Wiring harness repair?

[Bill Edwards]

Well welding a patch underneath a 1999 Mercedes hasn't gone very well.
The wiring loom at the left footwell went up in smoke.
Didn't think to take a picture but it's pretty bad.
Time can be spent on it if there's a chance for the car as it's a good motor.

I have two concerns:
1. There's a lot of wiring there but an awful lot of small stuff doesn't seem to have any trace markings. Most can probably be followed as there's copper left if it'll separate from the melted plastic but some have broken, so how to work out what goes where?
2. Everything being bare and covered in water to put the fire out - will things shorting have done damage to the ECU?
Is the car scrap?

 

[Robotstar5]

I'd split the loom either side of the damage, separate the wires and start marking what can be identified with white/light coloured tape and a marker pen, it reduces what's left to be identified. An electrical drawing may help, or seeing some of the modern ones, it may confuse things

 

[dobbslc]

As said I'd start by fixing what's obvious first then try to work out the rest.
Use some new wire and heat shrink it.
Is it a "cam bus" system on one of those? Could be more trouble if it is.

 

[The_Yellow_Ardvark]

Expose as much of the loom as you can.
Cut the tape or loom bindings and pull apart the loom.
Separate the wires. Some will pull apart, other will have melted together.

Use a new sharp blade and separate.
Now you will have al separated, modern looms when melted do not loose the thin wire in the thinner cables. That is the copper is a copper tin alloy.

You can obtain individual cables or as I have done jumped into a local breakers and cut out a decent length of matching loom.
Always go 50% longer on your salvaged length.

One cable at time, cut solder and heat shrink new wire in.
Stagger the joints and it will look neater.

You should be ok, as if you were welding you should of at least disconnected the battery.

A wiring drawing will give you colour and wire gauge. Then it will be a case of reading and tracing.

How big was the fire?

 

[WorkshopChris]

Without photos is tricky but i a guessing from your description you have a mess of burn copper and plastic with some obviously broken totally.

If you can carefully remove as much covering as possibly can then cut and number anything obviously broken to sort later.

Make up a couple to test cables for you multi meter with pins soldered on the ends so you can pierce the insulation.

Hopefully you can then pierce one side of the mess then work your way through the other side till you find any with continuity, note and number.

I have used the process in the past to help identify burnt conductors. It you can try to keep the burnt section as still as you can, burnt plastic seams to still insulate until disturbed then once the skin is cracked it may conduct.

 

[zx9]

Is it a "cam bus" system on one of those? Could be more trouble if it is.

I would think that there will be a data bus of some sort in there, my 2000 Vito had one, it had the round (12 pin?) connector. The data bus be the thin (white and brown stripe?) cables, I can not remember if it was a balanced signal requiring four connections (RX+ RX- and TX+ TX- ) , anyway good luck.

 

[Bill Edwards]

The battery was connected as I was using a surge protector.
For joining wires I was thinking of using the crimp terminals with integrated adhesive lined heat shrink, at least as good as soldering and quicker.

I will get a photo tonight but it's pretty bad.

 

[zx9]

And, they're much much bigger - how many wires are we talking here, 40? 50? That's going to be a fist sized lump either end of the repair section.

Crimps or solder joints, either way I would stagger them.

 

[Robotstar5]

Insulated crimps are going to make a bulky joint, I'd solder and heatshrink or use uninsulated butt connectors and heatshrink.

 

[piman]

Hello Bill,

crimps can make a good joint if they match the wire size, I suspect that many of your smaller section wires will be too small to make a sound joint though?

Alec

 

[brightspark]

solder and heat shrink . quicker and neater and a sound joint

 

[Spark plug]

As said get the soldering iron out if you want to make a good Permian to repair or you will end up with a massive ball of crimps.

 

[GW8IZR]

As said get the soldering iron out if you want to make a good Permian to repair or you will end up with a massive ball of crimps.

And a massive ball of potential failures in the future.

 

[Spark plug]

And a massive ball of potential failures in the future.

Guaranteed.

 

[the snooper]

solder and heat shrink . quicker and neater and a sound joint

Also stagger the joints so that it doesn't get thicker in one place

 

[normspanners]

Personally if it is that bad, I would follow the loom to the connector, possibly under dash then if possible get a secondhand loom of the bit that you need, cut it, then splice in the new section to save stripping out the rear of the car then you will only have one set of connections.

 

[Bill Edwards]

 

[zx9]

I think that is more than a localised repair, replacement look for the brakers as per Norm's comment above.

 

[GW8IZR]

I think that is more than a localised repair, replacement look for the brakers as per Norm's comment above.

Well if the loom is made from unobtainium or prohibitively expensive its certainly repairable but a new loom section as suggested above would be a better way.

Its a '99 merc so you have to weigh up the cost v long term, is it a quick fix or a car you plan to keep for life? If it were my lovingly restored 90 it would be getting a new loom.. if it were a nail of an E class used as my dog van it would be a repair job.

A repair to that loom using soldered connections and adhesive lined shrink can be as reliable as a new loom if done properly

 

[Robotstar5]

Can't see you driving that to work tomorrow