electrical fire - PeachParts Mercedes-Benz Forum

kenstfelix · #1 07-08-2016, 12:25 PM

well, i really did it this time. my power windows stopped working and i foolishly tried to remove the relay - without disconnecting the battery first! there was a spark, lots of smoke, i saw something glowing in there. luckily the whole thing didn't catch on fire but some wires were toasted. i'm going to try and assess the damage now (of course this happened while i'm out of town and parked in nyc).

you can see in the pics the wires that went to relay are burnt - and also some of those brown wires behind the steering wheel. Does anyone know what those are for?

if i can isolate these wires so they don't touch each other, would it be safe to drive? could the damage have gone where i can't see?

oldsinner111 · #2 07-08-2016, 12:39 PM

few years back,there was a guy with a beautiful w123,that he poured tons of money into.It caught fire and burned up,believe it was the climate control.Theres some way to add a 1 amp fuse to the switch on the firewall to prevent this from happening.But I think all old cars should have a battery disconnect like on drag cars.Pull window fuses to disable the windows till you repair.

jay_bob · #3 07-08-2016, 12:40 PM

You are really fortunate.

Red is battery voltage, a wire that thick is typically non fused in a 123.
Brown is ground.

You definitely sustained some major damage there. Probably more than you can see.

I would not drive the vehicle again (or leave the battery connected) until I could pull the entire dash and check for more damage.

gmog220d · #4 07-08-2016, 12:40 PM

Bummer. Gave yourself a nice little project, there.

The brown wires are for grounding. Yes, the damage could have reached places you can't see.

You need to open up the harness and follow the burnt wires to reveal the extent of the fry-age. I would consult a wiring diagram to see where those wires run and check them all. You'll need to re-wire to make a good repair. I suppose you could isolate the burnt wires and wrap them with tape in a pinch, but replacing them with good wiring would be better.

kenstfelix · #5 07-08-2016, 01:17 PM

oh boy. i guess i'm towing this thing home to deal with later. this is the icing on the cake. first the vacuum pump went out, no power brakes, transmission wasn't shifting. when i went to go fix that, i noticed the windows wouldn't go down. and the rest is history.

a few days ago i was just thinking how nicely this car was running. made an eight hour trip in it.

leathermang · #6 07-08-2016, 01:34 PM

Everyone should have a CO2 fire extinguisher securely fastened to there drive line hump .....
This can last forever... being refillable ... and does not make a mess like the ones with dry powder in them.... which are not refillable...

It can also be used for small things ....and reseals at the nozzle... the powder type can not reseal... so no matter how small the use... it is then trash....

vstech · #7 07-08-2016, 01:52 PM

Quote:

Originally Posted by leathermang

Everyone should have a CO2 fire extinguisher securely fastened to there drive line hump .....
This can last forever... being refillable ... and does not make a mess like the ones with dry powder in them.... which are not refillable...

It can also be used for small things ....and reseals at the nozzle... the powder type can not reseal... so no matter how small the use... it is then trash....

and having battery cables easily disconnected is a plus also!

leathermang · #8 07-08-2016, 01:58 PM

Quote:

Originally Posted by vstech

and having battery cables easily disconnected is a plus also!

Of Course.. but that refers to a planned event like starting to work on something...
sometimes stuff just gets hot going down the street.. or in contact with another vehicle... etc.... the CO2 will work for both...
And when one reconnects the battery cables... fun can happen then also....

jay_bob · #9 07-08-2016, 02:56 PM

Definitely get yourself the wiring diagram for a 123, should be available on the startek site.

I looked at your pictures again, the red wires that burned look like the dual 2.5 mm (approximately 12 AWG on this side of the pond) main hot feeds to terminal 30 on the key switch from the light switch terminal 30.

Definitely check carefully for shorts and be sure you found the root cause. I think something overheated the smaller wire bundled in with the two reds and caused it to melt into the red wires.

Also I see a couple burned grounds, be sure to trace back which devices these fed, since they saw the fault current.

Terminal 30 is unswitched battery in MB electrical notation.

Typical 123 power flow: battery to starter to light switch to key switch. All of this with no protection. Most of the fuses on the 123 are past either the key or the light switch.

kenstfelix · #10 07-08-2016, 05:11 PM

cool, thanks. yeah it's time to learn all about 300d wiring! hopefully i can get this towed home for not too much money but i am 100 miles away. the red wires were the ones attached to the power window relay. i was struggling to remove the relay and some of the wires must have touched each other or something. i got really scared that the whole dang thing was going to catch on fire!

Ceristimo · #11 07-08-2016, 06:00 PM

Ooh man! I am so so sorry!
Electrical fires are the worst. I wish you the best of luck and will keep my fingers crossed that nothing more got damaged. Maybe you got lucky and it is "only" the electrical windows.

However, speaking from experience: I once had an electrical fire because I also foolishly forgot to disconnect the battery on my little Citroen hatchback I owned years ago when I tried to fix the stuck ignition and steering lock.

At first I also thought that only my ground cable was roasted:

Turns out that everything burned... The only usable wires were the ones feeding the rear lights, everything else needed to be replaced.

It was a month long project:

It still gives me nightmares. Now I even disconnect the battery when I do an oil change...

jt20 · #12 07-08-2016, 06:29 PM

Looks like you made it out lucky.

I am really curious how trying to remove a relay causes this happen. What do you think?

vstech · #13 07-08-2016, 09:00 PM

My little brother now ALWAYS disconnects the battery when working on vehicles...

His newlywed wife entrusted him to change the carborator on her 85 buick...

The one in the box didnt fit so he took the old one and new one to store for a match...

Left keys in car, and my sisters husband needed to move the car... cranked it up, and it revved to redline and burst into flames...

Mechanical fuel pump... lines loose on motor... whatta mess...

The wife was NOT amused!

leathermang · #14 07-08-2016, 09:09 PM

Wives are so finicky !!!

Another thing undoing the battery first prevents... is getting a ring against the connections on the starter and melting the ring to your finger....
Mechanicing should be a no jewelry zone anyway....

Mxfrank · #15 07-08-2016, 09:56 PM

Some circuits...the electric fan and perhaps others, depending on model, have no external fuse. Instead, there's a fuse built into the relay. That's sort of ok if the load shorts, but if the relay shorts there's nothing protecting the battery circuit. If your car has a black plastic relay, this is probably internally fused, and the fuse can neither be examined nor replaced (there will be a schematic stamped into the plastic showing the fuse). Worse, they are compatible with the silver unfused relays, so a careless hand can easily leave you with a completely unprotected circuit. The correct new replacements for these relays have separate fuses built into the top. They're easily recognized by color: orange (40A), green (30A), blue (20a). Not saying that's what happened here, but it's worth a look.