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1983 300TD, 115,000 miles. Anthracite Gray
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Discussion Starter #1
I've done a lot of reading about this, on this forum and elsewhere. I've determined that the yellow tube feeds the door lock actuator, which then sends vacuum down yellow/green or yellow/orange tubing to each door lock. Vacuum is applied to one of those lines to lock, the other to unlock.

If I've got something wrong here, please let me know.

OK, the question: the vacuum in the yellow line--the line going to the actuator in the driver's door--should hold in either position, right? It should hold whether the doors are locked or unlocked.

I'm finding that it holds when the doors are locked, but not when they are unlocked.

Thanks,

Jim
 

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1982 300D Turbo with newer engine (KKK Turbo)
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90 Posts
Jim,

The yellow line comes from a vacuum tank that stores vacuum so that the locks will work when the engine is off. I'd suggest disconnecting the hoses from the valve in the driver's door to see if the switch itself has a leak in it. If the switch doesn't leak, then look for leaks on the locked side (the yellow/orange or what I think of as yellow/red lines). Typically with age the diaphragms at each lock around the car start to get leaky, and it's common that one will leak in only one position. If you confirm that you can't pump down the yellow/orange line to a vacuum or it won't hold for long, you'll need to go downstream to figure out which of the diaphragms is leaking. There are tees under the driver's carpet and the front passenger's carpet that distribute this vacuum out to all of the doors plus one line to the trunk and fuel door actuators. There is an additional tee in the trunk behind the right side fender well trim that splits that line out to go to fuel door and to the trunk lock.

I don't know how recently your system was working, but I recently bought a fairly rough '82 model and found that the only diaphragm that wasn't leaking badly was one that had obviously been replaced. So I ended up replacing absolutely everything, including the valve in the driver's door (which was also bad). With all new components mine will hold a vacuum for at least a week, so when you come back to it after a week and turn your key in the door, the other locks respond in kind. Experience with this one car showed that the leaks were at the master valve and lock actuator diaphragms- my tees and lines were actually fine, though I have heard reports of the lines breaking at the point where the flex with movement of opening and closing of the doors.
 

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1983 300TD, 115,000 miles. Anthracite Gray
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163 Posts
Discussion Starter #3
Jim,

The yellow line comes from a vacuum tank that stores vacuum so that the locks will work when the engine is off. I'd suggest disconnecting the hoses from the valve in the driver's door to see if the switch itself has a leak in it. If the switch doesn't leak, then look for leaks on the locked side (the yellow/orange or what I think of as yellow/red lines). Typically with age the diaphragms at each lock around the car start to get leaky, and it's common that one will leak in only one position. If you confirm that you can't pump down the yellow/orange line to a vacuum or it won't hold for long, you'll need to go downstream to figure out which of the diaphragms is leaking. There are tees under the driver's carpet and the front passenger's carpet that distribute this vacuum out to all of the doors plus one line to the trunk and fuel door actuators. There is an additional tee in the trunk behind the right side fender well trim that splits that line out to go to fuel door and to the trunk lock.

I don't know how recently your system was working, but I recently bought a fairly rough '82 model and found that the only diaphragm that wasn't leaking badly was one that had obviously been replaced. So I ended up replacing absolutely everything, including the valve in the driver's door (which was also bad). With all new components mine will hold a vacuum for at least a week, so when you come back to it after a week and turn your key in the door, the other locks respond in kind. Experience with this one car showed that the leaks were at the master valve and lock actuator diaphragms- my tees and lines were actually fine, though I have heard reports of the lines breaking at the point where the flex with movement of opening and closing of the doors.
Great information--Thanks. I've been gradually working through this, learning as I go (and gradually acquiring stuff like rubber hoses with the right ID that let me tap into the system.

