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:

Another way to track down vacuum leaks, as well as some info on replacing the floor flap:

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.

Tools/supplies you may want:
Mityvac kit
Rubber vacuum line
RC car boot used to fix fuel door lock pod
Vacuum Gauge kit


March 2024 Fixed incorrect legend colors, Green is unlock, Red is lock

July 2023 changed line styles because it was hard to see base color vs stripe color, I’m not sure if the dashed lines look good or will just be confusing.

24 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.

    1. Hello, Sorry I do not currently have a diagram for that model, I’m basing all my diagrams on the vehicles I own or have access to, and right now, that is just my 82 300D, Perhaps sometime in the future I will be able to get enough time with some other models to branch out and make some different diagrams

  4. Hi Jarod,
    I recently acquired a ‘83 300td and see you have done much troubleshooting and really appreciate your articles!
    So, I have a question on climate control. My heat turns on and off with the temperature wheel, but no control ( either hot or off). What do you think I should look at?
    Does the controller modulate dampers or the mono valve?
    Many thanks!

    1. The monovalve is meant to fail in a heat on mode, applying 12v to the monovalve is supposed to close the valve to turn the heat off.
      Take a look at this video of mine and see if you might be suffering a bad connection at the monovalve: https://youtu.be/cuYCvgt2WYI
      Or it is possible yours is stuck, I’ve had that happen sometimes. I have turned the controls to cold with the key on but engine off, and went and plugged and unplugged the monovalve, it should make a sound like a solenoid valve when you apply power. They are a finicky part of our W123s for sure.

      The temp dial is meant to control fan speed, the monovalve, and vacuum to various flaps.
      AC will enable some recirculation, I have a video on modifying the system so that AC enables full recirculation, which makes for much more comfortable trips in the summer. (https://youtu.be/0lYppFBIuzg)

  5. Yo, please update this, your channel is great and has been a huge help!
    Even bought some stuff with your links too 😉

    Thank you for all that you do and looking forward to watching more w123 content as I bring my own up to speed.

    1. Thank you very much!
      I’ve got some exciting things in the works, and some monumental things in the works, I’m not ready to share it all, but I’m going to make some huge waves in a very good way hopefully very soon!
      Stay tuned for sure!

  6. Dere Sir, kan you send me pictures from the vac lines in the engine bay ? mine is a mess and I can not read the diagrams need some pictures

    thanks in advance

    kind regards Gerwin de Groot from Holland 😃

    1. Hello Gerwin, I can send some photos of the vacuum lines under the hood to your email address, is it OK to use the one you posted here with? or you can send me an email, tom at trythistv.com and I’ll respond with photos of the vacuum stuff.

  7. Hi Jarod,
    You have been a wealth of information on 123 chassis.
    I have a vacuum problem I cannot find any information about.
    83 300d ; locking system will not lock. Tested all 3 lines from master control in drivers door. All hold vacuum. Drivers lock will lock but nothing else. When manual pushing down locks the vacuum is keeping them up.
    What am I missing. Please give me a hint if you have a theory.
    All the best

    1. Hello!
      have you checked to see if you are getting vacuum to the master switch while the engine is running? I have seen people put BBs in vacuum connectors because the car wouldn’t shut down but it would if that line was blocked.

      I’d also be curious if the switch itself has failed, or the linkage has fallen off or isn’t pushing / pulling the master switch to the right positions?

      If you aren’t vacuum to the master switch with the engine running it could be the yellow check valve under the hood of the car, It splits vacuum between the master switch and the reservoir in the trunk. (the first youtube video on this page shows where the reservoir is)
      If you are getting vacuum to the master switch, check if it is coming out to the right hoses, or if something has failed internally in the master switch.

      Let me know what you find!

  8. Never waste time drawing diagrams when you can photograph parts. Photos are real and accurate. Diagrams are abstract and unreal.

    1. I find it useful to know what colors and routing it is meant to be.
      Pictures are great, but after 40 years and who knows how many roadside repairs and questionable mechanics, My hoses aren’t all the right colors, some are outright missing, but I can look at the diagram and figure it out.
      I’d be happy to send you some photos under my hood and let you take a stab at figuring out what’s going on, It sure won’t help anyone else out though.

  9. Hi Jarod. I checked the lines like in your video with a vacuum gauge and the vacuum moved with all of the connections. The door locks have never worked since we bought it and won’t even push down actually. But the a/c stopped working too – none of the lower a/c fan speeds work, only the top defrost works. Anyway, if all of the line connections are moving the hand pump vacuum gauge, then do you have another idea of how to check? Thank you. We just replaced the fuel shut off valve because the car consistently did not shut off eventually and a test of the old valve showed it was failing. But now it shutters to a stop.

    1. The best bet you have I’d say is start eliminating portions of the vacuum systems. Find the 4 way connector under the hood, pull the big yellow check valve for the locks and stick a golf tee (or some other plug like thing to seal the vacuum) in the rubber fitting, that will essentially disable the locking system, then see if the heat/ac pods start working and the slow shutoff improves.

      If not, pull the green/yellow hose that feeds the HVAC pods, golf tee that outlet of the rubber 4 way, and see if the slow shutdown goes away. If it does, try removing one golf tee and reconnecting the door lock yellow check valve, then you’ll know if one or potentially both of those have leaks.

      The locking system is notorious for leaks, and the HVAC pods seem to be getting to the age that they are having issues as well.

  10. This is a great article. Just to add, when you start testing the actuators by removing the pipes from the junctions under the front seats, the 4-way (3-way on the passenger side) connector will probably develop a slow leak. So, either replace the x and y connectors, or make a plan to seal them, maybe use small cable ties with some Vaseline, or silicon that does not harden works as well.

  11. Massive thanks for all this work.
    Am working on a project that is small amount of MB vacuum and OM617 grafted to a Jeep vacuum/electrical system. Someone else did the swap. I am swapping to a better fresh OM617. all my ink on labels faded out where I labeled things

    It Has the MB vacuum reservoir ball and vacuum shut off system from MB. where does the two lines off the vacuum ball typically go I assume one has to be to something pull some vacuum so it should go to brake booster …. is other end likely my vacuum shut off or would that be the Alda orr?? Where is vacuum shut off… back of IP if I recall right?

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