My girlfriends '78 240d doors are locked all the time. To get out you have to muscle the locks up and then they pull right back down. It's a real PIA. I'd like to disconnect the vacuum and have them be manual. Will this work well? Where's the best place to plug off the vacuum?
If I could make them work as automatic that would be fine too but I won't put more than 1 hour into it since it just isn't worth it to have automatic door locks. I've never owned a car with auto door locks in the 30 years I've owned cars. We just don't need that...

Interesting problem. It sounds like the issue is with the master vacuum switch in the door panel. That or the previous owner messed with the vacuum system. Others can chime in and offer their assistance here.

You can read up on how to replace it here.
http://dieselgiant.com/mercedesmastervacuumswitchinstall.htm

To disconnect vacuum for the door look system look inside the engine bay for a small two legged dashpot. It resides near the oil filter housing. At least this is the case on my 85 300D.

This post has a diagram as well as a picture of the two legged dashpot I was referring to.
W123 Vacuum InterLock Diagram

Fixing the vacuum door lock system isn't hard at all, assuming a PO didn't screw with stuff. I still want to believe that the problem is with the master vacuum switch in the driver's door panel however.

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You also need to disconnect the reservoir. If you don't, the locks will reposition themselves with major temperature changes.


Just forward of the firewall.

Does this mean that under normal circumstances, if the vacuum becomes depleted, say, from repeated opening and closing of the locks before the car is started again to run the vacuum pump, the reservoir can become pressurized from a big increase in temperature (PV=nRT) and the increase in pressure can mess with the lock position?

Richard

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85 300D 75K Anthracite Grey 0-60 in 13 seconds **For Sale**
84 300D 333K Black (The Velveteen Rabbit) 0-60 in 14 seconds
00 Toyota Sienna 208K (Sold)
15 Subaru Outback 43K
11 Subaru Outback 67K
98 Ford Taurus 100K (Gertie - Was Grandma's - drove it to church and shopping - really) Daughter's car now.
30 Model A Ford 2 Door Sedan (Sold) 0-60 in . . . Never reached 60

the yellow vac lines with either red, green, or brown stripes have to do with the door locks. On the firewall, if you plug the three way connector and disconnect that little check valve thingy, you will cut the locks out of the system but not affect the shut off.

A little piece of a stick works great for this. I did the same thing because I am working on the floor where the lines run and needed to remove them, I also pulled the yellow and brown line out of the reservoir in the trunk, careful, cause there is a little grommet for that reservoir that is tenacious, and then suddenly will fly out and disappear as you try and pull the line out.

Its probably something really simple that would be worth fixing though, you have vacuum , just too much! I second looking at the master vacuum switch

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After you live with the MB locking system you get spoiled, I would fix it, its really convenient when you have it working. Get your self a "mity Vac" for trouble shooting, its worth the effort.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

I have a couple master switches if you need one. $15 shipped.

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1981 300TD "The Green Lantern"
1980 300TD
1983 300D Euro "China Cat"

Agreed, same goes for having keyless door locks

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That is correct. If the master switch is in the lock position, whatever the reservoir has to offer (vacuum or pressure) is routed to the "unlock" side of the actuators. Pressure will have the opposite affect as vacuum, so the doors will unlock with a temperature increase. Lock your doors in the morning and you might come back to an unlocked car in the afternoon.

You found a girlfriend that drives a 240d?

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1982 240d "butternut"
1991 F350 4x4 7.3 5spd

All the door locks in the car follow what the drivers door does. If you lock the drivers door they all lock. When you unlock the drivers door the rest follow suit. The best place to unplug them is on the engine side of the firewall. There will be a T that supplies the door locks and the door lock reservoir. Just unplug the line that supplies that T and you'll be set. But make sure you plug the side that comes from the vac pump, or you'll loose vac and all the rest of the vac stuff won't work.

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This seems like a flaw in the security system. In a climate that has hot days and cool nights, a car that is left locked will eventually become unlocked as vacuum leaks out (air leaks in) and temperature cycles up and down, causing pressure to cycle up and down.

So a Benz sitting outside in a southern state in the US, of the the era that used a vacuum locking system, will most likely, eventually unlock itself.

__________________
85 300D 75K Anthracite Grey 0-60 in 13 seconds **For Sale**
84 300D 333K Black (The Velveteen Rabbit) 0-60 in 14 seconds
00 Toyota Sienna 208K (Sold)
15 Subaru Outback 43K
11 Subaru Outback 67K
98 Ford Taurus 100K (Gertie - Was Grandma's - drove it to church and shopping - really) Daughter's car now.
30 Model A Ford 2 Door Sedan (Sold) 0-60 in . . . Never reached 60

None of the Vacuum-locks in my w123s are currently working, but so far they haven't randomly operated while my car was unattended. As for disabling them, First I disconnected and plugged the underhood vacuum supply line on both my 240Ds because a leak somewhere in the sysyem was preventing the diesel shut-off unless I locked the driver's lock button before exiting the car.
Next, on the 240D I drive the most, I've been pulling the door-panels (front passenger and both rear) to unhook and move/tie out of the way the actuating rod from the vacuum motors. These rods actually have a flip-flop spring-link that's supposed to allow overriding the vacuum motor, but in reality, the flip-flop link is so stiff that I'm constantly struggling against whatever position the vacuum motor happens to be in. Since the vacuum motor still wants to pull or push on the lock button, I got tired of fighting with it everytime I wanted to manually lock or unlock the car. Now, with the rods disconnected, the lock buttons all move very nice and easy.
For the purists, it's not a big deal to go back later and reconnect the rods if you wish to spend the time, sort out the leaky locks and make them whole again.

Happy Motoring, Mark

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DrDKW

So I just talked to her about it again and she says:

The front passenger door is always locked and the rear two doors can be stuck either way.

I'm going to try plugging off the vacuum supply and leaving the door lock system open to air and see if that frees up the locks - if not I'll pull the panels and disconnect the vacuum motors by the link.

Maybe I'll fix the whole system later but it needs to work now.

By the way Golf T's usually work great for vacuum line plugs.

Thanks for all the help!!!

+2

Once you understand the basics of the system and have good schematics (in color on this site).... It is very simple to troubleshoot. There used to be a really good color schematic for the door locks here on this site. I think one of the members made it. It is VERY easy to follow....

Having a mighty vac makes it easy and you can test the various components.


IE: I found a bad "door lock actuator diaphragm" in my drivers rear door. I disassembled the actuator and put a "Slime Skab" over the hole on both sides. That was over a year and a half ago and the system still works flawlessly.

Around that time frame I also found some bad "vacuum elbows" in the car with the mighty vac. 2 on the ignition switch and a few under the hood.

Edit: Here is the link to the Color diagram if you get around to it.

W123 Vacuum InterLock Diagram

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AJ

1985 300D (SOLD)