing of the sensor has cracked due to fatigue (vibration and chemical attack) and age, and is allowing oil inside the sensor from where it migrates through the connector pins and into the harness.