in my 240d, i have a disconnected wire that looks like it should plug into the battery (has a battery lead). It is a small red wire that is resting by the battery (not connected) and it goes into the firewall on the passengers side on the edge on the left side of the battery. What does this one red wire do? It looks like it has a ring as in one end should attach to the battery's positive connector, but is just hanging there?

Something to do with A/C, clock, cruise control? I can't seem to figure it out.

Thanks!

(240d manual 1982)

Could you throw a picture up for us? It sounds like it might have been added by somebody...

__________________
2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

What does not work on your dash ?

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

Post a photo, chances are its for an aftermarket radio, cigarette lighter or after market glow plugs....

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Definitely aftermarket.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Yeah that's not stock. Pull the kick panel and glove box liner if you need to and see where it goes. I'll bed it probably doesn't go to anything anymore. It could have been used for a million things by a previous owners (CB radios, aux outlets, etc, etc).

__________________
2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

I've got one of those; for me it's for the radio and it actually is active (disconnect the wire and I have no radio).
Not sure why the PO ever hooked the radio up like that; maybe the original wiring is shot, or maybe this was the easiest way to accommodate an after-market (though period-correct) stereo?

Whatever it is on your picture, it's definitely something added by a previous owner, and if everything in your car is working, that wire is no longer active. I would not just plug it back in: if the other end is cut off and makes contact with the metal body, you could be in for a nasty surprise when you plug that back in and it isn't fused properly. I'd just leave it alone.

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"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)