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1993 300E 2.8 (sold), 2012 C250 (wife's), 2004 C230 (sold), 2005 E320 (sister-in-law's)
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Discussion Starter #1
Dear all, first thanks for reading my first post. Really appreciate your help.

My mother's 1993 Mercedes 300E 2.8 recently has a "check engine light" problem. The car still runs fairly well, although couple of weeks ago she experienced engine stall when the car is going slight uphill, and when the car is started in cold weather, sometimes the engine would "stutter" (or the RPM of the engine will go like roller coaster). The car still have fairly low mileage (about 55K I think). We helped her to do some basic diagnostic (using the radio shack "home build" code scanner), and found that only Pin #6 (some minor SRS issues) and Pin #3 (Mass Air Sensor??) reported code errors. However, we were not able to reset the error codes with the code scanner (to see if they are really "hard" error codes), so we disconnected the battery for about one hour and reconnect it. The "check engine light" is still ON.

Questions:

  1. Is it long enough to clear all the memory and remaining electrical power for just disconnect the battery for one hour? Or should we disconnect it and wait for longer? How many hours then?
  2. I read a lot of posts in this forum, and seems a lot of people mentioned the culprits are probably (a) upper wiring harness, (b) ETA wiring insulator, and (c) OVP relay. Is there any step-by-step way to REALLY make sure any of these parts REALLY need to be replaced? Could we do the trouble-shooting by ourselves, or this is really a job for the professionals?
  3. Someone mentioned to check the OVP relay to see if it's 000-540-52-45, and if it is, then change it to the new part 000-540-67-45. HOW could I locate the OVP relay in this car and see the part number? Do I have to dissemble anything to reveal the OVP relay part#?
  4. Also what does the "ETA wiring insulator" looks like and how do I know whether it need to be replaced or not??
  5. How do I check the "tag" at the harness (what year) or splice the insulator and see if it's original? Could someone please post some pictures here in the forum??
Many many thanks!
 

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  1. Is it long enough to clear all the memory and remaining electrical power for just disconnect the battery for one hour? Or should we disconnect it and wait for longer? How many hours then?
  2. I read a lot of posts in this forum, and seems a lot of people mentioned the culprits are probably (a) upper wiring harness, (b) ETA wiring insulator, and (c) OVP relay. Is there any step-by-step way to REALLY make sure any of these parts REALLY need to be replaced? Could we do the trouble-shooting by ourselves, or this is really a job for the professionals?
  3. Someone mentioned to check the OVP relay to see if it's 000-540-52-45, and if it is, then change it to the new part 000-540-67-45. HOW could I locate the OVP relay in this car and see the part number? Do I have to dissemble anything to reveal the OVP relay part#?
  4. Also what does the "ETA wiring insulator" looks like and how do I know whether it need to be replaced or not??
  5. How do I check the "tag" at the harness (what year) or splice the insulator and see if it's original?


  1. What specific codes are you getting, by pin? You shouldn't have to disconnect the battery to clear them. They need to be cleared within a few seconds of the reading, then ignition off after each clear.

    The wiring harness is tested by inspection - do you see any crumbling insulation at the end points on the top front of the engine? On the ETA it's insulation, not insulator, in other words the same issue as the upper harness - the insulation crumbles and causes shorts. I'd focus first on reading and clearing the codes, and the upper harness inspection. The OVP is behind the battery, fairly easy to get to. Those two part numbers should both be good - the EPC shows them as interchangeable.
 

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93 300E 2.8, 79 450 SLC
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I just replaced my MAF in my 93 2.8 as well. It did cause a engine check light and occasional stall during idle and acceleration. I bought a new Bosch MAF online ($200) and the CEL disappear immediately and car runs fine ever since.
 

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1993 300E 2.8 (sold), 2012 C250 (wife's), 2004 C230 (sold), 2005 E320 (sister-in-law's)
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Discussion Starter #4 (Edited)
Hi bigwiki, thanks so much for the info! By the way, where did you get your MAF online ($200)? Also, did you try to use "CRC MAF cleaner" to clean MAF first and see what happens? I wonder whether I could just "clean" the MAF (or really need to replace the MAF) to get the problem resolved? Thanks much!
 

