On my 1980 300d I have disabled the vacuum lock system due to part failures and no desire to chase the gremlins. The lock related vacuum line is disconnected in the engine bay.

1. How do I make the driver door lock work in the fashion of not having to lock the door from the outside upon exiting the car. I want to be able to push down on the door lock knob and shut door and walk away with a door that is locked without having to use the key.

2. What is the deal with trunk locks. Sometimes I need my key to unlock the trunk and other times I do not. Sometimes within in 10 seconds of each other I can't make sense of it. What is the locking and unlocking sequences with the key? I am almost ready to hook up a remote trunk release

The answer to #1 is you can't. The lock mechanism is built to not allow you to lock the door with the door open. This prevents you from locking your keys in the car. You might be able to pull the mechanism out and figure out how it prevents you from pushing the lock down with the door open and somehow disable it.

The trunk lock follows the door locks, but it also has the option of always being locked. If you put the key in and turn it so the key is vertical and leave it that way it will alway be locked and you'll need the key to open it not matter what. If the key is in and turned horizontal and left that way it follows the door locks. If the door are locked you'll need a key to get in. If the doors are unlocked you don't need a key.

Also the actuator for the trunk lock is vacuum operated and if the system is leaking the trunk lock actuator often won't work or will take several seconds to actuate. So it may be remain locked for several second after you unlock the doors. But eventually unlock once the reduced vac finally gets it unlocked.

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#1--no can do.

#2--mystery. Should be disconnected from the vacuum system if you have the yellow line plugged under the hood.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

But it's not disconnected from the vacuum reservoir. Temp changes can lock/unlock the locks. Disconnecting the reservoir should eliminate the problem.

Yes can do if you open the rear door and push the knob down.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

From where does the reservoir get its vacuum? I've assumed it was coming via the yellow line. No?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Good info for anyone whose car did not come with an owner's manual!!!

Pressure on one side of a diaphragm has the same affect as vacuum on the other side.

For sure, if there is air in the reservoir, a locked car can unlock itself (and vice versa) after sitting in the sun. (The reservoir is isolated by check valves, so unplugging the vacuum supply line at the firewall won't vent the reservoir.)

I sound like Captain Obvious when you, quote a single sentance out of the paragraph.

Let me clarify. When the trunk lock is in the right position you always have to use the key to get into the trunk no matter which state the door locks are in. When it's in the other postion the trunk lock follows the door locks.

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I've never seen this happen. I think you'd have to go from sub freezing temps to up near the 100s to get that to happen. I wouldn't think the daily temp change would cause the door to lock or unlock.

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With the trunk, if there's vacuum sometimes it'll work with a push; if there's no vacuum, then sometimes it won't.

Unless the lock is mechanically busted you should always be able to turn to a 60 degree position and push. I forget if the starting position is vertical or horizontal.

If, as you describe, the trunk lock acts up within 10 seconds of opening or closing then that may be a clue that possibly the trunk lock has the leak in either the pod or the line.

I have. More than once.

It most certainly will. It doesn't take much of a pressure differential to move the door lock actuators.

Why do ppl actually lock their doors? I mean number of benz diesels stolen yearly has to be near zero.

More than one crackhead has taught me the valuable lesson of leaving doors unlocked and in most cases window down.

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1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42

I'm not so sure. Where would the air go? I presume it would follow the path of least resistance.

The check valves are oriented to isolate high pressure from low pressure areas when the vacuum pump isn't pulling air into it. So one check valve lets high pressure air flow INTO the pump (or the lowest pressure area) from the rest of the system, including the reservoir. When the pump isn't operating then the reservoir is the ersatz pump so high pressure air from the lock system needs to flow INTO the low pressure (vacuum) reservoir. So the check valve would not work the other way to reverse the flow FROM the (heated pressurized) reservoir to the locks. But it COULD leak from the reservoir to the pump, because the check valve lets air flow from the reservoir INTO the pump, just like normal operation.

The check valve between the reservoir and the locks is OPEN when the pump is running because they're both on the same side of the valve and the pump creates a low pressure area on the pump+reservoir side. When the pump stops, in theory the lock network stays low (no leaks) and the reservoir is slightly higher, so the check valve CLOSES. When you open/close the locks you let a little air INTO the system on that side so the pressure increases and pushes open that check valve momentarily - air is permitted to enter the reservoir in a brief controlled fashion via the check while the locks move. If you've got a leak in a pod or line, then air leaks in all the time so the reservoir "runs out" of vacuum (or more correctly the air flows from the high pressure leaky side past the check valve and fills up the low pressure reservoir). If the reservoir has a leak, then it fills up on its own and there is no "low pressure" area to create air flow and the locks won't work.

The basic premise would be true: high pressure on one side acts just like low pressure (vacuum) on the other side, but I don't think the check valves are oriented in such a fashion as to permit the build up of pressure on one side so in practice I don't think it would happen if the check valves are installed correctly.

It takes some amount of pressure differential to trip a check valve. Check valves do not necessarily guarantee a zero pressure differential. And it takes very little differential to move a lock actuator.

In any case, maybe the OP could disconnect his trunk actuator vacuum lines and report his findings. My money says that the mystery lock behavior ceases immediately.