Vacuum Lock Issue - Mercedes-Benz Forum
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post #1 of 11 (permalink) Old 05-14-2016, 02:05 PM Thread Starter
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Vacuum Lock Issue

I have an issue with the vacuum locks on my '84 W123. The locking only works on the right rear door, fuel door and trunk lock (although the trunk only locks and does not unlock again).

The locking ONLY works when the engine is running - so the vacuum must not be holding.

I am hearing a strange noise at the back of the car (not on any particular side - at the bottom of the car) - it sounds like air moving through a small gap. I was thinking this may be a leak in the vacuum locking system. Here is a video of the noise:

Could this be related to my vacuum lock issue? And if so, what could be making that noise?

Many thanks
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post #2 of 11 (permalink) Old 05-14-2016, 05:41 PM
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From your description I can say that, yes, you have an issue with your vacuum door lock system.

What is the issue? Too many things to make guesses from your description. Probably one or more vacuum actuators with bad diaphragms. However, this system has lots of working parts involved so some diagnosis will be necessary. Use this comprehensive guide that I wrote on how to diagnose the vacuum lock system:

https://www.ifixit.com/Guide/Mercede...echnique/20201

Then come back with any specific questions you have.
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post #3 of 11 (permalink) Old 05-15-2016, 10:57 AM Thread Starter
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Thanks! I'll follow the guide and see what I can find. Do you think that noise is related to my vacuum lock issue?
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post #4 of 11 (permalink) Old 05-15-2016, 12:32 PM
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The vacuum reservoir is located in the region of where that noise is coming from. But I can't really say for sure if that's vacuum related. If you shut the car off does it still make that sound for a few seconds?
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post #5 of 11 (permalink) Old 05-15-2016, 08:09 PM
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Originally Posted by jacquesblom View Post
Thanks! I'll follow the guide and see what I can find. Do you think that noise is related to my vacuum lock issue?
Is your car a station wagon? If so the vaccum reservoir is down beneath the car near the rear bumper. Could be leaking. If it's a sedan the reservoir is in the trunk and it's unlikely you'd hear it.
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post #6 of 11 (permalink) Old 05-15-2016, 11:39 PM
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Anywhere where vacuum tubes are connected by an elastic hose connector is a potential source for a vacuum leak. I'd start by testing the vacuum in the engine bay to see if those connectors aren't bad. Then I'd try the driver's door. Next I'd check the actuators in each door, an finally the egg crate reservoir in the back.

If those check out, you have a much worse problem: the hard vacuum lines...
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post #7 of 11 (permalink) Old 05-16-2016, 01:32 AM
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Hi will this help you in any way? May be a check valve
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post #8 of 11 (permalink) Old 05-16-2016, 10:53 AM
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def. a vacuum leak. Did the other locks ever work since you've had the car? I would start isolating the locks and working on one, one at a time. If the other doors don't work, the previous owner may have intentionally blocked them off as they may have been leaking. The pods are prone to tearing (and are old) as they are rubber. Also, the pods on an 84 are two way. What that means is that you will supply vacuum to lock on one nub and supply vacuum on another nub to unlock.

In a nutshell, you will access the 3 and 4 ways underneath the mats. Supply them vacuum in lock and unlock positions to determine if there is a leak. Do the same in the trunk. You will have to remove the liner and the panel at the trunk lock.
If all else fails, you can do what I did... get actuators on all doors including gas/trunk. After I spent countless hours trying to fix this, I determined that the actuator at the master was the culprit and it needed to be very precise. I ended up pulling the vacuum pods and putting electric actuators.
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post #9 of 11 (permalink) Old 05-26-2016, 04:38 AM Thread Starter
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This is what I've found:
- Locking/unlocking only works when car is on or about 10 seconds after switching off the engine.
- Passenger front door: Locking/unlocking works. But I need to help it with my hand a little before it starts moving.
- Rear right door: Works slowly without coaxing
- Rear left door: Doesn't work at all. However a car crashed into this door.

Could it be that the rear left door has a leaking vacuum line, which is causing the vacuum power of the entire system to be weak?
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post #10 of 11 (permalink) Old 05-26-2016, 04:59 AM
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It could be at any of the doors. It could, in fact, be at any of the connection points.

Use a golf tee to plug up the connector of the suspect element, then see how the rest of the system responds. There is no quick and easy way to diagnose the vacuum problem.

See if you can use a vacuum cleaner to apply vacuum if you don't have a vacuum hand pump - that'll remove the necessity to start and stop the engine so often.
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