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Old 06-10-2010, 04:15 PM   #1
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W124 wiring loom DIY?

Hi all,

Well the inevitable has happened and my wiring loom is kaput. I am aware that a new one is about 600 so, as I have some time but not much money, I was contemplating having a go at fixing it myself.

I'm no electrician but I'm reasonably competent with a soldering iron and I'm wondering just how hard it is. Clearly, the biggest problem would seem to be the various sealed connectors, which I believe you cannot buy on their own. Is it simply a case of cutting them open as neatly as possible, soldering in the new wires and then resealing them (glue?)? Are there any connectors that are really tricky?Are any types of wire more suitable than any other? What sort of glue should I use? What sort of casing should I put round my lovely new bunches of wire? Any other issues?

Or should I just start saving for a proper one?!

Any suggestions gratefully received, many thanks.

Yours perhaps naively,

Simon
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Old 06-10-2010, 04:52 PM   #2
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Hi there Simon,this was posted the other day by Nick Froome



The first part to fail is normally the loom from the ECU to the coil packs. The "valley" between the cams is a pretty hot place and the loom degrades and shorts out, eventually killing the output transistors of the ECU

MB make a loom replacement section for 7 and you'll need three for an M104 engine. The tails are long (2 metres approx) so you should have enough length to go back to the ECU. Alternatively you can join into the loom on the section between the engine & ECU

The replacement sections comprise a plug & two-core cable - which looks suspiciously like mains cable as it is a brown & blue twin-core cable in a black sheath

Nick Froome


Have a look at this thread W124 Engine bay wiring looms
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Old 06-10-2010, 05:09 PM   #3
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When you crack the loom open you will find plastic sheathing detaching itself from the wires throughout its length. Of course given enough time you can re make the loom but are you confident you will mirror everything exactly? You could, I guess, do a section at a time over a few hours.

I have my old one in the garage from when it was replaced with a brand new one. I intend to renew it but even if I do will I have the confidence to attach it to the car and try it...I do wonder.

ultimately is it worth your time against the money? Replacing it now will probably mean for the rest of the cars life you now it will not be a source of problems.
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Old 06-10-2010, 08:58 PM   #4
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A new loom (with all the right connectors) is just over 400 why would you want to try to repair an old loom bit by bit with all the risks this involves. Get in wrong and you will fry the ECU which costs a lot more than 400.
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Old 08-10-2010, 02:04 PM   #5
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Thanks for the replies.

Nick, I fear that simply replacing the bit that sits in the 'cam valley' won't really help as I can see considerable disintegration of individual wire insulation in at least 3 other places.

Scillyisles1, where can you get a new loom for 400?

Cheers,

Simon
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Old 08-10-2010, 04:42 PM   #6
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One thing to remember is that solder is strong but brittle. So although soldering in new sections of cable will work it might not work indefinitely. As the joints flex and vibrate with engine movement and heat you may find after a year or so you start to get weird issues that are caused by solder fractures.
Its usually best to use some sort of mechanically crimped connectors.
If you do want to solder you can buy self adhesive heat shrink tubing. It just slides over the cable before you solder. Once you have soldered and it has cooled then slide the tubing over the joint and heat upi with the tip of your soldering iron usually works. This shrinks the tubing and activates the adhesive thus sealing out moisture. The tubing also helps make the joint a bit stronger.

Good luck with what ever you decide. You could just replace a single cable at a time, that way you don't get anything wrong. It may be time consuming but you will get it right.
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Old 08-10-2010, 06:55 PM   #7
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Quote:
Originally Posted by Williamwoo View Post

Scillyisles1, where can you get a new loom for 400?

Cheers,

Simon
I wondered that too?

Simon. A few years ago I made a 320TE W124 loom using the duff ones from my car and one from an R129 I was kindly donated. The donor wires came from a complete and unused 124 main loom bought from BlueStarBill on ebay for 15.00. It came complete with miles of tube sleeving which I carefully cut and re-used. My rot went all the way back to the ECU plug. I'll post some pics up when I get them sorted but is was a fairly easy task if taken steadily one wire at a time. I made several diagrams and took pics of everything as I went along and noted pin numbers and colours. There are several cables in the loom made from co-axial wire with sealed plug ends and they were unaffected by the rot so were re-used. The biggest prob is opening some of the sealed plugs to get at the pins.

