Any (automotive) electricians here? - PeachParts Mercedes-Benz Forum

Stretch · #1 01-09-2016, 06:09 AM

G'day Folks,

I'm posting here because tech help these days is a bit of a quiet backwater.

I have been fault finding the WOT sensor wiring on my W201 and I have measured (and continue to measure) 0.4 ohms resistance from one end of a wire to the other. According to the FSM that is out of specification =>

<1 ohm ==> open circuit (I think that means short circuit)

So I busted out the nail scissors and cut away the outer sheaf on the wiring loom...

...and I found this

Not the best picture in the world but I've made worse!

There's visible blackening of the insulation through out this part of the loom. (Oh sorry just had a thought loom == harness!)

I am assuming that this is "burning out" but I would like to check with others who may have seen this before.

This part of the loom is close to the engine but actually far enough away for it to have been a heat from the engine problem. This blackening is more or less in a bend in the loom as it comes away from the bulkhead towards the engine.

Any help tips - "how to end it all" suggestions etc are welcome!

Stretch · #2 01-09-2016, 07:47 AM

I'm getting myself confused.

The specification in the FSM is as follows

The specification is that the resistance has to be lower than 1 ohm.

It is.

It is 0.4 ohms.

So it is OK.

However, even though I thought it wasn't there is the evidence of the blackened wires that I have found.

Does this look like burning?

pmckechnie · #3 01-09-2016, 10:38 AM

Stretch, I am not familiar with the 201 (never owned or worked on one) but worked on auto electrical systems for many years. From what I can make out in the pictures it is difficult saying exactly what caused the discoloration on the wires.

What I would do is look carefully at each wire and see if the discoloring goes all around the wire or only on one side. As you look, if the discoloration is only on one side, then it is not the problem, just at the wrong place at the wrong time.

If all the wires are discolored only on one side then the heat that caused it was from an external source.

If the discoloration is the same all the way around then that circuit has been very hot but if the insulation is still intact it is not causing a problem. You would need to trace that circuit to confirm what it does. In most cases, you will see the problem when you get there.

I know I didn't answer any question but I hope it helps you in some way.

That discoloration is not a problem, just possibly a clue to a problem. Also keep in mind the car is 23or 24 years old. Who knows what someone else has done that possibly caused the discoloration..

PaulM

Stretch · #4 01-09-2016, 10:59 AM

Thank you very much for your reply Paul.

The discolouration is only on one side. If the source of heat was from inside this bundle of wires I think the only wire it could be would be the one for the inlet air heater. Apparently this draws in excess of 21 amps for 1 or 2 seconds after the engine has been started. I can test that once I get everything back together.

Because I had gotten myself in a muddle with the specifications in the FSM I went back and tested the WOT switch and found that that is not in spec. So I at least have something to concentrate my frustrations on!

With your comments about the discolouration I think I'll wrap everything back up in self amalgamating tape and just keep an eye on it for now. (Fire extinguisher can sit up front from now on)