Limp Mode

[Andrew Rymer]

Hi All,

I have had an ongoing limp mode problem for the past4 months now with the car going into an auto electrician and eventually to MB themselves. During this time just about all the secondary air induction system has been replaced ( MAF, various valves/solenoids etc).It has been with them now for three days and I got a phone call from the service engineer to tell me that they have finally found the problem! It apparently was an oil leak from the camshaft seal that had found its way into the wiring harness. He then proceeded to tell me that it was ok because it had not got into the ECU so they could actually fit a "piggyback" harness, one presumes to bypass the affected bit of the loom! Now I know that some Mercedes wiring looms are prone to breaking down but for the life of me I just can’t see how oil in the loom and not the ECU can put the car into limp mode. Has any one come across this before or am I just being fed BS?

Regards,

Andrew

 

[television]

Hi All,

I have had an ongoing limp mode problem for the past4 months now with the car going into an auto electrician and eventually to MB themselves. During this time just about all the secondary air induction system has been replaced ( MAF, various valves/solenoids etc).It has been with them now for three days and I got a phone call from the service engineer to tell me that they have finally found the problem! It apparently was an oil leak from the camshaft seal that had found its way into the wiring harness. He then proceeded to tell me that it was ok because it had not got into the ECU so they could actually fit a "piggyback" harness, one presumes to bypass the affected bit of the loom! Now I know that some Mercedes wiring looms are prone to breaking down but for the life of me I just can’t see how oil in the loom and not the ECU can put the car into limp mode. Has any one come across this before or am I just being fed BS?

Regards,

Andrew

You are correct in your thinking and oil is non conductive.

It is the oil that goes through the wires, and ends up in the MAF that puts the light on.

Its a well known fault, fit the modified plug, and the wires could be washed and dried along with the ECU without further problems.

Many people do that

 

[Andrew Rymer]

Thanks Malcolm, I will ask about the modified plug and if they have infact replaced it. It will be great to have the power back!!!

Cheers,

Andrew

 

[howardkm]

hi
i have a problem with my merc c 200 elegant 1995 M reg, it all started with me parked at work talking on the phone, with the engine running coz it was a cold day when it switched off. i started it but had miss firing, called my electrician who diagnosed it and found that my ecu had been blown, i took it for repair and fitted it back with new coils and ht leeds with Full service. it moved for four month and now it is taking time to pick up speed and sometimes it miss fires on full throttle and hidling is not stable, it holds back when u need to speed up. i have pluged it in to diognistic machine but it is talling me that no faults found, what could be the proble?

 

[Andrew Rymer]

Hi all,
Right well I got the car back and, as Malcolm (television) said, they have replaced the leaking camshaft magnet, plug etc and all seems well! Now all I have to do is go and claim the £462.34 from the garage that sold it to me!! I must admit, if I was the garage, I would not be best pleased as they must have already paid out hundreds of pounds to try and get this sorted initially. It begs the question that if this is a common problem then why was that not looked for at first? Think I would be asking some questions of Mercedes Benz. Anyway I will keep all informed as to how the car performs over the next few weeks. Let’s hope the problem is cured for good.

Have a great weekend and thank you for your help.

Regards,

Andrew

 

[Autofix]

Hi Andrew,

That fault is extremely common and anyone who has worked on Mercs for more than a couple of weeks should diagnose that double quick.
Looks like you have suffered from "bolt on diagnostics".

As an aside, oil is conductive. When the oil gets into a connector it shorts out the pins. With this particular fault it is often the O2 sensors that suffer.
After the repair there is still quite a bit of oil in the wiring as it travels up the conductor and is impossible to clean out. While I have never seen this cause a problem i would have to wonder about the long term effect of this.

 

[television]

Hi Andrew,

That fault is extremely common and anyone who has worked on Mercs for more than a couple of weeks should diagnose that double quick.
Looks like you have suffered from "bolt on diagnostics".

As an aside, oil is conductive. When the oil gets into a connector it shorts out the pins. With this particular fault it is often the O2 sensors that suffer.
After the repair there is still quite a bit of oil in the wiring as it travels up the conductor and is impossible to clean out. While I have never seen this cause a problem i would have to wonder about the long term effect of this.

Sorry to argue the point on oil being conductive it is not. I did test 2 years back and with test leads spaced at 0.05 mm and 200 megohm test set, no leak what so ever.

Ignition coils are filled with it and so are the National grid transformers at(660.000 volts)

 

[Autofix]

Sorry to argue the point on oil being conductive it is not. I did test 2 years back and with test leads spaced at 0.05 mm and 200 megohm test set, no leak what so ever.

Ignition coils are filled with it and so are the National grid transformers at(660.000 volts)

Hi TV,

Never be sorry to argue (or discuss even) a point

Good points. But, from experience, it does seem to conduct. Maybe its all the carbon in the used oil??

An example of the experience I mention is the case in point, the Merc loom. Often the O2 sensor signals degrade and cause running concers. Same for the MAF. How could the oil in the loom cause that unless the pins in the connectors were shorted?

 

[television]

Hi TV,

Never be sorry to argue (or discuss even) a point

Good points. But, from experience, it does seem to conduct. Maybe its all the carbon in the used oil??

