1978 450 SEL... power locks?

[skatinblind815]

Dear w116 users, greetings!
I am finally the proud owner of a 1978 450 SEL and a new w116.org user. It is awesome, but one thing is worrying me. I cannot get the doors to lock! They wont lock using the key, and you cant push them down. Seems to me that they are automated in some way. Does anybody know the source of my problem? Any advice on a good starting point would be gladly appreciated. Please help, I know there has to be some 450 gurus on this website.
Thanks, PEACE

[flutes]

Hello...and welcome to the forums!

The central locking system of the w116 is controlled by a convoluted but surprisingly effective pneumatic system.  A pump on the motor maintains vacuum in a supply tank, and a switch in the driver's door then controls the system through vacuum elements in each of the other doors, the fuel cap and boot.  The driver's door has the master cylinder that drives everything else.

In a well functioning system it will hold its vacuum for a couple of weeks.

Usually the system develops leaks over time and will only hold the vacuum for a short period of time (my 450 holds for about 6 seconds!) but problems presumably with the master cylinder causing the whole system to remain locked seem to be common.

Search for "doors wont lock" or "Can't lock doors" on this forum yields quite a bit of info and some debugging steps but nothing conclusive.  Here's an example.

There is also the Service Manual Library (link in the footer) but these usually cover specific steps for removal/installation of parts, not debugging steps.

Someone else with more experience please chime in....

Regards,
Matt.

[koan]

It.s a pretty simple system, use engine vacuum (on petrol engines) to "suck" on vacuum actuators to lock and unlock doors etc.

What complicates it is the need to "store" vacuum (tank under parcel shelf), and one way valves to keep the vacuum in when the engine is off.

Many places leaks can occur, starting with the blue/black one way valves at the fire wall, the drivers door vacuum switch. the vacuum lines where they pass from body to doors and the rubber diaphragms themselves.

The first test would be run the engine and use the driver's side door button to lock and unlock the doors.

koan

[TJ 450]

When the locks aren't working properly, they can do some bizarre things. In my 6.9, they go up and down constantly when idling, making strange sounds!

I would try locking the driver's door with the key, then go around to all the others and push the buttons down manually. See how it goes.

Tim

[wbrian63]

The fact that you can't lock the doors is a good sign, actually.

There is no way to lock the open driver's door of an MB and close the door. You have to close the door and lock from the outside with a key.

If the driver's door is unlocked, attempting to lock any of the other doors should be met with resistance and if you're successful in pressing the lock knob down, it should pop back up when released.  This is because the driver's door is unlocked, and the control valve is ported to the open position. When you press the lock pin down on the other doors, the diaphragm actuator in the door forces the pin back up again. This only happens with a actuator that's intact (no leaks).

As noted by others, get in, start the car, close the driver's door and attempt to lock the door by pressing down on the lock pin. All 3 remaining doors should lock at that point. When you pull on the door handle to open the driver's door, all the other doors should unlock. Turn the car off, exit and close the driver's door. Attempt to lock with key (it will feel "resistive" to the turn of the key, not what you'll be used to with other makes of autos) - when you lock the driver's door, the remaining 3 doors should follow.

The one-way valves underhood are cheap and available from a variety of sources. They can be tested by removing them (pay attention to which side of the valve is attached to what piece of hose) - they're bi-colored. They should only allow airflow in one direction. In the case of the valve for the locking system, you should be able to draw vacuum thru the nipple that was attached to the hose that attaches to the motor, but not the other way around. Failure of the one-way valve will only prevent engine-off operation of the locks - it shouldn't have an effect on the system with the engine running, unless the valve is completely blocked, which is a rare condition.

The "egg-crate" that's under the rear package shelf stores the reserve vacuum for engine-off actuation. There are also 3 sets of dual-action diaphragms, one in every door except the driver's. They can be rebuilt with kits available from www.mercedessource.com. The kits rebuild 1/2 of each valve, so you need 6 kits. Last I looked, the part # was VCM-116LDK and they cost $16.95US each.

