Failure of wiring harness in engine compartment leading to shorting and failure of Fuse 57.
Root cause was the wiring harness rubbing against one of the engine compartment insulation fixing screws spot welded to the bulkhead and abrading the insulation of the engine control unit wire until it shorted.
MB are proposing a repair by "cutting" out the damaged section of wire and soldering in a new section, the joints being protected by heat shrink tubing.
Anyone else encounter a similar situation ?
Would like comments on the proposed repair method; personally I have misgivings with regard to the long term effectiveness of the insualtion properties of the heat shrink tubing and it's weathertightness.
Edited by Pugugly on 18/08/2008 at 21:41
|
If done properly; a soldered, heat-shrunk, repair is perfectly acceptable. It's a lot more durable than many of the standard in-loom joints.
|
>If done properly; a soldered, heat-shrunk, repair is perfectly acceptable.
That is the key - a properly soldered joint. How often your average MB auto electrician picks up a soldering iron is another matter.
If there are only a few wires the best solution is to use uninsulated butt crimp joints to give mechanical strength (and a few extra mm of slack) and then solder and heatshrink them.
In harsh or hazardous environments then a Scotchcast can/should be used around the whole loom.
Kevin...
|
Kevin
I think a Scotchcast is a bit OTT on an engine-bay loom. You might be surprised how often an auto-sparks uses one of his many soldering devices - and how good they are with them.
A wire bundled in a loom suffers little mechanical stress and is unlikely to even get wet; quite frankly, even a well done twist'n'tape will outlast a modern Mercedes.
|
>>and how good they are with them
It's dead easy on a well lit bench, with a magnifier and a nice temperature controlled solder station, but not so good when you're upside down in the footwell struggling under the dash.
>>A wire bundled in a loom suffers little mechanical stress...
for most of the engine loom, I agree with that. I would, however, be keen to avoid a joint of any kind in the part of the loom that spans between the engine and body.
|
|
|
>I think a Scotchcast is a bit OTT on an engine-bay loom.
Oh it's way OTT on an engine-bay loom unless it's something like marine or earthmoving kit. It was additional info that I really should have qualified.
>A wire bundled in a loom suffers little mechanical stress..a well done twist'n'tape will outlast a
>modern Mercedes.
But it's hardly professional for the extra 30 seconds it will take to crimp and solder, is it?
I can imagine the colour of the air in your workshop if you'd spent half a day tracing a problem to find a dodgy twist'n'tape.
Kevin...
|
Yes; it wasn't a serious proposal; as you say, crimp ferrule and solder is nearly always best - although I've done hundreds of temporary twist'n'tape joints over the years and, if done with an inch-and-a-half of greased tail, they're surprisingly durable/conductive and they'll never pull apart.
The colour of the workshop air would depend on who did the bad joint...? As long at it was nothing to do with me, I don't mind spending half a day - at £80 an hour......
|
|
|
|
|
|
>>MB are proposing a repair by "cutting" out the damaged section of wire and soldering in a new section, the joints being protected by heat shrink tubing.
If it's just one or two wires, the MB proposal is an effective repair. If the work is done properly, it will be fine, and will last for many years. Depending upon which wire it is, and where the short is, it might be better if the wire were replaced from before the short to the nearest connector, hence, the repaired wire would have only one solder joint in its run, rather than two.
The other thing to do is to check to see if there are repair sections of the wiring loom available - I know coil wiring repair kits are available for older E320 engines, which save the owners fitting entire engine wiring harnesses.
What would be of more concern is if the shorting has caused any damage to control units which are connected to the affected wires.
|
If there is enough slack in the existing wire, just one cut and solder joint will be required, but the repair must leave sufficient slack in the remaining wire so as not to cause stress in the remaining connections. Two joints is also perfectly acceptable. The ends of the wires to be jointed must be tinned first to ensure best possible soldering. Dirty wires do not solder well causing a resistance in the wire with possible heating and melting of the insulation.
Heatshrink over the jointed sections is common practice as long as there is plenty of overlap of the heatshrink onto the remaining sound wire insulation.
After they have done the solder repair with two lap joints, I would put a large blob of clear silicone sealant over the the errant screw head to protect the repaired wire.
This type of repair is perfectly acceptable on aircraft, never mind cars.
|
What runs off fuse 57?
As the car is a 2007, you'll still have at least 18 months warranty left. This should be plenty of time to make sure that the shorting has not resulted in consequential damage elsewhere in the electrical/electronic system.
|
What runs off fuse 57?
I think, the engine management ECU.
|
|
|
|
