W123 Vacuum Diagrams and Troubleshooting Information.

This is the start of my project of making vacuum diagrams for my W123 Mercedes.

I’m quite confident in saying its rough. VERY ROUGH. Take these with a grain of salt… maybe more than a grain, probably a pound or two would be more fitting.

I’m fairly sure my vacuum system on both of my cars has been tampered with extensively, so I’m having to piece this together from looking at other diagrams and chasing hoses to try to determine how it was meant to be.

I’ve seen many diagrams and they have helped me track down multiple problems, but they all seem to be grainy and hard to read.

So while I realize this is going to take me a long time, I feel it will be worth it. I’m drawing these in Inkscape, so they will be available in SVG format. I will break down each major section into smaller printable images to be used when troubleshooting.

First video on the most common central locking issues I see is right here:

Disclaimer: This may be entirely incorrect. The longer I look at my diagrams the more confused I get. Which proves why this needs to be done, but it also proves why it hasn’t been done yet.

I’m fairly confident everything drawn from the firewall back to the trunk in the locking system is correct, but the engine bay and all that is nowhere near complete, I’m going to break the diagram up into pieces, so the locking system will be one part, then HVAC, transmission, emissions, etc in different areas/segments so nothing gets muddled together as far as color codes and such.

7 Comments on “W123 Vacuum Diagrams and Troubleshooting Information.”

  1. Dear Sire,
    I have a 1987 300d automatic W124 deliver to US September from Germany.
    VIN: WDBEB33D8HA367779.
    I wonder if I can buy from you one Amplifier and one Actuator which you have checked it’s OK.
    I have looked on yours performance on YouTube and it’s looked good.
    Thanks in for hand for a quick replay.

    Best Regards
    Jan Åke Olsson

  2. Jarod, through fortune (good or bad, I have yet to determine), I have been acquired by a 1985 W123 300TD wagon named “Olivia”. She was neglected for years and abandoned, but with four year-old diesel in her lines, she fired up almost immediately with a new battery. Vacuum lines are brittle/disintegrated, locking system not functioning, cruise control does not function, odometer stopped working, dash dimmer non functional, etc. The fuse in the over-voltage relay was broken…I am installing a new module to see if this gets the tachometer to come to life. The vacuum system is my next intended system to address. Just want you to know that your videos are very helpful to a neophyte. Greatly appreciate what you are doing.

    1. The wagons are awesome. I’m working on more vacuum stuff slowly, and at some point I have acquired some things related to the dash lighting, and should do a video on the odometer repair, every Mercedes I’ve ever owned needed the odometer fixed.
      Sorry I didn’t see your comments sooner, I was on vacation for about a week, took my w123 about 1500 miles round trip to the beach and back, wonderful to get some time to relax.

  3. Jarod, I am attempting to get the cruise control on my ’85 W123 300TD functioning again after years of neglect. After watching your videos, I found that the engagement solonoid on the mechanical actuator was not functioning (I could hear the motor turn on that part of the test). I opened the beast up, cleaned the contacts..still nothing. Like your lesson on repairing the cruise control amplifyier, is there a way to repair/replace the solonoid?
    A separate note: my tachometer was not functioning…found my tach amplifier was missing from the plug. But after replacing the overvoltage relay module, the tachometer is functioning WITHOUT an amplifier.
    I am pleased, but surprised…what gives? Again, thanks for what you are doing…really helpful and pragmatic lessons!

    1. So the 85 models got a slightly different tach amp setup from what I understand, it runs off the EGR computer I think, so the amplifier plug would just be empty.
      As far as repairing the cruise actuator, I’ve got a spare that I disassembled and was hoping to find potential replacement parts, possibly even making some 3d printable replacements for the plastic gears, but I have not made much progress on either, Work keeps me crazy busy during the summer months, I did not have much luck finding replacement solenoids or motors, I bet there is something that could be made to work, I just haven’t really explored that yet.
      Glad the videos have been helpful, I’m working on more amongst the chaos, some vacuum stuff and I’m going to be poking my transmission soon too, between the external adjustments and maybe dig deeper if needed, I noticed some weird stacked shifts on a trip, where it would start in 1st gear, then rapidly shift 2nd-3rd-4th within a few miles per hour. I’ve dealt with that issue before on my other w123, so this time I’m going to make a video on it.

      1. Thanks for the clue on the ’85 tachometer not needing an amplifier. The learning curve is steep, but I am way ahead of where I was a few weeks ago. Olivia runs great. I repaired my odometer…a bit tedious and intimidating, but fairly easy. Dashlight rheostat cleaned, but still not functioning. Your attempt at a vacuum schematic helped. I have replaced many of the rubber connections and some of the hard plastic vacuum lines…all doors (except the rear hatch) now slowly function. But I still have questions under the hood about what vacuum line does what…if the guy before me did an imperfect job, I am only replicating his errors. Some lines are not connected (a black hard plastic line runs from the bottom of the clear-white fuel thingy above the fuel cutoff switch through the firewall and is unconnected under the dashboard) and I make the assumption that these are meant as vent lines. Using your test, the cruise control actuator I ordered from ebay had a good motor but a bad engagement solonoid…must be a common problem. Once I get another actuator, I will be able to see if my cruise amplifier is functioning. Anyway, you are a big help.

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