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Oooh, that smell!

3K views 20 replies 9 participants last post by  dude99 
#1 ·
Guys, I'm a little anxious about what might be something wrong. Yesterday while driving the 300SE home I smelled what I thought might be a forest fire—or maybe an overheated clutch...like you might smell from a big, loaded bus with a manual transmission traversing a mountain. Not quite a burning wood smell, but definitely not oil, either. Thought it was outside the car.

Thought nothing of it until I realized, when I got home, that it’s my car. I popped the hood and looked around but can’t see anything. No evidence of a burn. I looked at all the fuses—nothing. However, when I stick my nose into the space between the fuse box and wiring harness, there was a reasonably strong something-burned smell. There was no evidence of smoke. Smells eletrical-ish, but I’m no expert. This morning the smell was mostly gone—but certainly not entirely.

So it’s a mystery. Anyone have any ideas what it could be? The only other visible anomaly I noticed while driving was that the temp gauge was about 10ºC warmer than usual (around 90ºC), and it wasn’t an overly hot day—80ºF. I had driven the car only a couple of miles to Best Buy and then back.

On a potentially related note (but not likely), in poking around I discovered a weird wire splice coming off the harness; it’s attached to a blue-and-green wire and seems to extend in front of the brake booster and then into another group of wires. However, when I tugged on it a bit, it pulled righto out—and is evidently attached to nothing. I have zero idea whether it’s related to the smell. I assume this has been there for a long time.

I’m afraid to drive the car now because (a) I don’t know what’s causing the burn smell; and (b) this funky wire thing has me concerned as the splice evidently went to another, uh, splice that, as you can see, goes to nothing.

I’m not even remotely qualified as an electrician, so anyone who has a clue and has an opinion or answer is really appreciated. It smells electrical, but then what do I know.

Any help is much appreciated! :beerchugr:

Oh, and happy Father's Day to the dads out there.
 

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#2 ·
My experience with three W126 SEL 560s is that the original owners spliced in telephones and radar detectors. I have done my best to strip out all of the non-original wiring. Look under your fuse box. That is an area of access often used.
The next issue that I always encountered is previous owners put in new stereo systems with hack job wiring. This issue is painful since it is not always practical to go back to original equipment. Sometimes yes, sometimes no.
You may never figure out the purpose of that wire.
 
#4 ·
Thanks, Tom. The only funky wiring that I'm aware of this car was a poor job fixing the AC about 15 years ago and as a result, it was hard-wired in the on position--which burned it out pretty fast. This car was bought new by my mother, so I know that the only after-market thing done to it was my addition of a Pioneer head unit about four years ago--and thus far no problems. This is why this problem is kinda baffling; there would be no reason for the installer to splice in a wire from the harness as the old stereo had both power and light/antenna inputs. That spliced wire is odd.

Anyone know what that particular blue/green wire did? I'm sure the wiring diagram shows it, but I have no clue where to get one or how to read it. That would at least help understand what someone was trying to do with that splice--back whenever it was done. Could have been any time during the last thirty years.
 
#3 ·
I ran over a tarp in the road on New Years day. I didn't think a lot of it at the time. A couple days later, while approaching my parked car, I noticed the tarp was still under the car.

Turns out it was stuck (melted) to the tail pipe and cat converter.

I pulled it off.

To this day, after a long drive, I still get the burnt plastic smell when exiting the car.
 
#5 ·
Good idea. I looked with a flashlight and can't see anything. But I'll have another look tomorrow with daylight. That would please me no end to find out it's just something that wound up melting on my cat or exhaust pipe. Somehow I'm not terribly optimistic, though if one applies Occam's theorem, then I might be pleasantly surprised.
 
#7 ·
That's exactly what I was thinking. To keep that area clean I vacuum it each time I vac the interior during a wash job. Use a crevice nozzle and go in where it will fit, different angles to get all the way to the bottom
 
#8 ·
Drive it around the block a couple times and see if any of the rims get hotter to the touch than the others. Might be a sticking calliper.
 
