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  #1  
Old 06-23-2019, 02:46 PM
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Cool 79 300sd vacuum line question

My question in concerning a vacuum line that I cannot figure where it should connect. There is a Y that comes out of the top of the VCV. One connection couples with another Y. The other is a black line that goes left behind the valve cover and is just hanging loose under the engine compartment.

The transmission is also shifting rough and I was hoping this was related to this disconnected vacuum line.

I have looked at many diagrams and cannot figure it out. FYI this is not the vent line that goes under the dash. I am fairly certain this line should connect somewhere.

If anyone could help I would really appreciate it with any pictures of where this should connect. Thank you.
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Old 06-23-2019, 11:07 PM
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On my year of W123 the Black line is not supposed to be connected to anything. It is a vent line that is supposed to go to the inside of the car but it remains open.
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  #3  
Old 06-24-2019, 12:21 AM
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There is a black line that comes out of the SIDE of the VCV and enters the dash on the driver side. From what I have researched, without removing the dash board, this is the vacuum vent line.

I am asking about the black line coming from the TOP of the VCV and I suspect connects to the transmission somehow
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  #4  
Old 06-24-2019, 10:13 AM
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That line should go down and connect to the shift modulator on the right side of the transmission. Also, this black vacuum line should snap into a black plastic clip which is located on the top of the transmission, just behind the bellhousing, and on the left side.
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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 347,000+ Miles
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  #5  
Old 06-24-2019, 10:24 AM
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This is the line pushed into the black plastic clip on the transmission:


And this is the shift modulator to which it attaches:
Attached Thumbnails
79 300sd vacuum line question-lineclip.jpg   79 300sd vacuum line question-modulator.jpg  
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DON'T MESS WITH MY MERCEDES!


1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 347,000+ Miles
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  #6  
Old 06-24-2019, 10:35 AM
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Concerning the transmission module when I accidently disconnected that line there was only a tiny extra harshness on up shifts but it was the down shifts that were extremely hard. Again I have a different year, model and Transmission.
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Last edited by Diesel911; 06-24-2019 at 10:48 AM.
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  #7  
Old 06-24-2019, 11:01 AM
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Thank you Squig and 911 for your help. Real clean engine there Squig. I'll get there someday.

I'm gonna reconnect that line and hopefully I see a big difference
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  #8  
Old 06-24-2019, 05:25 PM
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Diesel911 - you are exactly right - vac only needed for downshifts.
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