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Mercedes 420 SEC 1989
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Discussion Starter #1 (Edited)
Hi guys!

I finally decided to do the engine wiring harness on my 300 SE 1991 w140, it has a 3,2 Litre motor, so I guess it's the same as the later S320. I'm not sure if all the steps are the same on a 2,8 Litre motor.

It's a job well worth doing, as it costs from about $550-750 to buy a new engine wiring harness online, MB part no: 140 540 10 32. The job of taking it out and putting it back in was about 2 hours in total, but it's easy and if you decide on just buying a new harness instead of rebuilding yours, you can at least save some bucks there. Rebuilding the harness itself is kind of a daunting task, and at one point I seriously regretted not just having bought a new one, but you'll be happy in the end when you see the results.

There are two DIY instructions out there, on how to rebuild the engine wiring harness / MAF wiring. They were great help for me, so I recommend taking a look, but I found there were some steps that could have been covered better in those manuals. Or well, I like making things idiot-proof :)

By ScrapingScrap: http://www.benzworld.org/forums/attachments/w140-s-class/178460d1211131170-knowlege-base-1992-500se-loom-rebuild.pdf

By V12uberalles: Rewiring the MAF Sensor in a W140 Mercedes Benz

Okay. I would recommend that you tackle this job at a time where you won't need your car for some time, as it will take much longer than you expect it to. I'm guessing you could do this faster than I did it (about 16-17 hours in total), but I wanted to be meticulous :)

You will need:
About 50-60 meters of silicone based wiring, 0,75 mm^2. Where I bought mine, it only came in 100 m rolls. Set me back about 70 dollars. I bought all black wire, as 20 multicoloured ones would have been more expensive than buying a new harness.
Various tools, T20 torx bits, lots and lots of electrical tape (+self vulcanizing tape).
A digital camera to take loads of pictures of everything you do, so you know how to put it all back together right!
A soldering iron + tin
A good place to work on this for a few evenings in a row.

First off:
Find the main connector on the passenger side and slide it off, with the lever on top. I also removed the ETA unit to see the condition of the wiring for that.

Remove the weather protection over the wires leading to the middle of the engine bay.

Remove the air intake housing (two 10mm hex-nuts on the sides, then wiggle it off).
Remove the MAF connector.


On inspecting the ETA wiring loom, it was the updated version, with good solid rubber insulation. No need to do anything more than tape it back up and put it back in the car.


There are three other connectors on the same wiring loom as the MAF connector, their part numbers are:
0195455428 (unknown function)
0125450428 (water temperature sensor? REMEMBER TO TAKE NOTICE OF IT'S POSITION. It can go on both ways.)
126540308 (unkown function, 4 pin sensor, REMEMBER TO TAKE NOTICE OF IT'S POSITION. It can go in any way you want it, so it's important the right sockets go on the right pins, when you install it back in the car).


The wiring harness can be split in two sections:
1.the section going to the above mentioned MAF sensor and other various sensors.
2. the section that contains the injector connectors and the changeover valve connector (I think.. maybe it's the camshaft position sensor?).

There are a lot of plastic straps that hold the wiring in place, just remove those, buy some new ones in your local hardware store.

The Changeover valve connector is behind the plastic cover on the front of the engine, with the star on it. It's simple to remove the cover, just push in the plastic pins on the sides.


With that removed you'll see the sparkplug wires and a connector on top of the (camshaft?). Disconnect that connector.


Mine had obviously had some sort of an accident and someone had tried to fix it, without much luck. It was also shorting, and I have a sneaky suspicion it is what caused my transmission to be rather harsh between gears. The wiring to the connector, leads behind a plastic cover on the front of the engine, to protect the spark plug leads from the belts underneath. It is held in place by 4 screws, locations shown in the picture.


