So, the time has come to trouble shoot my lock system. All in all, system is working fairly well....all doors, trunk and gas door will lock and stay locked save for the gas door which bleeds off after about 7 minutes (at which time it "opens" or the pintle retracts and everything else stays locked). After pulling that actuator, I found the external rubber boot had a split and using the Mityvac, it would only actuate to the "unlock" position. I was able to repair that with some fine mesh and silicone. Checked with Mityvac and it worked in both positions after the repair. Prior to the repair it would bleed down after about 20 seconds, so 7 minutes is a step in the right direction, I guess? For some unknown reason, the car's vacuum system would actuate the gas door to the lock position but the Mityvac would not (more powerful vacuum signal?). Anyway, I'm guessing it may have a leak on the internal diaphragm. Trying to save 50 bucks. I realize the gas door actuator could simply be removed and the lines plugged (which I did) solving my problem, but I think there are other leaks as the doors lock and unlock very slowly (even with the gas door actuator removed). Time to dig a little deeper. One other observation is the right rear door lock is the slowest of the bunch (right in front of the gas door actuator). Not sure if this is relevant. Stumbled onto this site and thought I'd share. Good info and a methodical approach. Links to other related pages, too. This may have already been posted (more than likely) but I was "there" and thought I'd pass it along....cheers!

https://www.ifixit.com/Guide/Mercedes+W123+Vacuum+Lock+System+Diagnosis+Technique/20201

Wanted to update this thread and close it out. My "repair" to the actuator was unsuccessful and lasted only a short while. Even after the repair, fuel door still wouldn't remain locked very long. Realized I'd have to replace the part. Found a used one "the bay" for $25 (I was reluctant to spend $50 on a new one). Installed and all is well. Fuel door stays locked as long as the car is locked. Entire locking system now works as designed (gotta love it). Still can't figure out how the old actuator was bleeding down (remember, it's spring loaded to retract), but I wasn't losing vacuum anywhere else in the system? Doors/trunk, etc all stayed locked, locked and unlocked normally, even with the bad actuator. Not one to look a gift horse in the mouth, I'll take it. Really amazing, to me, how these systems still function flawlessly after 40 years. Thanks. Over and out.

Good to hear you sorted it out .

I fooled around with used vacuum actuators, they always failed after a year or so so I decided to bite the bullet and buy new from the M-B Classic Center as they're cheaper than the dealer by far including shipping to your door .

A slow door unlock is usually a tiny pin hole just beginning to go .

Remember : any of the short rubber connecting hose bits that pull off without serious twisting are bad and leaking .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

I'd say "got lucky", more than "sorted out"...heh heh. I never did go through the whole system as everything was functioning properly EXCEPT the fuel door actuator. I've always been curious as to how "fast" the door locks should "open and close". I mean, they've gotta be slower than their American, solenoid operated counterparts, yes? I'm assuming, even when new, they don't really "snap up" and "snap down"....just by the nature of the mechanisms, they gotta be a little more "relaxed", yes? I'm also quite impressed by how long it will hold vacuum in storage, ready to go, when you lock the car for the night, after hours of sitting in the driveway, etc. Very cool system!

I did not read all of the threads. Depending on the type of actuator you have the exterior boot can be replaced with a finger from a rubber glove. Somewhere there is a thread that shows Bosch replacement boots and the number.

The first post has the Bosch Boots and post 18 has a pick of one of the rubber glove finger repairs.
I found the thread: w123 door lock actuator diaphragm

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84 300D, 82 Volvo 244Gl Diesel

Nice work on your system. I think the only working vacuum lock on my '83 240D is the driver door (I can't push it down, can only lock it using the key, does that mean it's working?). Other than that the system seems to have been abandoned. I still need to get me a mityvac to trouble shoot, It's on my list of things to do but not top priority. I'm excited for when I get mine working though.

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1983 240D Manual Transmission
123.123 - 616.912

thanks for the info and link! This was a "shorty". I just posted 1 about my fuel door actuator, that's what was leaking. That's a unique piece, I think. Not sure how the diaphragms are set up for it. It doesn't come apart and has some internal passages or something. It was failing in the unlock position, but wasn't causing any issues with any other part of the locking system. I attempted a repair, but it was short lived. It's designed to fail "open", i guess, which makes sense so you can get to your fuel filler neck.
Found a used one, put it in, and everything is working "as advertised" now.

If it holds vacuum for a few days you're good to go, stop touching it .