This afternoon I took out the floor in the back and put a T in, then turned the car on to pump it down. It held vacuum fine (maxed out at about 18 inHg) with the driver's side door locked; went down to zero (or near enough) when the driver's side door was unlocked. And yes, when the doors are locked (and the car is allowed to run for a while) the engine shut-off works.

So it's definitely somewhere in the door locking system. I'll do some more work--probably get into the driver's side door--this weekend.

Thanks again.

Jim
 

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European 1974 350SLC ,'78 300CD& '80 300CD sold , '81 240D SWMBO's Car '84 300CD & Euro
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666 Posts
Central Locking Testing

Go to Harbor Freight Et Al and buy a " Mity - Vac " tool , it's a small hand held vacuum pump with dial and it allows you to test each part individually .

Don't go taking the driver's side door aprt willy-nilly unless you're ready to drop over $200 replacing every vacuum chamber in the system .

Take up the passenger's side carpet then gently release and tip up the black plastic cover so you can access the rubber vauum T's , test from there ~ once you figure out which part of the circuit is bad , doors or trunk / fuel flap lock , then you can begin taking things apart .
 

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W-1-2-3 Go!
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16,161 Posts
When you are unlocking the doors, your observation states that the vacuum does not hold. If so, then look for the door lock pin that rises the last. This usually is a sign that the actuator in that door is leaking, because in principle they should all receive vacuum to unlock the doors at the same time.

If you want to isolate the driver door master switch, I would plumb another line to the unlocking nipple, to the handheld vacuum pump (or a container that is known to hold vacuum), and simulate the same scenario. If the vacuum gauge reads that it does hold vacuum, then the problem is on the other 3 doors, trunk or fuel filler actuator.

Everything else will be a matter of removing the door panels or access panel in the trunk. The fuel filler actuator is the most difficult one to access due to its configuration.

Don't forget how the lines were installed, so you don't reinstall them backwards, if you take both lines off for testing/diagnosis.
 

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1983 300TD, 115,000 miles. Anthracite Gray
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163 Posts
Discussion Starter #6
Hello all--problem solved, and a good time too.

By locating most of the Ts I was able to trace the problem to the passenger-side rear door vacuum element without removing any panels. Hoping (but not expecting) to finish the job this weekend, I called the dealer on the unlikely chance that they had the part in stock. Nope.

So I called up the local independent that's supposed to be closed on Saturday. Dave picked up on the second ring. Thought they might have something around, but wasn't sure. Boss will be in soon. He'll know. I called twice more and the boss still wasn't in, so I got in my car and headed south. It's about a 45 minute drive (to German Auto Service in Saco, Maine).

This is one of those places with corpses of a couple hundred benzes scattered about and used parts in cardboard boxes in out-buildings, the attic, and anywhere else there's a corner. Dave and I hunted around, found a big box full of the old style vacuum elements (I knew what they were from the PeachParts picture). But couldn't find the later style my car takes. So I asked Dave if they had any later-model 123 among the corpses outside. We went hunting and soon found an 85 300D, with one door panel still intact.

"Can I get my tools out of my car and dig it out?"

"Sure, go ahead."

So I did--and got to rehearse my repair in a junker before trying it on my car. 15 minutes later I had a vacuum element. Took it back to my car where my MityVac was stashed to test it out. Tested great.

Boss still hasn't turned up, so I ask Dave, "What do I owe you?" "No idea," he says. Give Larry (the boss) a call next week and work it out with him.

I've given these guys my business before, so I was probably a somewhat familiar face. But I'm not THAT familiar, and he didn't even write down my name.

Drove home, employed my new door-panel-removal skills, took out the old element and popped in the new, etc.

So now my 300TDs doors lock, the engine shuts off, and all the other good things that go with having a healthy vacuum system.

If you have an old Benz and you live near me, you already know about German Auto. But if you don't, give 'em your business. The place is a mess, reaks, and the walls are covered by political stuff that's borderline offensive (but funny!) But they're great to work with. Recommended.

Thanks, everyone, for your help.

Jim
 
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