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Old 07-18-2001, 02:20 AM
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While poking around under my dash (driver side) I noticed at least two vacuum lines that come through the firewall and just end in space (not connected to anything) about six inches inside car. One is slightly larger than the other and has a clear filter on it inside the engine compartment on other side of firewall.

Is this normal? What are they for? I thought maybe electric seat option, which I don't have. But why aren't they stopped up?

This vacuum stuff is very worrisome. I am having no problems that I know of but I often read that others do.

'85 300CD @ 142k miles
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Old 07-18-2001, 02:31 AM
The Bob
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Vacuum lines are trouble,
Be careful when changing your oil as you can easily knock some of them loose.

Any how, on my 84 300d going into the firewall is a group of vacuum lines, The colors of these lines are
brown ignition shut off to the fuel pump
green climate control vacuum for the vents
yellow door locks
yellow with black strip vacuum resevior for the door locks

there is a black one the goes into the cabin that is a bleeding line it goes to nothing.

I would say that as long as your door locks work, all the vents work, transmission shifts smoothly and brake pedal is firm you are probably ok

If you like to screw around with cars and want to master the vacuum system on you benz buy a vacuum meter and start testing all of the circuits to check their integrety.

You may end up breaking it though so be careful..

Have fun

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Old 07-18-2001, 03:20 AM
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Thanks for prompt response. As far as I can tell, all is well with my system. My center vents work erratically so I assume the vacuum valve on the center vents is tired. Would replace but can't get to it. Locks, brakes, windows, etc. work fine. Car shifts strangely (jumps from third to fourth too quickly) unless peddle pressure is steady but no real problem.

I wish there was a clear and simple guide to the vacuum system available. I would like to understand it all. Have purchased a section of black rubber vacuum hose that is used in many places (verses the plastic lines). Those rubber sections are somewhat cracked and I'm after them. I'm also going to replace vacuum pump itself at next major mechanical event.....gotta get that mighty vac thing too.

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Old 07-18-2001, 06:07 AM
The Bob
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vacuum system is not that complicated as long as you separate out what you are doing. Test one thing at a time and progress from there. When you get your vacuum pump test the source vacuum at the vacuum line that goes to the brake booster. Then check each system to see if it is holding vacuum. The color codes represent the systems that i wrote to you earlier. Also just for fun seal off your ERG valve vacuum connection as that may be giving you leaks.

I also wrote on an earlier thread how to get out the center vent vacuum element in about one hour. You are not ready for that yet so when you are just contact me and i can help you out.

good luck

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Old 07-20-2001, 09:33 PM
lrg lrg is offline
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Join Date: Jul 2001
Location: San Francisco
Posts: 1,163

I can second Bob's comments. My center vent servo on my '85 300 td went bad and I was able to fix it without removing the dash. In my case the rubber bladder inside the servo split and caused the vent to act erratically, opening sometimes but not others. My car is in a fairly hot climate so I think the heat got to it. I lived with it for several months until the system couldn't maintain enough vacuum to work the locks. The vacuum system sounds really complicated but it's really not if you have a cheap vacuum pump and tester. You simply isolate one line at a time and test to see if it will hold a vacuum. I bought a cheap plastic Mighty Vac at the local parts store some years ago to help bleed my brakes and while it proved to be worthless for bleeding brakes, it's great for testing MBZ vacuum systems. Go figure. The vacuum servos aren't that cheap so it's worth the investment just to avoid unnecessarily replacing a vent servo. Good luck.
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Old 01-20-2005, 02:39 AM
Jimmy Joe's Avatar
peace out
Join Date: May 2004
Location: California central coast
Posts: 1,004
Was searching for vac line colors, and found this, tht!
any additions?
Think Alternative Energy!
300CD '80 (now gone but not forgotten...)
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Old 01-20-2005, 02:58 AM
BoostnBenz's Avatar
Benötigt Mehr Druck!
Join Date: Aug 2002
Posts: 1,855
I don't know if there are two or not but the vacuum lines which vent the whole system end on the other side of the firewall. Generally a black line means it is vent, this is with the exception of the line going to the transmission's modulator. Lol, looking at dates I think you either have figured this out by now or have given up.
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k
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