I would check that wire out. Fuse c is the aux fan power feed. One side of that fuse goes directly to the battery without any protection. I don't know which side of this fuse is the protected side on a 123 but if you are on the unprotected side with that tap, it could cause you a major problem. It looks like the insulation is pinched in one of your photos.

I suspect it might be part of your aftermarket audio setup to get unswitched power for the head unit.

However in later 123s there is a 2 pin connector under the radio location that was meant for the electronic Becker 612 to provide illumination and memory backup power. The analog 599 just used switched power and ground.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Speedometer cable is going bad or just might need a little lube (a few drops of ATF from the head end works good).

Fuel tank sender can be cleaned, I posted on this thread with how to do it:
w123 out of diesel at 1/8 tank

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Some updates: took her on a ~700 mile round-trip to Burlington and back (which, in hindsight, was pretty ballsy [edit: dumb is more like it], considering my overall uncertainty regarding the condition of the vehicle).

The good news:

• No major fluid leaks
• Proper (~30) MPG for highway driving
• Handled pretty well on the highway. Needs new tires as well as an alignment, but good other than that
• Made it there and back

The bad news

• LEAKS. SOMEWHERE. I'd had some experience driving her in rainy conditions, and things seemed perfectly fine. However, the rain this past weekend was TORRENTIAL, and I discovered a small lake forming in the rear passenger side footwell. The wheel wells and floors are rust-free, so I'm suspecting a rear window seal leak, or a sunroof-related leak.
• Confirmed the presence of gunk in my fuel tank. Made the mistake of parking her nose-down on a sharp incline, and the same rough/no start and power-loss returned. Parked her on the same incline nose-up the next day, and she started and ran like a top.
• Transmission is still a little wonky. Essentially completely unpredictable. Seems to be happiest on long highway stretches, and seems to run smoother cold (??). In city driving, will sometimes behave just fine, but occasionally flares, clunks and shifts hard on a seemingly random basis (generally only once the transmission has warmed up, however). I don't suspect that the transmission is completely gone, since I know that it's capable of shifting perfectly when it feels like it.
• Fuel cap seal seems to work well. When filling up, the car starts to leak diesel fuel out of the tank lines and from another unidentified point under the rear passenger fender. Once the cap goes back on, the leak stops. I'm guessing the pressure created by the seal might be holding the fuel in?
• Rough start after sitting for the evening. For the first start of the day, the engine starts a little rough (as if it was a little short on fuel). Give her some gas, however, and she's fine for the rest of the day.

I f you have a leak of fuel under the rear passenger side of the engine, there is a very good chance the rubber portion of the fuel line is rotted and is leaking, it will also cause air to get sucked in and cause your other issues you are having...

The drain for the door area, also runs under there...so another possibility is the seal is leaking and running down the drain, causing a leak...

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Some photos:

Thanks. I'm suspecting I'll have to do a piecemeal replacement of the rubber lines throughout the car. I know that my fuel and vacuum lines in the engine bay aren't in good shape, and the fuel lines in the rear don't seem to be doing great either. What door drain seal are you talking about?

Thanks. I've been experiencing a few electrical gremlins (dash/IP lights quit on me during the trip), so I'll have to figure out what's going on in there. The current head unit is a panasonic Radio-CD player, which seems to have been installed in a somewhat hamfisted way. Whatever aftermarket audio system used to be in the car has since been removed, as the speakers are stock.

Yeah, I'm guessing that some of my erratic transmission behavior may be related to the butchered vacuum line. I'll have to take the time to go over (or ideally replace) all of the lines once I get the time.

Again, many thanks for everyone's help on this forum. It's been a fantastic amount of help.

Is that covered bridge in Waitsfield?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Not too far. This specific one is actually right off of rte 14 on Kingsbury Road (we were driving to Barre from Rutland).

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Planning to give her some love tomorrow. I have a boatload of fuel hoses and vacuum bits that just came via UPS, which (along with the handpump) will hopefully allow me to track down any leaks in there.

Changed the fuel return lines last week, which unfortunately hasn't solved the leaking around the injectors. As of right now, it looks like 3 out of my 5 injectors are leaking. It doesn't seem to affect things too much, but does mean that I'll have to rebuild the injectors at some point in the future.

I'm hoping to get to the larger fuel lines tomorrow (the current ones are cracked and hardened). Forgot to order a new cigar hose, may stop by the dealer for that. By the way, pulling into Mercedes Benz of Manhattan's futuristic 5-story deep garage/showroom in an old W123 is a completely surreal experience. They also quoted me $250 to change my fuel filters, so..................

Picked up some new tires, but the slight negative camber of the front wheels makes me worried that they'll wear unevenly. Based on the threads on the subject around here, the most likely culprit is the upper control arm. One more thing to add to the list!

Going to attempt changing the engine belts tomorrow. Wish me luck! There's lots left to do, but it's starting to take shape.

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P.S. I've been having limited success with the built-in "cupholders" so far. They've been fine around the city, but I can't imagine the mess that hitting a pothole on the highway would cause. For the most part, aftermarket solutions don't seem particularly elegant, so I may try to custom-build something for the center console.

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Luck with the belts.

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83 SD

84 CD

Could be leaking from the injector hard line flare interface due to over torque. Completely dry the area with toilet paper. Wrap the area of the line nut to injector with toilet paper and start engine. If the toilet paper gets wet, you found the leak.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

That big electrical cable looks like it was from a a big powered subwoofer or amp. I had one on a suby I bought once looks like it says sound?? something on it. The lump is a fusible link to protect the BOOMin system man. Goodluck with the project.
Michael