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1993 300E 2.8 (sold), 2012 C250 (wife's), 2004 C230 (sold), 2005 E320 (sister-in-law's)
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Discussion Starter #5 (Edited)
What specific codes are you getting, by pin? You shouldn't have to disconnect the battery to clear them. They need to be cleared within a few seconds of the reading, then ignition off after each clear.

The wiring harness is tested by inspection - do you see any crumbling insulation at the end points on the top front of the engine? On the ETA it's insulation, not insulator, in other words the same issue as the upper harness - the insulation crumbles and causes shorts. I'd focus first on reading and clearing the codes, and the upper harness inspection. The OVP is behind the battery, fairly easy to get to. Those two part numbers should both be good - the EPC shows them as interchangeable.
Hi deanyel, the codes that I was getting was NOT consistent. Here are the detailed list:


Pin #3 (continuous fuel injection): code 10 (only showed once, the 2nd time I tried to read it, the code was gone without me even tried to reset it)

Pin #6 (SRS): code 8, 9
  • Code 8: voltage supply interrupted
  • Code 9: SRS Warning Lamp (with flashing SRS warning lamp Impulse counter scan tool button held too little time to read out the DTC memory or too long or erase DTC codes. Reread codes).
Pin #7 (air conditioning): code 1 (should be "no fault found" right?)


As for the wiring harness, I do not see any serious "crumbling" insulation at the end points on the top front of the engine. They just look a bit rusty and old at the end points (I will try to post some photos later). The car has always been garaged, and never met any snow - living in sunny cupertino california area for the past 17 years. I agree with you that I also want to focus first on reading and clearing the codes, but I can't seem to be able to read the codes consistently with the same results every time, and I can't seem to be able to "clear" the codes (even by disconnecting the battery for more than 1 hour). Thoughts or comments on this?

I will try to see if I could locate the OVP (behind the battery) by myself. When you said the EPC shows them as interchangeable, what do you mean by EPC (sorry about this stupid question)? Also why would people recommend the new part 000-540-67-45 in other threads? Is there a special reason for this? Thanks again!
 

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Photos of the harness would be good. Also check the label near the source plug for a production date. Eratic codes can be a sign of a bad harness.

For engine operating problems concern yourself only with pins 3, 8 and 14. Forgot about the others for now (although voltage supply interrupted has an OVP sound to it).

The EPC is electronic parts catalog from Mercedes, the factory's view of what part goes where, very authoritative but can be wrong. When a part number is outdated they show it as such, or if not as interchangeable as in this case. AutohausAZ.com is selling 52 45 for your car. Possible that they are both wrong but the chances are pretty slim.
 

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1993 300E 2.8 (sold), 2012 C250 (wife's), 2004 C230 (sold), 2005 E320 (sister-in-law's)
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Discussion Starter #7 (Edited)
Photos of the harness would be good. Also check the label near the source plug for a production date. Eratic codes can be a sign of a bad harness.

For engine operating problems concern yourself only with pins 3, 8 and 14. Forgot about the others for now (although voltage supply interrupted has an OVP sound to it).

The EPC is electronic parts catalog from Mercedes, the factory's view of what part goes where, very authoritative but can be wrong. When a part number is outdated they show it as such, or if not as interchangeable as in this case. AutohausAZ.com is selling 52 45 for your car. Possible that they are both wrong but the chances are pretty slim.
Hi deanvel, thanks so much for your help! Here is a photo of the harness:



Uploaded with ImageShack.us



Uploaded with ImageShack.us

I also noticed that the number on the harness is 124 440 3908 KS 02 06 92. So I guess the first 10 digits (124 440 3908) are the part number, and the 02 06 92 is the manufacturing date? Am I correct on this? Is the 02 06 92 stands for 1992 June 02, or 1992 February 06?? Also what does that "KS" stands for? Does this harness looks like "it's gone" and need a replacement? Thanks

