Pictures are forthcoming, as they are on my digi camera and I haven't downloaded that software onto my laptop at home yet. But curiousity can't wait and maybe someone can ID it based on description.
I was looking around the engine compartment today after checking the oil, and noticed a wire that crosses from the pass. side of the engine to a plug just above the t-stat housing. Well, the wire crosses through the belt tensioner parts and it really just looks like it doesn't belong in that area. I felt the wire and noticed that the "sheath" on it is all worn off, and one portion of it is tattered, with little shreds of metal sticking out ... as though it's been chewed on, which I have to assume happened because of its proximity to the belt tensioner and all that jazz. It goes from the top of the passenger side threads downward past the tensioner and then enters on the other side of the engine above the t-stat. What is this, what does it do, and is it supposed to be hanging there like that? There is also a broken clip clinging to the wire, but I'm not sure where it had been clipped.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

Possibly the temp sensor on the thermostat housing? (That is the one that delays the CCU and blower until sufficient heating.)

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Sam

84 300SD 350K+ miles ( Blue Belle )

That would make sense ... the CC has been a little early to kick in lately ... it delays a while but always starts blowing cold air out before it gets warm.

Another odd thing that's happening ... as soon as the CC heat kicks in, the temp gauge on the dash suddenly drops about 5 degrees. After that burp it continues back up to normal temp.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

Not uncommon...

Just consider the cold water that just got introduced into the engine, thermostat closed, from the etire heater core/assocaited lines.

Photos ... let's see if I can get this to work ...
Here is the wire ... this is the view from the front of the car facing the engine compartment ... you can see the belt at front (drivers side to the right, passenger's to the left. The arrow points to the chewed up portion of the wire.

__________________
1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

Here is a shot of the wire's two ends ... the main photo shows where it enters above the t-stat ... I am pretty sure this is the water temp sensor (the blue thing). The inset of the photo (white outline) shows where it connects on the drivers side near the top of the engine ... the green switch is the temp sender that actuates (I think) the aux fans, at least that's what i always thought ... the wire I refer to is traced with a yellow line in both images.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

It definitely looks like the temperature sensor wire. It also looks like it either got caught in the belt or rubbed against the pulley. Someone with a W124 should be able to take a look and tell you where it should be secured so it stays away from the belt. Are you fans still working correctly?

I had a similar situation on the 617. The wires that reach around the engine get old and brittle from the heat and the insulation cracks. In this case, it appears that it was rubbing on the bracket directly below the wire.

Not a big deal but it should be fixed by getting out a roll of electrical tape and carefully wrapping the wire from one end to the other. Stretch the tape as you do this to get a tight seal to the wire. Naturally removing the wire from the switch will make the job much easier.

While this particular instance may not need it, I found out something within the last year that bears mentioning. Many of the connectors used on my car that have a plastic housing at the end can actually be easily disassembled and repaired. Looking carefully at the housing, it can usually be opened with just a slight pry from your pocketknife. Then, exposed are a number of pin connectors with a wire soldered to it. A little heat removes the wire, exposing a hole drilled in the pin. A new wire, with about a pin diameter's length of insulation stripped back can be inserted in the hole and soldered on. Then snap the cover back on.

I've done this a few times and been very pleased with the results. My point is that it's actually fairly easy and fast and the results are great.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

As far as I know, the fans work fine ... at least that's what my mechanic said on last check ... I had the works done as far as checking out the cooling system this summer, because I thought my car was running too hot (90-100 on hot days) ... it turned out that my temp gauge was off about five degrees and it was not running hot. During that investigation I had him check the aux fans and he said they were engaging. Now, what I don't know is how he tested them (i.e., up or downstream from the damaged wire). Does this wire in fact run the aux fans? I know there are a smattering of temp sensors in the engine compartment and have yet to figure out which one does what.

__________________
1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

Thanks Brian. Will the tape do the trick even though there's actual damage to the wire, though? The wire itself appears shredded, so I wasn't sure if it should perhaps be replaced. Or will the tape hold it together enough that the damage won't matter?

Also, is there any particular brand of electrical tape I should buy? Obviously I want to make sure I use something that's going to stand up to whatever devoured the original insulation.

__________________
1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

If there are many broken strands in the wire, it might be prudent to replace it. You can do this if you follow Pete's recommendation above. I'm sure that 16 gauge wire will be more than adequate. At the opposite end, where it enters the wiring harness, you'll need to solder it and tape it carefully.

No special electrical tape needed. Just stretch the tape as you wrap it. The distance between each wrap should be about 1/8". You'll lay down about four layers of tape over the top of the soldered joint to thoroughly protect it. Make sure that you start the tape at least 1" away from the joint and end the tape at least 1" beyond the joint. The tighter you can pull and wrap the tape........the better.

I've also had some success with heat shrink tubing. However, the irregularity of the soldered joint doesn't readily lend itself to a very clean heat shrink job.........so........the tape will probably be best.

I did mine, on a 617, with heat shrink tubing as the heat of the engine caused the electrical tape to eventually pull away and left a sticky mess. It was a little more trouble, unsoldering the end connector, but was a litttle happier with the end result.

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )

I use heat shrink on all my connections now. My time is worth more than supplies, and solder and heat shrink means never having to say you're sorry.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

Heat skrink is the PROPER thing to use...notice that there is very little to NO electrical tape on a stock MB wiring harness...

I've been wiring up a bunch of marine stuff and heat shrink really does the trick.

Just use a heat gun to warm it up (lighter/match works good too)