#9 ·
tip on cleaning all of those areas back by the firewall, under brake booster, crevices behind the battery, etc...Vaccum usually isnt strong enough to get ALL the leaves, needles from pine trees, etc.
But, an air compressor with a skinny blower attachment can get ALL of the stuff blown up and out from back/under there. I usually toss an old bedsheet over the entire engine area before using this method so that all of the various dirt, leaves, etc end up on top of the sheet for easy disposal vs falling back down into the engine bay
 
#12 ·
Thanks, gents. I'm gonna try this with my shop vac in reverse (blowing) and see what happens. Also will check the wheels to see if any of them are hotter than the others.

Seems odd that it would be electrical unless a wire came loose and shorted something--but even in that case I'd expect a fuse to blow. Electrical fires usually happen because a fuse fails to blow first--or someone does something like arc the fuse contacts with a solid piece of metal that far exceeds the circuit capacity. Does that make any sense?
 
#13 ·
So I went out and bought an air compressor (nothing like a problem to justify a new tool, right?), and tonight I'm going to try blowing out all the leaves and see if the smell goes away when driving. However, in digging around, I see this bolted to the body. For those of you who talk about a capacitor that gets hot enough to burn leaves, is this the one you refer to?
 

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#17 ·
This forum is honestly amazing. Thank you all for your responses. It was in fact some sort of debris buried down there that must have smoldered...and thankfully not long enough to actually start a fire. On the plus side, solving for the burn problem only cost me the $110 for the air compressor, and the bonus is that it nicely satisfies my OCD side to know that all the small, dried leaves and decades-old parking stubs down there are now removed and it's dirt-free in that crevice.

However, as those who remember 1980s Saturday Night Live's Roseanne Roseannadanna would agree: "It's always sumpin'. If it's not this, it's that." So solving this problem raise another question: Should a resistor in such a location get that hot? I know very little about electrical stuff, but it strikes me that if a resistor is getting that hot, isn't its capacity too low for that application? Or, conversely, if it is getting too hot, perhaps the resistor has failed and needs replacement?

I did verify that this resistor does control the auxiliary fan (thank you, @luckymike): Pelican Parts sells the original MB part for about $35. But I also noticed that during my "what's that smell?!" session, the water temp reached 90C during a short, three-mile drive--about 10C hotter than normal (after said blow-job, however, it seems to be back to normal). Does anyone here know if (a) the high heat generated by the resistor and (b) the elevated water temperature suggest that the resistor which helps control the auxiliary fan (speed?) may be intermittently operating improperly? If the auxiliary fan is used to help cool the radiator (presumably due either to ambient temperature or to AC compressor running--or both), and the auxiliary fan resistor helps control its speed, then this sort of makes sense.

Or am I all wet and there's nothing to worry about.

Overly-analytic much? Moi? Never!
 
#19 ·
is getting too hot, perhaps the resistor has failed and needs replacement?
High wattage resistors get hot. Its normal. The placement of this one I will admit is not ideal for cooling so it gets toasty.

The Aux fan is supposed to kick on under one of two conditions if I understand it correctly (still new to the gasser W126's) Either when the A\C is switched on and the high side hits a certain pressure (this triggers low speed) or when the engine temp hits 110*C (this triggers high speed).

Basically if the low speed works on your car, then there is nothing wrong with the resistor itself.

However, it could be getter hotter than it should if the fan is drawing excessive current. This could happen for example if the the bearings in the fan are on their way out.
 
#20 ·
This is good to know, dude. Thank you. Meanwhile, I'm aside from understanding some of what the A/C system components do, can you explain what "high side" and "low side" mean?

I'll test my aux fan by turning on the AC to ensure it goes on, but unless I somehow can get the water temp to 110C, then it's hard to test for high speed...no?
 
#21 ·
The high side is the high pressure half of the A/C system. It is the side that supplies the evaporator in your car from the compressor. The low side, also known as he suction side, is the half of the system after the evaporator that feeds back to the compressor.

You should be able to force the fan to kick onto high speed by unplugging the 2 wire temperature sensor on the thermostat housing (you didn’t say what year your w126 was, but I later cars I think this may be a 4 wire sensor, I think it should work either way).

Keep in mind that the high speed doesn’t use the resistor, it’s only for low speed.

Also, with the car off, make sure the aux fan turns easily by hand and doesn’t make any bad noises (basically check the bearings in it)
 
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