One of the screws, closest to the sensor, is hard to remove, hidden well inside a hole. I had to use a T20 torx bit to get to it, as it was the only thing that could reach it. With the plastic cover removed you can get the wire out. When I installed the plastic cover back, I could only put 2 of the 4 screws back. The other 2 wouldn't go in :p


Push on the sides of the injector connectors and pull gently upwards. They can be a bit hard to get off (especially injector 1, closest to the windscreen).




With the wiring harness out, I could start mapping out the connectors on the Main connector and where the pins lead to. It actually took a good while, so I'm supplying you with a map. But for good measure, you should have a voltmeter handy to check your own harness. I found out that the connector for injector 3 was shorting, and the changeover valve connector. My car was thus running on just 5 cylinders :p



Now to work on dismantling the connectors. The MAF connector is covered in v12uberalles manual. ScrapingScrap has some interesting solutions on the other connectors.

I found that using a hairpin, that I cut down to equal length on both ends, functions very well when you have to get the pins out on the main connector.


The wiring is held in place with some white goo, when you pull the rubber off on the end. Try to get as much of that off as you can.



Then you will begin to see some yellow hard foamy plastic. That needs to get out as well, but it's a pain in the a***. Get a sharp knife and just cut into the black plastic portion of the main connector. You'll be able to glue it back together nicely later with superglue :)



Remove the one screw holding the plastic retainer inside the main connector. Gently pry the retainer out, while pushing the wiring in. It's fiddly.


The retainer is comprised of 3 parts. One outer part, and two inner.


Push something over the little plastic nipplets preventing the inner retainer to be released. I found that old auto fuses were perfect, but also you can just slip a piece of hard paper over each niplet.




Push the hairpin in on each of the sides of each connector pin on the main connector, then pull on the wire. Do this with each pin.


Pry the wires off each pin. You'll solder the new wires back on the pins, but be careful not to damage the "claws" at the back of the pins, as those are useful holding the new wires in place.
This is a picture of the injector sockets, but they are pretty much the same as the pins on the main connector.



Part 2: Removing the injector connector sockets, rebuilding the connectors and putting all the wiring back together.

==> Next post
 

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Mercedes 420 SEC 1989
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Discussion Starter #2 (Edited)
The MAF connector is covered in v12uberalles guide. Refer to that. Just remove the retension ring, then cut it in two where the joining lines are. It took me a good while (1½ hour), I had only a knife doing this. A dremel is probably a lot faster.

Lots of cracked insulation:


The Injector sockets are actually marked with connector numbers under the protective rubber on top of them. I didn't know that from the start, so I numbered them 1 and 2 like so:


Thankfully, it was the same numbering as on the injector sockets :) This is a picture of injector socket no. 3. It was showing a short when I measured it with a multimeter. Wonder why? Notice the numbering (1 and 2) on the tip of the socket. Important for when putting this all back together.. which is the fun part :) When removing the connectors from the injector sockets, push the hairpin REALLY hard in on each side of the connector, until you can feel it almost "clicking" in place. Then you can pull on the wire, it shouldn't take much force. If you have to use a lot of force, and it won't come out, then you know it's because you haven't pushed the hairpin in deep enough. It's harder to remove these than the pins on the main connector.


The injector sockets and the changeover valve sensor, have a common "ground" connection, that leads to pin 16 on the main connector. When cutting the wires down to correct length, I recommend you use your own measuring but you can use my table (see earlier post) as a reference. The wire for pin 16 was 100 cm from the changeover valve sensor connector 2, to the common point for all the wires. From there you can just join them together and then spice in a wire (about 80 cm) that leads the rest of the way to pin 16 on the main connector. You'll understand when you get there ;)


All the old wiring, with the rubber protection removed (I kept those and reused them, but you can just wrap it up in self vulcanizing tape or buy new rubber hosing).


The changeover valve connector. Mine was completely shot, it was shorting and someone had tried fixing it with some glue/paste. It has a part number: 0115457128. It costs about 4 dollars to buy it on the internet, but I couldn't wait for it to come in the mail. Maybe I'll replace it later, but for now I just fixed it up a little bit better myself.



