Strange central locking problem - PeachParts Mercedes-Benz Forum

DieselAddict · #1 06-04-2006, 03:56 AM

Recently I replaced the rubber boots that protect the vacuum hoses in the front door jambs of my 240D as part of my campaign to waterproof the car. Before that I had a working central locking system but now I don't. To replace those boots I had to take off the interior door panels and disconnect the vacuum hoses that run through those boots and I made sure to connect them back properly. The engine shuts off properly and all other vacuum features work. Here's the strange part. If I unlock all the doors and run the engine, the doors will slowly lock themselves (except the driver's door of course), but they will not lock if I unlock them manually and then lock the driver's door. Before I messed with the system, the opposite used to happen, i.e. if the doors were locked, they would slowly unlock themselves when the engine was running, but they would lock if I locked the driver's door. Today I had the door panels off again and I tried swapping the red and green hoses in each front door just to see if I had made a mistake somewhere, but after any such swapping the engine wouldn't turn off and the central locking system wasn't working at all. So it seems like the hoses were hooked up properly, but why is the system now acting in reverse compared to before? Any ideas?

yellit · #2 06-04-2006, 05:02 AM

Have you checked the lock/unlock valve in the driver door...this is what steers the vacuum to the lock or unlock hose pairs common to all of the vacuum motors...if it is loose in its bracket or leaking or has become detached from its pull arm then this is where to start...provided you have a solid vacuum source to the valve from the vacuum tank....hope this helps....kev

#3 06-04-2006, 09:32 AM

This is a guess, but I would suspect the three connections to the valve in the drivers door are somehow not correct. The vacuum "supply" line from the engine compartment connects to the center connection, and the open and close (green and red) lines connect to the two end connections, I don't remember which is which. First verify the center line is the supply, then try switching the other two connections until the other doors "follow" the drivers door in the correct direction. If the car does not shut off, there is a leak someplace. The reason I suspect the 3-way valve, is that the valve is designed to vent the "closed" line when vacuum is applied to the "open" line and vent the "open" line when vacuum is applied to the "closed" line. If these connections are not in order, it will create a "leak" in the system.

DieselAddict · #4 06-05-2006, 12:51 PM

Thanks guys for the replies. The 3-way vacuum switch in the driver's door doesn't seem to be the problem. Yesterday I only had limited time to do some more testing, but this and the passenger door are the places where I did the hose swapping and each time it resulted in not being able to shut off the engine. The hoses were in the correct configuration the first time. Yesterday I made sure my vacuum tank was leak free by using my MityVac on the blue hose that runs through the firewall. As I thought about it some more I think I now may be onto something. The driver's door lock seems to control whether vacuum is created in the vacuum tank or in the lines that control the locking. I'm thinking maybe I have to drive the car around with the driver's door locked to "recharge" the vacuum tank. Does that theory hold any water? I'll give it a try the next time I have some free time.

tangofox007 · #5 06-05-2006, 01:35 PM

Quote:

Originally Posted by DieselAddict

The driver's door lock seems to control whether vacuum is created in the vacuum tank or in the lines that control the locking. I'm thinking maybe I have to drive the car around with the driver's door locked to "recharge" the vacuum tank.

The vacuum reservoir is always connected to either the lock or unlock vacuum circuit, as determined by the selector switch in the driver's door. If the reservoir is not holding vacuum with the doors unlocked, you must have a leak in the unlock circuit.

Maroon 300D · #6 06-05-2006, 01:42 PM

I would have to concur with the others, it seems like the problem must be with the 3-way door valve. I would suggest borrowing a working one and testing it in your car if possible.

Please keep us posted. I'm interested in finding out exactly what's going on.

Maroon 300D · #7 06-05-2006, 01:50 PM

Quote:

Originally Posted by tangofox007

The vacuum reservoir is always connected to either the lock or unlock vacuum circuit, as determined by the selector switch in the driver's door. If the reservoir is not holding vacuum with the doors unlocked, you must have a leak in the unlock circuit.

Hmm, that seems to make sense, too. Would a leak in the unlock system create a 'venting' effect like was described above? If so then I guess a leak could cause the doors to lock when unlocked.