An example of the experience I mention is the case in point, the Merc loom. Often the O2 sensor signals degrade and cause running concers. Same for the MAF. How could the oil in the loom cause that unless the pins in the connectors were shorted?

Thank you
I think it is the presence of oil on the 02 that triggers it.In the case of hot wire MAFs the oil burns itself into the wire, with the sensor type it can be washed off

From an electronic point of view I have done test with all types of oils and fluids both old and new and have never got any leakage measurements at all.I resorted to old oil with the thoughts on any metallic particles being in it

During my last test I used a working Sky box on the bench,, these things have the 240v AC/ 450DC/ 1000v ac/ more important 10 microprocessors running at speeds far in excess of any car ECU .. The unit stayed working the whole time while I poured oil all over it.

Sure MB say that these things must be replaced but the teckies from 2 MB sites no longer replace the ECU, they wash it. They do replace the harness on grounds that the oil can take for ever to work its way down the copper wires,,, and this could be a nuisance later making a re call.

I think that MB say always replace to stop comebacks.

Its an interesting subject and thanks for reading it

 

[Autofix]

It certainly is interesting.

Lets take another example. Land Rover TD5 injector loom leaks the same way as the Merc ones do and fills the plug with oil. The result is that it will go off a few cyls. If you read the fault memory there will be codes for inj shorts. If you do no more than clean the plug it will then fire on all cyls. The only thing that has changed is the oil in the plug so that would suggest the oil was shorting the pins.

Another common one is the Merc trans loom leak. Sometimes, if caught early enough, the oil will only be in the plug/loom and just cleaning the plug out cures the speed sensor/solenoid faults. Again the only thing to change is the oil in the plug.

If the oil cannot cause the short, what does?

Over to you (p.s. I hope you have the answer cause I dont )

 

[Autofix]

Actually, i may have a theory, more later

 

[television]

It certainly is interesting.

Lets take another example. Land Rover TD5 injector loom leaks the same way as the Merc ones do and fills the plug with oil. The result is that it will go off a few cyls. If you read the fault memory there will be codes for inj shorts. If you do no more than clean the plug it will then fire on all cyls. The only thing that has changed is the oil in the plug so that would suggest the oil was shorting the pins.

Another common one is the Merc trans loom leak. Sometimes, if caught early enough, the oil will only be in the plug/loom and just cleaning the plug out cures the speed sensor/solenoid faults. Again the only thing to change is the oil in the plug.

If the oil cannot cause the short, what does?

Over to you (p.s. I hope you have the answer cause I dont )

Yes I am aware of the gearbox faults thats why I tested with transmission fluid. is it being low on fluid the gives the problem in the end. As I said ignition coils are full of oil to stop arcing,, the oil does not hurt the varnish either on the widings.


I am happy to set up any test and put up the pictures and results

Cheers

 

[Autofix]

Hi again,

I can state 100% that the trans faults, in the case of the leaking seal, are not caused by low fluid level. If the pins are cleaned and nothing else is done (ie no fluid added) the faults clear. At least until the plug fills up again. The faults are caused by the circuits that are covered in the trans loom so you get codes for speed signal missing, solenoid open/short etc.
In the case of the Merc trans plug, I have on many occasions cleaned the plug then cleared the codes so as to check if the fault was there or in the trans circuit board. With the Land Rovers I have often cleaned the plug to get them running on all cyls just to keep a customer going for a few days while I wait for the new loom.

I have always assumed that the oil caused a short and never questioned it, until now.
So thinking about this it strikes me that if you check the code setting criteria for the logged codes they can be caused by either open or short.
While I stated earlier that oil is a conductor that was merely because I had never thought about it. I am not arguing that it is a conductor just trying to figure why it effects the circuits.

My theory now is that the oil acts as an insulator. The quality of the male and female connectors is very poor and a film of insulating oil could well cause an open circuit or high resistance.

Theres a new set of tests for you

cheers

 

[television]

Hi again,

I can state 100% that the trans faults, in the case of the leaking seal, are not caused by low fluid level. If the pins are cleaned and nothing else is done (ie no fluid added) the faults clear. At least until the plug fills up again. The faults are caused by the circuits that are covered in the trans loom so you get codes for speed signal missing, solenoid open/short etc.
In the case of the Merc trans plug, I have on many occasions cleaned the plug then cleared the codes so as to check if the fault was there or in the trans circuit board. With the Land Rovers I have often cleaned the plug to get them running on all cyls just to keep a customer going for a few days while I wait for the new loom.

I have always assumed that the oil caused a short and never questioned it, until now.
So thinking about this it strikes me that if you check the code setting criteria for the logged codes they can be caused by either open or short.
While I stated earlier that oil is a conductor that was merely because I had never thought about it. I am not arguing that it is a conductor just trying to figure why it effects the circuits.

My theory now is that the oil acts as an insulator. The quality of the male and female connectors is very poor and a film of insulating oil could well cause an open circuit or high resistance.

Theres a new set of tests for you

cheers

Thank you, the main thing is that all test are carried out correctly.

The points that puzzled you have puzzled me and yes I know about the poor fitting of the ECU plugs,, the Can bus ones on the end especially .

With the heat and expansion of the sockets this could be the answer.

I know that the oil brings up fault codes as you said, but I could not tie it in with the fact that oil has an unmeasurable resistance.

Thank you for talking about this ,