The water temperature sensor? Part number 0125450428. Easy to pry open and close up again. Nothing to it really. I soldered out the old wiring and soldered in the new wiring. Easy PC :) All these connectors had numbering on them to indicate where each wire should go.



There was a connector next to it, with just 1 wire in it. Part number: 0195455428. Don't know what it does. It was as easy to open as the other one and the same procedure to replace the wire.


ScrapingScrap covered this connector. Refer to his guide. It's just cutting it over near the middle, where it is thinnest. Didn't take that long with a bread knife :)

There was this yellow, hard foamy plastic protecting the wires, just as in the main connector. PAIN IN THE FUC**** A***!!! Pushed it in and with patience it came out:



MAF sensor all wired up:


Like I said, I used all black wires, so it is important to keep a track of each wire. It takes alot of time though, you could just wire all the wires in, don't think about which one is to which pin on the main connector, and then in the end when you got all the wires near the main connector, mark them up by testing each wire with a multimeter. That will probably save you about 2 hours work, trying to keep tabs on each wire and where it's going. I figured that out in the end :p

Put some silicone glue on the connector and plastic straps on the end to hold it in place. It needs minimum 24 hours to get stiff. Don't do anything to it for at least 24 hours.

This happened to me because I used too much silicone glue. Not to worry though. Be sure it's really "dried" and not sticky. If you try to remove it when it's still sticky, you'll just be removing some of the glue and the connector won't conduct current sufficiently from the MAF sensor. I used a paperclip to carefully pry out the silicone, then pliers to grip it and pull it out.

After I got the silicone out (be patient, it took me a while):


A good advice had maybe been: Don't use too much silicone glue! :)

Part 3: NEXT ==>
 

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Mercedes 420 SEC 1989
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Discussion Starter #3 (Edited)
With all the wires soldered to each connector, the rubber hosing put on, all the wires and connectors lined up as before (that is why it's important to take a lot of pictures), you can start pulling the wires through the main connector. Solder the pins from the main connector on. Then align each pin to the correct number on the main connector. It helps to have the inner parts of the harness to guide the pins in place. Also keeps them from touching eachother and short :p Put it all back together! Make sure the harness is securely and correctly in place (as it can be a bit hard to get it back in).

Quick note on this next picture: I made the mistake of putting the wrong part of the inner harness on first. You should put it on as shown in an earlier photo. I made quite a bit of mistakes doing this, and that is also why it takes time to do this job :(

Like so:





My workplace for the last week:

All done:

Back in the car, process reverse of removal :) That is also why it's SO important to take pictures!!


Tested that it worked and it did! Put everything back on:


Have driven the car for two days now. It has made a HUGE difference to the car, now it's responsive, feels light, accelerates like a rocketship (or well.. a lot faster than before) and the gearshifting is smooth. I LOVE IT! :)

There is still a problem with a rough idle, when it gets hot. The rpms fall to about 450 when it's in gear, and sometimes feels like it's about to choke/die. The oilpressure also falls to about 1½ when it's in gear, and is only about 2 when it's in idle. I would have thought 3 is the optimum all the time, regardless of if it's in gear or not? I know that the car is leaking a bit of oil, probably from the headgasket, but I top it op with oil every other month or so, and the oillevel is fine right now. Don't know what's wrong :p

But the job of engine wiring harness rebuild was well worth it!!

Hope this helps someone out there do their rebuild.
 

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1994 S420
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Great work. Your photos and notes illustrate how detailed and complex the work is.

At a sixteen hours of bench work....
If you paid a typical mechanic $50 per per hour, that is an $800 rebuild
If you can find anyone to do it for $20 an hour cash, it would cost $320 to do.

If you could do it in 12 hours instead of 16 ?
$600 on the high side
$240 on the low side.

Kind of explains why the darn things costs so much !!!
 

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216 with everything. 2002 SL500 with everything. 2009 SL500 with everything.
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Congratulations and very nice work to have done it the way you have.
 

