M104 C280 Ignition Wire End Repair - PeachParts Mercedes-Benz Forum

Steve M · #1 09-05-2011, 11:19 AM

While doing routine spark plug replacement on 1996 C280 yesterday, I managed to damage either the wire, insulation or connector which attaches the wiring harness to the coil pack on the #4 cylinder. I'm hoping someone can tell me what to do next.

Background: car has 285,000 miles and was running fine with what I believe are all of the original coil packs, spark plug wires and connectors, and wiring harness. I am aware that some people routinely replace connectors to the three spark plugs under the coil packs. The insulation on visible wires at the thermostat looks fine. I installed the genuine MB Plugs.

After replacement I quickly determined that plugs 3 and 4 weren't firing, though there was no CEL. Since plug 3 is fed by the coil on plug 4 I focused in on the wiring feeding the coil pack, since I could see some copper through the insulation about an inch away from the connector. When I pull the wires out of their trough, the car runs fine. I unwrapped the fabric insulation (which was deteriorated) around the wire , and found two separate stranded copper conductors going to the connector. I tried to wrap each conductor separately with electrical tape up to the connector, but the car doesn't want to run when the wires are tucked back into place.

I have glanced through some of the posts about "waste spark" systems, but I really don't know how these things work. I am assuming that neither of the wires should come into contact with ground, but are they supposed to be touching each other? Are they connected together at the connector? I can see copper through the back of the connector, so it is possible that something isn't right in there, and that something moves when I rotate the connector on the coil pack to put the wires where they belong.

Fastlane sells the connectors individually (W0133-1634589) for a few dollars. Would they just crimp on to the end of the wire? Should I be unravelling these wires to some point away from the engine and replacing them somehow? I believe that these wires are part of the expensive engine wiring harness, which I don't want to replace, as I have no other problems.

mbdoc · #2 09-06-2011, 09:02 AM

Sounds like a bad engine wiring harness

Steve M · #3 09-06-2011, 09:22 AM

Went to the dealer and ordered three parts which make the connection between the wiring harness and the coil pack:

202 540 60 81 Plug
002 545 99 26 Bushing
004 545 09 26 Bushing

Total cost around $20, or something like that. I should have them tomorrow. When I take off the old connector, I will try to re-wrap the wires with tape to make sure they are not touching anything. I think that the new connector will just crimp on. I am obviously not the first person who has done this, which gives me hope that the connector may in fact be my problem. A new harness lists for $ 1,290.

I will post my results.

porkface · #4 09-06-2011, 11:24 AM

if the harness is coming apart where you can see it, imagine what you can't see. the more you move it, the worse the damage will be. go here and look at the pictures-

http://www.mercedesdefects.com/2008/04/wire-harness-defect.html

i'm not involved in this site and don't have any agenda regarding the damage. just trying to point out the damage is worse than what you see. good luck, chuck.

Steve M · #5 09-07-2011, 01:24 PM

First the Background: After reviewing my files, I realized that this car had thrown a CEL 4 years and 30,000 miles ago, due to a P0304, cylinder 4 misfire. The indy who worked on it replaced the plugs and the wires ("OEM Beru Wire Set $164.00") at that time. He expressed questions and concerns about the wiring harness. But like I said before, it's worked fine since then-in fact, if I had just not done the scheduled plug replacement, I would probably be fine. This all makes me think that I did not carelessly break something when I changed the plugs, but rather just disturbed something that wasn't able to handle it.

Also, I tried plugging my handheld scanner into the OBDII connection, and it doesn't want to work. I don't think I'd ever personally checked codes on this car, but others have.

Now for the plan: I went to the dealers to get my three parts today. First I asked him to double check the price on the harness. Now he tells me it is $1,840 - part # 202 440 18 05. I am obviously more determined than before to avoid this expenditure.

The main part that I ordered yesterday was the plug, part# 202 540 60 81, which cost $16.00. It was actually substituted with 026 545 90 28. Interestingly, it is the exact plug as I have, but it has a really nice 2 meter long pigtail on it! So there is no need for the two bushings, which go in the plug. It is apparent to me that I can use this assembly to bypass the wiring harness completely (avoiding some difficult close quarters work on fragile wiring by the coil) and going directly to, .....uh, ...uhm, the ECU?

I have some homework to do. I will get the wiring diagrams in front of me, but I expect that I will have to get this pigtail attached to the engine side plug that attaches the wiring harness to the ECU. This may be difficult.

As always, thanks in advance for any thoughts on this approach.

Steve M · #6 09-08-2011, 12:52 PM

I went through the diagrams and descriptions in Alldata, and am a bit reluctant to move forward. It turns out that one of the conductors in this plug goes to pin 9 on the 44 pin ECU connector. The other is spliced within a few inches of the engine (within the harness) to the other 2 coil packs, and then goes to the other side of the engine compartment where it is attached to an unfused terminal block directly in front of the fusebox.

The way the sytem works is one side of the coil pack always has 12V, and the ECU grounds the other side when spark is required. Both plugs on the coil pack spark at the same time.

There's no way I'm touching the wiring harness near the engine. I can run one conductor from my new plug over to the terminal block with no problem. From what I've read on this forum, though, it may be very difficult to disassemble the 44 pin connector at the ECU to attach my other conductor to pin 9. My alternative might be to unwrap the wire bundle a couple inches from the connector, and cut the wire (it should be the only black&white one in there), and splice there.

That should work, but it leaves a couple of useless wires in the harness, one of which is tied to 12V, and they are likely to be touching each other somewhere (my original problem, I think). I have to worry about them becoming grounded (if the're not already) and discharging the battery.