I searched online and seems that replacing the harness could be a DIY project. As we would like to keep the repair cost down as much as possible, so I also "ebay" the part and found some "used" harness for this model (93 300E 2.8). Some quick questions:

  1. Would people recommend buying "used" harness from eBay? why or why not?
  2. If you recommend buying new ones (including OEM?), which online store might have the best price for this particular part? Is there any quality differences between an genuine OEM one and a authentic Mercedes part (does Mercedes even make this part anymore?)
The car runs VERY smoothly on local city roads and highways, it's just that we really annoying that the "check engine light" is on and we can't even clear the light by disconnecting the battery for more than an hour.. Maybe I should leave the batter disconnected for a even longer time, like 8 hours or more? Would that help if it's the harness problem? Thanks again!
 

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Those wires look pretty good but I don't think you've actually got any pictures of the upper engine wiring harness there. The part number would be either 124 543 90 33 or 124 440 56 32. It runs from behind the battery to the fuel injectors, 3 coils and a number of sensors on the front top of the engine. I think a used harness is a bad idea unless it's really cheap. Parts.com is about $650. But you need to focus on the check engine light - what are the codes, why won't they erase. I'm guessing you're doing something a little bit wrong.
 

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Those wires look pretty good but I don't think you've actually got any pictures of the upper engine wiring harness there. The part number would be either 124 543 90 33 or 124 440 56 32. It runs from behind the battery to the fuel injectors, 3 coils and a number of sensors on the front top of the engine. I think a used harness is a bad idea unless it's really cheap. Parts.com is about $650. But you need to focus on the check engine light - what are the codes, why won't they erase. I'm guessing you're doing something a little bit wrong.
Hi deanyel, thanks again for your help! Sorry I'm still kind of new as a Merc DIYer, so I'm having a bit trouble to really locate the "upper engine wiring harness." I tried to look for it again this morning in the garage (kind of dark), but still can't really find those two part # (124 543 90 33 or 124 440 56 32). Is that part located in the following picture?



Also I found this article, thought that I would just share it with everyone here:

PeachPartsWiki: W124 Engine Wiring Harness Replacement
 

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No, wrong side of the engine. You need to be over on the other side behind the battery. There may be a plastic cover over the source plug with the part number. Follow the wires from fuel injectors to the rear of the engine then over to the passenger side rear.
 

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No, wrong side of the engine. You need to be over on the other side behind the battery. There may be a plastic cover over the source plug with the part number. Follow the wires from fuel injectors to the rear of the engine then over to the passenger side rear.
Hmm.. on the other side of the engine and behind the battery, there is no room for any harness wiring, see:



Did you mean "under" the battery maybe? Should I take the whole battery out to find the part number, and is the OVP relay underneath the battery?

Near the battery there is the diagnostic unit (where I put in the banana head for code reading), is this the "source plug" that you were talking about? BTW, all the wires seems to be in good shape (no signs of insulation cracking and flaking off, and can't see any bare wire exposed)...



BTW, long time ago (maybe in 2004), I think the battery exploded. We were able to neutralized the acid right away, but it still did some minor damages to the nearby parts (some type of electric motors?), see:



Should we be worried about that? Would this shorten the lifespan of the harness wiring in some ways? What would be the best way to clean those battery corrosion around this area? Thanks again!
 

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Should be a removable shield behind the battery there.
Thanks deanyel! By the way, do I have to remove the battery first before I could remove the "removable plastic shield"? What tools do I need to remove the "removable plastic shield"? After I open up the "removable plastic shield," do I need to do anything to confirm the upper wiring harness is bad (e.g. crumbling), or pretty much just visual inspection would be good enough? Sorry for the newbie questions. Many thanks!
 

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No and no, should just be able to pull the shield out. You're looking for a part tag with an FD date. That's the worst place for a visual inspection - need to look at the end points on the top front of the engine.
 

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No and no, should just be able to pull the shield out. You're looking for a part tag with an FD date. That's the worst place for a visual inspection - need to look at the end points on the top front of the engine.
Wow deanyel, you are the best! Thanks so much for all of your help. I really appreciate that.

So I was able to pull the shield out, but now I'm having trouble to put it back :)surrender:)...is the orientation of the following picture correct (to put the shield back)??