On one car I re positioned the fuel locking rod spring so it was held locked and only released when I had the engine running and the doors unlocked....

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

I can live without the fuel door lock, indeed my 2002 Chrysler T&C doesn't have one and it came fully optioned (even 5 CD changer!), but I've never found one bad. Most leaks have been in the trunk lock, which I could likely also live without (key works) but I have spares. Perhaps the best place to start searching for leaks is at the right front door threshold (pull up carpet and plastic tray) since many vacuum tube junctions there to pop open and test each way, both for actuation and for holding a vacuum. Don't forget leaks at the vacuum reservoir in the trunk "ceiling", though I've never had issues there.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Thanks for the compliment, but I can't really take credit. I'm "guessing" my car was pretty well cared for (evil rust aside), so everything is in tact. As I mentioned, the lock system was working fine, save for the fuel door actuator. Replaced that and "all is well". Not sure if you noticed the link in my first post. I realize you're not gonna tackle your system right now, but, take a look, it's very well done and a great "guide/resource" when you work on yours. It was a HUGE help, to me, just getting a handle on the entire system. It's actually fairly simple (how else could it last 40 years and continue to operate?). Cheers to the MB engineers and PO's! https://www.ifixit.com/Guide/Mercedes+W123+Vacuum+Lock+System+Diagnosis+Technique/20201

Normally, I'd agree with your "fuel door" view. But, with diesel quickly approaching $4/gal. (that's FOUR DOLLARS PER GALLON!!) and our society quickly devolving into anarchy (I'll forgo the political rant and personal opinions of "why"), I've decided I like the small bit of security the fuel door actuator provides when my car is parked anywhere but in my driveway. $70 to fill my little 17 gal. tank with diesel is NUTS!
Your '02 T&C is 20 years old. Twenty years ago, things were "peaches and cream" in this country. No worries about "gas thieves". Sadly, those days are gone, forever. As for the "CD changer", I remember when I got my first car with one. It was a used '98 Ford Exploder (in 2005). Little did I realize, the "CD craze" had already come and gone. They've gone the way of the vinyl record. Cars don't normally have them any longer, haven't in a while.

To enable the front doors to be locked and unlocked by the key the vertical actuating rod is two piece with a cute butterfly looking spring in it, this spring fails every so often and isn't available as a separate part .

The one in my '82 240D recently failed, I took the vacuum actuator off the car and hand bent a paper clip to replace the broken spring, nearly two years now and it's working fine .

I agree, when it works the silent vacuum central locking works a treat .

My son who never wants to do the full job on anything hates the system simply because he learned about it when I was sorting out my very first W123 300CD...

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Personally I got tired of the various lock vacuum issues and just disconnected it. All locking and unlocking is done manually. I only have the vacuum shutoff left and what goes to the Transmission modulator.

The heating defaults to defrost when not connected to vacuum.

No one has tried to get any Diesel Fuel out of my Car with the unlocked lid.

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84 300D, 82 Volvo 244Gl Diesel

I can understand disconnecting it all in the case of constant issues. More than likely, I'd do the same. My system seems to be all there and functioning as intended. Which, to me, is amazing in an almost 40 year old car that, obviously, hasn't been pampered it's entire life.
The only malfunction I had was the fuel door actuator. Didn't wanna drop $50 on a new one, but found a used one for half that (which I also had reservations about), so I gave it a shot. Once I replaced that, the entire system is 100%. Gives me a warm and fuzzy feeling....dunno why.

"defrost only" is not an option in my climate. I would love to live in an area where that would suffice. Lived in N. CA for about a year, about 20 yrs ago. It DID NOT give me a warm and fuzzy feeling. I can only imagine what it's like now. Clearly, the situation hasn't improved.....

As for "fuel theft". I agree, not very likely (yet). But, once again, it's great when the entire system is working like new. Quite a testament to MB and the engineering of that era.

I've had a mostly-functional vacuum door lock system since buying my car. I got so dang annoyed at my one problem, and the lingering question of "will it break or go bad soon?", that I disconnected the locks at the firewall and at the door actuators (to make the up/down easy). Amazing. I love it. This vehicle is meant to be simple and reliable. I can't stand having such a silly Achilles heel item as the door locks. If leaning over and unlocking a pin is a problem for anyone - I'd definitely suggest owning another vehicle all together. I feel like manual locks should be standard issue in 2021. But hey that's just my opinion.