Hey all,

I Recently discovered that my 79 300td will not shut off with the doors locked. The vacuum locks work very well and follow the master but the master element vents vacuum from the end of the barrel opposite the plunger when I depress the lock. Is it supposed to vent vacuum? If not I'm going to order up a replacement. If it so I will dig in to the rest of the doors, Thanks,
Michael

Yes it is supposed to vent some vacuum. It has to release the locks from what what ever position they are in so they can go into the other position. It is a function of switching. If it is constantly venting then you have an issue.

Thank you, I'll start digging.

There is a bit of adjustment in the placement/mounting of the master switch. If it's mounted a bit off it's possible it is never moving fully past the "venting" part of its travel.

However there is a pretty good chance you have a large leak on the lock side if the engine won't shut off when the doors are locked, but will when they are locked.

On these older W123's there is a greater chance for the unlock side to have leaks due to the design of the door and gas flap actuators. On the door actuators the pair of lock side diaphragms are exposed to the elements that come in past the window scraper. Notice how the upper diaphragms are not covered or protected in any way.



It could be you have multiple leaking diaphragms.

Additionally, the gas flap lock only works on the unlock side on these earlier cars. It is spring loaded to return to unlock when vacuum is removed. So on these cars the unlock side has one additional leak source that doesn't affect the unlock side.

That said, so far the most common spot I've found for large leaks on these is the trunk lock actuator. Its outer diaphragm seems to be prone to ripping. This was what lead to major leaks on both my '79 and my '85.

__________________
1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

http://i.imgur.com/LslW733.jpg

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I suspect that the last word in that sentence should be "unlocked."

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

In fact, either the lock circuit or the unlock circuit (one or the other) is vented to the atmosphere all times.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Yes, thanks for clarifying, was a typo.

__________________
1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

http://i.imgur.com/LslW733.jpg

The Baja Arizona Oil Burners Send a message if you'd like to join the fun
Left to Right - UberWasser, Iridium, Stuttgart-->Seattle,, mannys9130

Visit the W123 page on iFixit for over 70 helpful DIY guides!

Isolating the actuators via the vacuum lines might be an easier option. That way, you can identify any problem actuator(s) without "digging."

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Indeed, that is good advice. Here is a guide that covers that systematic approach.

It starts in the engine compartment, where you test the locks and reservoir separately.

Then it moves in to the driver's door, which lets you test the two branches on the system isolated. This may help confirm that your unlock side is tight, so you can focus on the lock side.

After that you move under the carpet to test the lines that lead to the various doors, and back to the trunk and gas flap locks, before opening anything up.

Hopefully this leads you to the specific area of the leak without having to pull off every panel.

https://www.ifixit.com/Guide/Mercedes+W123+Vacuum+Lock+System+Diagnosis+Technique/20201

Note - this guide was written on a sedan. Your layout will be different to some extent in your wagon. The methods will be similar, though.

__________________
1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

http://i.imgur.com/LslW733.jpg

The Baja Arizona Oil Burners Send a message if you'd like to join the fun
Left to Right - UberWasser, Iridium, Stuttgart-->Seattle,, mannys9130

Visit the W123 page on iFixit for over 70 helpful DIY guides!

Yes. But you should not hear it all the time as it reaches equilibrium and there is no pumping on the one that is vented.

Thanks for the link uberwasser, I can now try out my Tool Aid Mano-Meter I picked up at fleabay. Very nice looking tool lets see if it works....

Yep the tool works, check valves are fine, vac reservoir is fine, lock line leaks...now to diagnose the rest of the lock system but that'll have to wait till tomorrow after sheetrocking... Did the rear hatch have an actuator as well?
Thanks,
Michael

Well today I tracked the vacuum leak to the lock side of the vac system and all the door actuators are good but the line for the hatch lock/ gas lock has a big leak. I pulled the emergency panel and found this



The unlock line is gone and the unlock diaphragm is torn. The lock diaphragm is good. So the big leak is most likely in the hatch lock. I am going to tear into that tomorrow as I need to reinstall the door panels and I'm loosing light. Could some one let me know where the Y that splits the trunk/ gas door lock is located on a 300td and where the unlock line for the gas door originally ran? I see a large unlock line above the gas door actuator which I assume goes to the trunk lock but I do not see the lock line running to the trunk lock. Any help is appreciated.

Oh also would anyone happen to know what type of fastener held this joint on the passenger front door, it is the link between the actuator arm and the door lock.

which explains why that door only worked on occasion....

Well not being one to leave things alone I busted out the flashlight and pulled the hatch cover. The lock actuator is working fine although it leaks down slowly(10min) on the lock side. The leak is in the line between the x connector in the passenger side floor pan and the hatch. Now to find that y in the vacuum diagram so I can isolate the two legs and see which I need to replace. My secret hope is that one leg of that y is detached and I will not have to snake new vac lines, but I am not holding out too much hope on that one. But the passenger side rear seat back was replaced by the mechanics so maybe...

As I noted earlier, I don't believe your gas door has an unlock side. It's the early style. It uses vacuum to lock it, and a spring to return it to unlock when vacuum is removed from the lock side.

So I think your gas lock is set up as designed. It may be leaking down slowly, but it's not missing anything. And since there is no unlock side, it's not part of the problem for the unlock portion of the system.

If you are finding that all your door locks are good, the leak on the unlock side is probably from the trunk actuator.

__________________
1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

http://i.imgur.com/LslW733.jpg

The Baja Arizona Oil Burners Send a message if you'd like to join the fun
Left to Right - UberWasser, Iridium, Stuttgart-->Seattle,, mannys9130

Visit the W123 page on iFixit for over 70 helpful DIY guides!

Thanks I see that the gas door lock has only a lock line on the diagram now

The hatch lock actuator is working well with only a very slow leak, the big leak is in the line(s) between the X fitting in the passenger side floor pan and the hatch. If I block two of the three lines it will not hold vacuum at all and seems like it is open or cracked badly. Any Wagon owners happen to know where the Y that spits the hatch and fuel door vac lines is located? Thanks,
Michael