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1978 300CD - Diesel Coupe
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13 Posts
Discussion Starter #1
well lets start off, my name is Jeremy, i just aquired a 1978 300CD Benz. This car is in very good shape and has no rust, the interior is almost imacculate, i just have some electrical problems. It is a cali car that i bought from an old gentleman who had this for 10 years (has not been driven for 5) and brought it back to canada. it has 166,000 Miles on it and is silver with dark blue interior. The engine fires up great and purs like a kitten.

Drivers window is the only one that works, Blinkers + hazard does not work(they turn on but will not blink by them selves so im thinking its the flasher), the defrost work but the other heater things do not... i have no idea how this climate system works so im not sure if it is 100% functional or not, i need to do some more diggin and playing..... i will keep this updated as to what i fix and do as much as i can :) This is my first benz and i have been a mustang guy for a while so this is a big change for me lol. I still own my mustangs but i needed a good on gas commute car for the fall/spring seasons and i got a smokin' deal on this car.
 

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1984 300D TurboDiesel
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18 Posts
I had issues with the windows on my W123 when I bought it as well, though a good teardown and cleaning of the window switches fixed that right up, though from what I remember the switches are different on the older W123's.
 

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my 84 flashers and turn signals stuck. Some will tell you to either clean or replace hazard switch (assume its just behind shifter). Since crud, coffee, etc gets in there. Same for window switches. What caused my flasher problem was the relay connection was poor. I pulled shift console and i believe the ash tray and the relay lays right under there. I cushioned the relay with rubber to keep it tight (stopped jiggling) and haven't had a problem since. good luck
 

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congrats on the purchase!!!! i had a 1978 240D...was my first car. drove like a tank. had it until the tranny went out but the motor was still strong. these cars can go up to 500,000 miles! looking for another. i drove a 2000 VW for awhile but now i am going back to the w123... =)
 

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1978 300CD - Diesel Coupe
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13 Posts
Discussion Starter #6
thanks guys, well i also found the that the whole right side of the car's lights are not working, onnly the high beam works (low beam works fine on drivers side, same with marker and brake lights) and the 2 middle "Fog Lights" i cannot get to turn on although im not sure how to do so.... none of my interior dash lights work except for the radio will light up when i turn it on. Also, on the dash on the right side above the climate control, there is a button that looks like it is either related to glow plugs or is a rear defrost, any idea?? 1 more thing, what is the thing that is by the window switches that rolls, i thought it was a dimmer switch but it seems to do nothing? what should this beast control... i cannot find any owners manual or anything like that which will tell me what switches do what.
 

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2013 MINI Copper S Clubman, '84 300CD-weekend car
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10,152 Posts
To turn the fogs on, turn the head lights on low beam, then pull the knob out.

The button is rear defrost.

The roller on the center stack beside the window switches is the radio front to back fader.

The no dash light is probably due to a failed rheostat, or dimmer switch, spin it a few times, that may help.

The round plastic knurled knob, #13 in the pic.
 

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1978 300CD - Diesel Coupe
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13 Posts
Discussion Starter #8
ok ill try that. well the radio lights up but doesnt make sound so will have to try once i get that sorted out.

Are there a fuse box anywhere?
 

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2013 MINI Copper S Clubman, '84 300CD-weekend car
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10,152 Posts
Yes, the fuse box cover is held in place by the large round knobs, unscrew 'em, the cover comes off.

Theses are torpedo fuses, you need to replace them all.

They can appear to be fine, but once removed, the ends may have failed.
 

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1989 300E
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702 Posts
The fuse box is located right under the hood and directly in front of your steering wheel basically.

If you are able to, and after you have changed any bad fuses, cleaning the switches can help a ton. I alleviated a lot of my own problems by simply cleaning the contacts on the switches. Not that hard and kinda fun.
 

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Mercedes 300te
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1,970 Posts
Replaces all the fuses first with copper ones. They usually are not that great anymore even though they look good.

In my opinion no turn signals, hazards it should be the relay that conked out.
 

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1978 300CD - Diesel Coupe
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Discussion Starter #13
LOL i was wondering what that was just didnt wanna take it apart quite yet.... ok so the hazard button is what makes the lights flash?? all marker lights come on when i hit the hazard button but they just wont blink.
 

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1978 300CD - Diesel Coupe
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Discussion Starter #14
also, should the back windows roll down? or do they push out with the window switches? it does neither for me, only drives works, and its really tricky to get it to go up... just bought a bunch of fuses earlier this evening and will replace them all tomorrow and see how things go.
 

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1978 300CD - Diesel Coupe
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13 Posts
Discussion Starter #16
Update:

Replaced all fuses except a couple which i need to buy more bascially, now my console/shifter light works but the dash light does not, so i suspect the dimmer? which i need to pull gauges any way as the left side blinker marker does not work and it bugs me.
The blinkers started to flash but they stopped working today.... also my battery died which was from 02 (the sticker/Napa brand) so i grabbed my optima from the stang and threw it in for now and is working fine for the moment.
 

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1978 300CD - Diesel Coupe
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13 Posts
Discussion Starter #17
well another update: this one about the battery...

Basically i dont think the optima is going to cut it... the battery i pulled out was fairly big and my optima is small compared to this.... doing i quick search i found something about using a group 49 battery not a deep cell like my optima is. What is happening is the optima starts it fine, but if im not driving or have the rev's high, they battery light is coming on as if its not getting enough juice..... any one have any idea if this is alternator related or battery... i assume battery cause when im driving the light goes away as if the alternator is charging it. If my lights are on the battery light comes on quick.... if not driving, it takes a minute out 2 for it to come on, but it will come on if its just idling sitting there even with no accesories running.

Thanks!
 

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2013 MINI Copper S Clubman, '84 300CD-weekend car
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10,152 Posts
Yes, probably your dimmer switch, much has been discussed on the subject, search rheostat.

A deep cycle battery is not designed to be constantly charged.

Obtain a group 49 unit, you'll be fine.
 

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1977 W123.123 1995 W124.034
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3,868 Posts
I would recommend either a Interstate battery or the MB dealer battery. I have had bad luck with others.
 

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1978 300CD - Diesel Coupe
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13 Posts
Discussion Starter #20
before i go buy a new battery, just thought i would mention this.... when idleing the battery is only at 11.79 volts but has no problem starting ect.... just fired up great this morning..... i think my alternator be shitty cause it should read around 13.5 correct? the car was sitting for 5 years before i got ahold of it so it very well could be....
 
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