DieselAddict, the way you describe the doors slowly unlocking when the engine is running is what's supposed to happen provided the driver's door is unlocked at the time. On my car it's not slow, though, so that might suggest that you always had less vacuum, due to a leak or some other problem, in the system than you should have had.

tangofox007 · #8 06-05-2006, 02:32 PM

Quote:

Originally Posted by Maroon 300D

I guess a leak could cause the doors to lock when unlocked.

I don't see how a leak would cause that condition. The door lock actuators are not "spring loaded" to either position. If things are working in reverse, something must be connected backwards.

Maroon 300D · #9 06-05-2006, 03:02 PM

I'm thinking that if the door valve operates by venting the 'unlock' vacuum line in order to allow the doors to lock, then maybe a leak in the 'unlock' line could also cause doors to lock. I guess the leak would have to be similar in size to the amount that would normally be vented. Perhaps this amount would be small enough that while the engine is running, other vacuum functions would not be effected.

I'm not sure exactly how the resorvoir system works, so I wonder if it would be possible to lock/unlock the doors a few times after shutoff even if there was a small leak.

Even if this is true I'm not sure it would explain why the other 3 locks won't engage if they are unlocked manually and then the driver's door is locked.

tangofox007 · #10 06-05-2006, 03:09 PM

Quote:

Originally Posted by Maroon 300D

I'm not sure exactly how the resorvoir system works, so I wonder if it would be possible to lock/unlock the doors a few times after shutoff even if there was a small leak.

The reservoir should enough capacity to operate the locks 6-8 times with the engine off. So a small leak certainly will permit a number of cycles shortly after shutdown. But not hours after shut down.

DieselAddict · #11 06-05-2006, 03:10 PM

Quote:

Originally Posted by tangofox007

The vacuum reservoir is always connected to either the lock or unlock vacuum circuit, as determined by the selector switch in the driver's door. If the reservoir is not holding vacuum with the doors unlocked, you must have a leak in the unlock circuit.

So you're saying that if I apply vacuum to the entire locking system (at the T in front of the firewall that connects the yellow and blue lines) I should be able to build it up and hold it regardless of whether the driver's door lock is up or down, correct? I did this test on the first day and the results were as follows:

* With everything unlocked, applying vacuum locked everything (but the driver's door of course).
* If the driver's door was locked, nothing happened.

Also, even before I messed with the system, I was never able to unlock the other doors by unlocking the driver's door. Only the locking feature ever worked. Does this information help in the diagnosis?

tangofox007 · #12 06-05-2006, 03:21 PM

Quote:

Originally Posted by DieselAddict

So you're saying that if I apply vacuum to the entire locking system (at the T in front of the firewall that connects the yellow and blue lines) I should be able to build it up and hold it regardless of whether the driver's door lock is up or down, correct?

Correct. The switch in the driver's door just routes vacuum to either the "lock" circuit or the "unlock" circuit. So your problem could be at the switch or somewhere downstream from the switch. Or some combination thereof.

tangofox007 · #13 06-05-2006, 03:27 PM

Quote:

Originally Posted by DieselAddict

Also, even before I messed with the system, I was never able to unlock the other doors by unlocking the driver's door. Only the locking feature ever worked. Does this information help in the diagnosis?

Oh... that's a little different than when you said "Before that I had a working central locking system but now I don't."

See if the "unlock" line coming from the driver's door holds vacuum with the botton in the "unlock" position.

DieselAddict · #14 06-05-2006, 05:22 PM

Quote:

Originally Posted by tangofox007

Oh... that's a little different than when you said "Before that I had a working central locking system but now I don't."

See if the "unlock" line coming from the driver's door holds vacuum with the botton in the "unlock" position.

Sorry, I realize I initially wasn't as clear as I should have been. I was so used to the one-way central locking system that I thought it was normal that way, but now I realize that it probably wasn't. I'll let you know what I find out.

DieselAddict · #15 06-05-2006, 05:32 PM

BTW, which line is the "unlock" line? The one that runs to the center of the driver door switch (plain yellow) or the one that seems to run straight to the vacuum tank (with the blue stripe)? There are also the red and green lines that run through the doors. I'm still somewhat unclear on what each line does.