The main ECU plug with the top cut off. For those who think the rot stops at the coil section.



The plug for the MAF



And opened up.



The main culprit. This is the plug to one of the HT coils. When you switch on the ignition the big pink/red is instantly live. When the insulation breaks down and it touches the trigger black/yellow....pop goes the ECU.





Two looms. The old at the rear and the new taking shape with it's new plastic casing.



Sections for the coil packs made from heatproof cable (Maplins) with fire resistant sheathing.



The rewired Maf plug with reclaimed sheathing and tiewrap securing before the top is glued back on.





.

Last edited by Druk; 08-10-2010 at 07:24 PM.
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Old 08-10-2010, 09:46 PM   #8
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If you know the part number for the wiring loom then you can type the part number into the Inchcape Mercedes Benz parts on-line system.
I did that recently for my 1996 E320 Cabriolet which has A/c. 5 speed gearbox etc and the price quoted was just over 400 inc VAT.
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Old 08-10-2010, 10:12 PM   #9
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Fair enough if that is what they're selling it for. But I ordered a small part for a 107 flagged as costing 1.00 and they emailed back saying it wasn't stocked and if I wanted one ordered it would now cost 13.00. (and the colour they said was available now wasn't').

I'm not saying making a DIY loom is for everyone, but I'm no electronics guru...just slightly handy with a solder iron and a pen and paper. It's actually not that difficult once you get started.

Last edited by Druk; 08-10-2010 at 10:15 PM.
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Old 08-10-2010, 10:27 PM   #10
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Didn't someone (Andy k IIRC) have some kind of deal with his local dealer for discounted OEM W124 looms?
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Old 08-10-2010, 10:30 PM   #11
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Didn't someone (Andy k IIRC) have some kind of deal with his local dealer for discounted OEM W124 looms?
500+vat W124 Engine bay wiring looms
Still good enough though
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Old 08-10-2010, 10:44 PM   #12
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Thanks, I couldn't find that thread.
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Old 09-10-2010, 01:06 PM   #13
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Fair enough if that is what they're selling it for. But I ordered a small part for a 107 flagged as costing 1.00 and they emailed back saying it wasn't stocked and if I wanted one ordered it would now cost 13.00. (and the colour they said was available now wasn't').

I'm not saying making a DIY loom is for everyone, but I'm no electronics guru...just slightly handy with a solder iron and a pen and paper. It's actually not that difficult once you get started.

I suppose that is a possibility but I have been buying a lot of parts recently and before ordering from the dealer, I check the parts price online at Inchcape Mercedes Benz parts site. In every occasion they have been spot on in terms of price. The parts I have been ordering are not your run of the mill parts either - all parts have been special order with many having to come from Germany.
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Old 17-10-2010, 06:41 PM   #14
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Thanks for your various comments, expecially Druk for the photos.

Ok, I've got the old loom out and things are not actually too bad apart from near the engine. The MAF lead looks very similar to yours Druk and the insulation is almost powder-like for the first foot or so. However, from there, it is fine and indeed all the wires by the time they get to the main ECU plug show no signs of any cracking. Is it wrong to assume therefore that it is solely because of the heat that the insulation disintegration (I quite like that phrase!) occurs? In other words, my plan is to simply replace all sections that are bad but joining them in near the ECU plug but not in the actual ECU plug itself as I feel I'm more likely to do more harm than good if I mess around with the latter. Incidentally, my ECU plug looks very different to yours, Druk - could it be because mine is a slightly newer E320 as opposed to 320TE?
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Old 18-10-2010, 05:31 PM   #15
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If I remember correctly, In early 2006 I sent to Derek in Bathgate (Druk?) the old wiring loom from my 1994 W124 E320 cabriolet as he said that he is going to make up a new loom. If Druk is Derek then perhaps that loom he refers to came from my W124 E320 and not a W129.

The new engine wiring harness in 2006 cost me 496.33 from MB Exeter and I asked for a total replacement even though the garage recommended a part replacement.

It is nice to know that old parts of cars are being recycled.
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