Recently the engine on this car began to run very rough. It was mainly at low rpm's. At first I thought the head gasket was blown, but looking further into the situation, I think it is due to the engine harness breakdown. There are a number of articles online about this, but little technical investigation.
1. There are a number of harness part numbers. Be sure to check yours before buying one to replace, apparently they are quite vehicle specific. Mine was 124 440 29 33.
2. The harness has a number of layers which have their own behaviour over time. It is the inner wire insulations which seem to dramatically lose their elasticity over time-at-temperature exposure. Thus, the hottest parts of the harness break down first. The section inside the engine head which is enclosed under the aluminum cover has a rubber moulding over the wires going to the primary side of the 3 ignition coils. The insulation inside this moulded rubber breaks down first, and likely provides all kinds of shorting and cross-signalling to the ignition coils. Outward signs are minimal since the rubber moulding covers the break-down of insulation. Anyway, once you find a breakdown in this area, the rest of hte harness is well on its way to interior insulation disintegration. There are some narrower signal wires which are part of the harness. they go the the crank position sensor, knock sensors, etc. These are shielded 2-conductor cables with urethane outer jacket. The outer jacket remains in top shape while the individual wire insulations inside are seriously breaking down.
3. Dealer price for this harness in Toronto is $1780 new (jobber price is 15% less). Ebay prices are $900 new and $300-450 used. Quite a range. Price of harness repair by DIY - $100 at most. The problem with the repair is you need to buy high-temperature wire in various colours - That alone is a major deterrent for most DIY, I think, but the wire is easily purchased from Allied Electronics, or Newark, or perhaps Digikey, even Mcmaster Carr. I recommend teflon or urethane insulated stranded copper. I would take silver tinned, but not tinned is fine as it will be more flexible.
It seems most of the plugs used in the Mercedes are not too bad to rewire and solder. The problem is some plugs have completely disintegrated and fallen off, leaving only silver contacts.
In addition, the wires in the electronic throttle actuator are probably also breaking down if the part hasn't ben replaced in the past. Note this component differs based on whether the vehicle has ASR or not. Get the right part number. Seldom mentioned is the break down of the plastic vacuum lines as well. I found mind are cracking easily now. These are 4mm outside diameter. in some places, short lengths are emploed as couplings between rubber hoses. Replace these with copper or brass lines available from RC hobby shops or K&S tubing. Couple of dollars. If anyone knows a good source for plastic vacuum lines, please leave a comment.
Will post more if and when I repair this damn snake of a harness. Someone needs to find the components and provide an aftermarket kit or something to make this a whole lot cheaper.....no?
