I recently rebuilt the wiring loom on my E320 W124, which I presume is the same engine as yours. It's not difficult as such (you just need to be handy with a soldering iron) but it does take a while and I'll be happy to explain in detail what I did, but I thought the disintegrating insulation issue only applied to W124s, not the W140?
All W140s of every engine variant had the dodgy soy bean wiring insualtion up to model year 1995. 96-98 model years the problem was fixed from the factory with better wiring.
I've seen some cosmetically superb looking 500 and 600 models scrapped due to the wiring looms disintergrating. As has been noted many times, when they the main engine fails it has a habit of taking the ecu(s) with it.
For some strange reason, the w140 wing mirror wiring looms continued to use the dodgy wiring all through the production run even ont he final cars built in 1998.
I believe the loom for the w124 is slightly different for the w140 even though it's the same 104.992 engine. probably just the engine bay layout being different. Either way, the principles of fixing/replacing will be same.
Hi again pyth0n, This is what I did to replace/rebuild the loom on my W124 1995 E320 Take plenty of photos and ideally some video before you start, and intermittently as you progress. (Thank god for digital photography!). The wiring loom in question (at least on a W124) starts from the ECU that sits behind the battery on the right-hand side (viewed from the front) of the bulkhead. To remove the old loom: ·Remove the battery ·Disconnect the main plug into the ECU. This is a large black plug with all the wires from the loom going into it. It is held in place by a metal lever (and was very awkward to remove on my car) ·Remove all trim necessary to free the main cable as it nears the engine block. This is quite likely to be different on the W140. ·As the main cable divides into its constituent parts and each wire branches off, remove all clips and cable ties etc, and disconnect each plug in turn, ideally taking a photo of each for later reference. ·There are many different types of plug, and consequently differing ways of removing them. Some are a lot easier than others, but all are removable with a bit of perseverance and/or logic. With a W140, you might be at an advantage here as there is presumably more room under the bonnet. In any event, a small inspection mirror attached to a rod with a universal joint is very useful occasionally. Lay the old loom out, and take more photos, including close-ups of each plug, showing the colour of each wire going into it if possible. I also found it useful to draw a diagram and note the length of each set of wires. It’s now a case of replacing each individual wire. I bought new wire from Maplins, and always tried to replace like with like in terms of thickness (number of strands). I could get no more than 6 different colours though. The loom has many more than this but, using several different colours of permanent marker, I could create the same number of unique colour combinations as in the original, by ‘dabbing’ the marker every 5 mms or so along the length of each new wire. I of course kept a record of what colour wire I replaced with what (it was often not realistic to replace like with like – purple/black is hard to replicate for example). ·The coaxial wires do not seem to disintegrate and do not need to be replaced ·The leads to the coil packs pass to them through a rubber ‘block’. I did not reuse the block; instead I connected wires directly, ensuring I sealed them at the point they entered the top of the engine. As Nick quite rightly points out, many of the plugs are moulded. You cannot apparently buy these new separately so they must be reused. How you do this is up to you, but I cut each plug off leaving about 3 or 4 cms protruding, and soldered the new wire to this. Everything I soldered I subsequently sealed using heat-shrinkable sealing, which again is available from Maplins, in various different diameters. I also used plenty of Servisol silicone sealant wherever necessary. Some of the plugs are not moulded though and these can be prised apart, the old wires released using the soldering iron and new wires soldered into place. I sealed each new bunch of wires together using self-amalgamating tape. This is like insulating tape except that it is stretchy and, when wound round back on itself sticks and creates a water-proof seal. In the great tradition of Haynes manuals, replacement is simply a reversal of the removal process – well, more or less, anyway. I used small cable ties to try to keep it neat, and had to improvise on occasion where wires are now thicker than they were due to the application of the heat-shrink stuff and tape etc referred to above. Purists may no doubt frown at some of the above and I know that there are many other posts from more knowledgeable folk than me on this forum covering this issue, and of course a new replacement loom is preferable (I couldn’t afford one) and would certainly look neater. I have no particular automotive skills other than perseverance and a bit of common sense, but the car started first time after I put the loom back in (luck? – yes, probably), and has run faultlessly ever since (about 5000 miles in 8 months).