Still need to drive it out of the garage so that I could better look for the part tage with an FD date... it's too dark in the garage. I will remember to look at the end points on the top front of the engine... thanks again for the tip!! :)
 

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'91 C124 300CE, '06 W164 ML500, '00 BMW MCOUPE, '65 COBRA REPL.
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The ECU is behind that shield and thus behind the battery. Remove the battery.
Forget abou the shield for now. Before you put the battery back in, you can install the shield and you will figure out the proper orientation.
You see a metallic box with a large connector and a bunch of wires going to it.
Take close up pictures starting there. Then follow that big, thick bunch of wires.
The majority will land on your engine. Take pictures of the exposed wires. Not the ones that we can not tell the condition because they are wrapped.
Check the condition of the wires. Are they freid? are they crumbling? etc.
Do not worry about replacing the harness if you are not sure it is bad. Concentrate on finding the cause of your problem.
Start with the simplest first as it is usually cheaper. The MAF might be the cause of rough, uneven idle.
We need more diagnosis to help much more.
Re do the code scanning. Try to clear the codes again. Do not worry about AC, SRS etc.
Concentrate on engine electronics.
Report back.


Look here for correct orientation of shield

dscn0312.jpg - Rozmiar oryginalny - Fotosik.pl
 

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The ECU is behind that shield and thus behind the battery. Remove the battery.
Concentrate on finding the cause of your problem.
Start with the simplest first as it is usually cheaper. The MAF might be the cause of rough, uneven idle.
Dear semis1, thanks so much for your help! I will post some photos later.
By the way, I'm glad that you mentioned the MAF. I remember reading another older discussion thread which finally found the rough and uneven engine idle (even stall) was caused by the MAF, not the engine wiring harness. Couple of quick questions here:

  • If it's an MAF problem, usually for this year/model of Merc (93, 300E), can it be fixed by just using CRC MAF cleaner to clean it up?? Would I be able to confirm it's an MAF problem by using the home-built code reader??
  • If it can NOT be simply cleaned, how much would it cost to replace the MAF in 93 300E? Can this be a easy DIY type of project? How much the MAF part would cost?
Many thanks!
 

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There are indeed some faults for mass airflow sensor. Cleaning isn't usually a long term fix for a hot film sensor - pretty much needs to be replaced. You can price out most of your parts on the internet - sites like autohausaz.com. It is a very easy replacement, easy to get to, maybe 10 minutes. But you need to diagnose the problem first.
 

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There are indeed some faults for mass airflow sensor. Cleaning isn't usually a long term fix for a hot film sensor - pretty much needs to be replaced. You can price out most of your parts on the internet - sites like autohausaz.com. It is a very easy replacement, easy to get to, maybe 10 minutes. But you need to diagnose the problem first.
Sorry it took me so long to get back to this thread. I took the car to an independent shop and they used the MODIS "snap-on" diagnostic system to find out the error codes, and here are what they found:

17: CAN; com data exchange -> ABS; EA; HEM
16: Closed Throttle Position (CTP) idle information
10: Mass Air flow signal; low voltage

They were not able to further diagnose as I was in a hurry to get back to work, but I asked them to remove the battery and was able to get some more photos after the batter is removed (not sure if this helps or not):





I guess I still can NOT really see the part# of the ECU (and I don't know what ECU is for), and the wiring seems to be fine. The mechanic did say that based on this diagnostic, it seems that the MAF need to be replaced. Funny thing is, with this powerful "MODIS snap-on" system, it still can NOT reset/clear the error codes so my "check engine light" is still on.

What should be my next step? Get the MAF replaced? Thoughts or comments? Again the car only have about 54K miles on it, living in California for all of his life (never see snow), and have always been kept in a garage. Thanks!
 

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The ECU is the brain (Engine Control Unit). This is what makes the fuel injection know what to do when the engine is cold, when it is warm, when you accelerate etc.

From the diagnosis, an MAF (mass airflow sensor) is probably the cause.
MAF senses how much air enters the engine and tells the ECU what to do.
 
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