The rigid plastic hose that flows the vacuum from point to point isn't usually a problem, it's the rubber hose that connects each hard line to it's related device that usually cracks and leaks. There's also a dual-acting valve that's attached to the driver's door lock mechanism, this is what sends vacuum to the door pods to pull the door lock down, or pull the door lock up, depending on which direction the driver's door lock pin is pressed (or which direction the key is turned).

There's also 2 more actuators - one that locks the fuel door, and one that deactivates the button that opens the trunk. I don't think these are rebuildable, but they are available as parts for reasonable $.

[skatinblind815]

I took your advices. here is where i am at. When the doors are closed, and you push down the driver side lock pin (from the inside), all of the locks (inclunding the trunk and fuel door ones) lock perfectly and tightly. When the drivers door is open and you try to push any of the locks down, you are met with heavy resistance which seems like a good thing. But when you try to lock the doors with the key (from the outside) the key just kind of turns inside the locking mechanism without any resistance. This leads me to believe that the lock in the handle is disconnected or messed up. Going to check it now. Just got my new manuals 

[koan]

When the locks aren't working properly, they can do some bizarre things. In my 6.9, they go up and down constantly when idling, making strange sounds!

That's very weird.

koan

[koan]

Sounds like you are on the right track.

Was the engine running or not for both inside and outside tests?

The only time the door button can be pushed down is with the door closed.

There's a spring arrangement on all passenger door lock buttons. This allows the vacuum actuator to move to the lock position while the door is open, when the door is closed it is locked and the button pulls down.

koan

[Papalangi]

I took your advices. here is where i am at. When the doors are closed, and you push down the driver side lock pin (from the inside), all of the locks (inclunding the trunk and fuel door ones) lock perfectly and tightly. When the drivers door is open and you try to push any of the locks down, you are met with heavy resistance which seems like a good thing. But when you try to lock the doors with the key (from the outside) the key just kind of turns inside the locking mechanism without any resistance. This leads me to believe that the lock in the handle is disconnected or messed up. Going to check it now. Just got my new manuals  

On a W114, the end of the lock cylinder has and adjustable cam that if not adjusted correctly will let the cylinder spin around without locking or unlocking the door.  I'd guess that the W116 is the same.

See http://handbook.w116.org/chassis_73_4l.htm
Click more (on left side) till you see 72 Doors, click it
Click more (on right side) till you get to 240 Removing outside door handle and click that

Michael

[TJ 450]

It could also be that someone has removed the external handle in the past, and not reinstalled it correctly.

Yes, the oscillating locks issue is very strange indeed.

Tim

[skatinblind815]

w116 bro's,
To answer Koan, I tested the locks from the inside with the engine ON and OFF. They work both ways.
Also my key doesnt do anything in the drivers side or passenger side front doors. But when you put it in and turn it either way, there Is a slight springy feeling that pushes it back to the center. But I still cant unlock anything. I still haven't actually taken off the door handles and checked, but I was wondering if anybody knows if there is a fuse or something that disengages the door locks? Going to check again. Didnt see anythningabout door locks on the fuse box sheet..
-nick

[koan]

w116 bro's,
To answer Koan, I tested the locks from the inside with the engine ON and OFF. They work both ways.

Can only be the linkage in the drivers door. When you use the key in either front door it should move the lock button up and down.

if there is a fuse or something that disengages the door locks? Going to check again. Didnt see anythningabout door locks on the fuse box sheet..

No fuses, just vacuum.

koan

[skatinblind815]

Cool! I took the door panel off and took a look inside the door and I now see that the very tip of the key mechanism isn't connected to anything. Am i missing a connceting piece that goes somewhere, maybe up towards the locking pin or the lock or something? I look at the lock from the inside and the tip is just wiggling with the turn of the key and has nothing attached. Also I'm having trouble removing the whole thing because i cant get the stripped screws out. 

[koan]

Ages since I've been in there can't remember what connects to what.

Have a look at job 72-250 Doors-Removal and installation of door lock, there are a couple of pictures which might help.

Which screw are stripped (or is it the + heads damaged)?

koan

[koan]

Ages since I've been in there can't remember what connects to what.

The link might be in the bottom of the door.

Have a look at job 72-250 Doors-Removal and installation of door lock, there are a couple of pictures which might help.

Which screw are stripped (or is it the + heads damaged)?

koan