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1992 300 se /245 45 18
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can any one help, i lost position of the (4 pin sensor) im almost done with my harness.
 

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1992 300 se /245 45 18
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im sorry to bother you but i lost the position of my (4 pin sensor) as you mentioned to take notice of could please help
 

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Mercedes 420 SEC 1989
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Discussion Starter #10
Do you mean which way it should go on?

If you take a look at one of the first few photos in this guide, you'll see the 4 pin connector, as it is supposed to be fitted. There is a long line along the top of the connector, that should point upwards and towards the windscreen.

Hope this helps.
 

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Got it, I can see as you posted i can go on anyway. there are two lines on the sensor,one with the part no, and one that is just blank that leaves me with two ways this can go on.


Thanks again man you saved the day and me $800-$1000 that the dealership quoted me.
 

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Discussion Starter #13
Got it, I can see as you posted i can go on anyway. there are two lines on the sensor,one with the part no, and one that is just blank that leaves me with two ways this can go on.


Thanks again man you saved the day and me $800-$1000 that the dealership quoted me.
Glad I could help.

From my high res photos from the project, I think the "blank" line goes up and the part number is facing down. My car is outside in the -11 degree frost, so I really don't want to go outside to check :p
 

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:bowdown::bowdown::bowdown::bowdown::bowdown:

Paul, my respect goes out to you comrade!

I have to do the exact same job it seems, since I have not been successful at finding a replacement..

Thank you for documenting it so well, it might just come in handy for many people out there, including me :) I saw Scarping's write up, but that was for an S500. This is a good guidline for me since you have the exact same engine as me :D

Cant wait to get this wire harness issue out of the way!
 

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Discussion Starter #15
:bowdown::bowdown::bowdown::bowdown::bowdown:

Paul, my respect goes out to you comrade!

I have to do the exact same job it seems, since I have not been successful at finding a replacement..

Thank you for documenting it so well, it might just come in handy for many people out there, including me :) I saw Scarping's write up, but that was for an S500. This is a good guidline for me since you have the exact same engine as me :D

Cant wait to get this wire harness issue out of the way!
Yeah, the wiring harness is a bitch :) But it's not a difficult task, takes time and patience, not that much skill (since I could do it).

Even though you have my photos to help guide you, I'd recommend taking your own pictures. I used the photos alot during the project to try to remember how everything goes back together.

Good luck! :)
 

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Great write up there sir, far more detailed than mine!

So we now have a how-to for a v8 and a 6 cylinder car - who wants to rebuild thread one for a v12?! :)

Could a mod please copy this to the Knowledge base?
 

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Discussion Starter #18
Great write up there sir, far more detailed than mine!

So we now have a how-to for a v8 and a 6 cylinder car - who wants to rebuild thread one for a v12?! :)

Could a mod please copy this to the Knowledge base?
Thank you! But I would never have done this if I hadn't seen your guide in the first place. You're brave to have taken it on, without having any real help.
 

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as paul said scrapingscrap execellent job i was back and forth looking at both of your how to's. im just waiting for the silicone to dry so i can pop it back in. I will post my results
 

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finally its done. scrapingscrap and paul Ax you guys need a donation setup, i know it may not be much but i got $25 dollars a piece for you guys just let me know where to send it to.Now it took me 2 weeks mainly because i din't need my car really i also kind of took a short cut because i got a harness from a junk yard i took out of a 1993 300 ce i just cut off the excess wires and sensors i didn't need. I admit i should have used new wires because they were the old plastic caoted/insulated ones but they were in good condition.The injector wires were all i used, i just cut the fuel injector plugs off the old harness and soldered them on the new harness and ran all new wires to my MAF,water temp sensor ect.. Only thing is my silicone would not not stay even after 24hrs so i used some 60 second epoxy glue by versachem i let it dry clamped tight for 24hrs just to be safe.Well enough of that, that car runs like a champ. my transmission shifts smoother the car gets up like never before. Thanks alot again you guys. And im dead serious about that $25 dollars.
 
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