Another approach would be to cut away the old plug, slide heat shrink tubing as far down the existing wires as I can, tuck it away and then check to make sure that they are neither shorted, grounded, open, etc. Then maybe connecting my new pigtail to the old wires right next to the coil pack may do the trick.

Which reminds me, the wires on the new plug are tiny compared to the original wires. I'm hoping the originals were designed that way just to make them durable, rather than to handle sufficient current.

Any thoughts on which of these approaches is best are welcome. I'm thinking that I can try the splice at the coil pack approach rather quickly, and if it works I'm done without disturbing the rest of the vehicle. If it doesn't work (which means the wires are screwed up within the harness), then I probably have to try the other approach.

Steve M · #7 10-03-2011, 12:35 PM

Deep pocketed purists and NTSB petition enthusiasts should skip this post.

The upper engine wiring harness as it comes from the factory is really two wiring harnesses that come together at the 44 pin connector that plugs into the ECU. Mine is now three harnesses that come together at the ECU.

One of the replies to my original post challenged me to imagine what the wires I couldn't see must look like. I decided (with the harness in place) to start unwrapping the wiring harness to find out. Because of the way it is wrapped, I unwrapped from the ECU end, but I only unwrapped the bundle that included the coil pack wires (the one with the black, black & white and black & yellow wires.) I basically went all the way past the engine, stopping at the thermostat. The second bundle remains unexplored.

The insulation on the wires associated with the coil packs looked terrible, and all of the other wires looked brand new. I decided to replace just the coil connectors and the wires attached to them.

As I mentioned in a previous post, the dealer supplied connectors now come with wires attached. Because they are of a much smaller gauge than the factory wires I was reluctant to use them, but the parts guy said that this is the only application for the connector, so I went with it.

I was afraid to mess with the 44 pin connector at the ECU, so I cut the three wires in the plastic trough at the top of the engine compartment and soldered the brown wires from my new connectors there.

I ran the blue wires from the new connectors along the original harness over to the switched 12 volt terminal behind the washer bottle on the left fender, and put them on a lug which goes on the same post as the existing wires, but I have it coming in from the front of the car, rather than the left, so that it fits on the post. In the same area, I cut an inch out of the large pink and red wire so that I don't have stray 12 volts floating around in the harness. I removed the large pink and red wire from the coil packs to roughly the thermostat, so I basically have abandoned some pink and red wire in the undisturbed portion of the harness. If I had it to do over again, I would not have unwrapped the harness from the thermostat back to where the coil pack wires exit the engine.

Because I could see cracks in the insulation all the way to the white goop that all of the ECU connector wires disappear into, before I soldered the three wires near the ECU, I slid heat shrink tubing down as close to the connector as possible. I did not shrink it, though. I did put heat shrink over the soldered connections themselves, which I did shrink.

When all the wiring was done I wrapped it with Tesa wiring harness wrap that I got from the dealer - the same stuff the factory used. The car seems to work fine.

A few notes: I stupidly failed to disconnect the negative terminal from the battery before I disconnected the ECU the first time. I wound up with an SRS light that stayed on for 2 minutes each time I started the car. The dealer read some undervoltage codes and cleared them out, reset the SRS, and it seems fine.

Different people, going from the VIN, quoted three different prices for a new harness. They were, IIRC, $690. $1,290 and $ 1,840. The last one seems to be the correct one. It seems that you really need the P/N from the harness. I found my P/N on a tag on two brown ground wires attached to a terminal a few inches in front of the ECU. I understand that some people find theirs on a big tag on the ECU connector itself. The date on my harness was early 1995.

In addition to the indy who worked on this car four years ago, I have found reference online to other people who have had problems after changing spark plugs, which they resolve by pushing the wires around until the engine runs ok, at which time they close it up and cross their fingers. I chose not to do it this way, even knowing that it may work for years. In looking at my wires, I'm surprised that plugs 6 and 1 weren't mis-firing also.

Two years ago while tightening down the cross tube from the MAF to the throttle body on this car while it was running, the car backfired and the crosstube exploded, sending plastic shrapnel throughout my garage. I was fortunate that I was barely injured. I now attribute this event to a mis-fire caused by the harness.

Steve M · #8 03-06-2012, 08:33 AM

These are some pictures I took at the time - they might be helpful.

First is a picture of the new coil plug and pigtail, with the labels from it and the package it came in. It shows the part number that you order, which is different than the stamping number on the plug itself. You don't need the Bushing or Pin, and you don't have to solder anything near the engine.

Second is the new plug with the sheathing stripped back to the point where the two conductors go their separate ways at the wiring harness.

Third is my electrical test, before I wrapped everything up. Blue conductors go to 12 volts, brown to the ECU.

Fourth shows (on the right side of the bundle going into the ECU plug) how I slid heatshrink down as far as I could on the three wires that go to the coil plugs. The actual splice is about 18 inches away (I didn't want to traumatize the wires where they are smooshed together going into the plug).

This was done several months ago - the car runs fine.

gabzor · #9 03-07-2012, 01:47 AM

well your lucky it was only those coil plugs, and thanks for letting me know.. i'm attempting to repair my whole harness, seems my knock sensors have huge resistance, and weird ohm readings.. any idea if itll fix if i repair the wires? or can it be the plug itself? ohm readings are weird, but continuity is great.

Steve M · #10 09-16-2012, 07:52 AM

Just wanted to let you all know that the car was traded this summer, about 9 months and 6,000 miles after the repair to the harness. It was working perfectly at the time, so I would encourage anybody to consider my approach and repair only the damaged part of their harnesses.