How I (mostly) fixed the W108 Central Locking System

[Stereolab42]

So... I rebuilt the completely nonfunctional central locking system on my W108 (1973 280SE 4.5) over the last week. It's probably been non-functional for decades. Result was a perfectly working system, at the cost of much more time than I planned on spending. It's not going to hold vacuum for weeks like it did fresh from the factory, but good enough for (I think) a couple days. First thing I did was search for a vacuum routing diagram for the W108 on the 'net. Found lots of threads where people asked for this, but nobody actually had such a diagram, and several claimed it didn't exist. I gave up on this search temporarily but I did find the wonderful ifixit site for the W123 central locking. It was clear on the surface that the systems shared a common design so this taught me a lot. However, the location of the Y-connectors was completely different, there was nothing under the passenger mat. So I still did not have a full picture.

Completely by chance I came up on a 1972 parts manual for a W108 and believe it or not, found a diagram of the W108 central locking system (see below)! The biggest takeaway here was that most of the Y-connectors were buried deeply in the dash or behind pillar trim, unlike with later Mercedes models. I decided I wasn't going to inspect or replace those and hoped none were bad.



Opening up the doors, it became clear nobody had ever been in here before since the factory clear film was still present with the original tape. (Believe it or not, the electric window regulars still work perfectly.) As expected, most of the diaphragms on the actuators were torn. Heading to the trunk, I pulled the gas door and trunk luck actuators. Those still held vacuum but not ideally. I decided I would have to refresh them as well. There is only one style of replacement diaphragm out there. It has a "combo nub" that is a combination screw hole and notch. You can get them from eBay or Mercedes Source, at a ridiculous cost, but it is what it is.



After they arrived, I ran into the first complication. They are slightly smaller than the stock door actuator diaphragms and did not easily fit. After much struggle I managed to shoehorn them on with the retainer ring, but the ring would not go down all the way on all sides. Some testing with a Mityvac (get one!) showed they still leaked slowly. I concluded I was going to have to assist the diaphragms in their job with some Permatex black sealant. This stuff is awesome and I could not have completed this job without it. It's basically glue that turns into rubber after a few hours, and is air and water-tight.

But before I applied the sealant, I decided to test-fix the actuator in the door. Now I ran into the second problem: The lock rod was highly constrained in its up-and-down range and did not work properly. After some thought it was clear the problem was the "combo nub" was longer than the OEM screw-hole nub. Out came the hand saw and I took off the tops of the nubs. Luckily the screw hole went very deep and there was still sufficient thread. A second test-fit showed that the rod now had proper travel. Now I assembled all the door actuators and slathered the inside and outside lips with Permatex. After drying, they held vacuum just fine. I buttoned-up the doors and tested the vacuum of the entire system from the firewall tube. Since I hadn't done the trunk yet, I capped off those lines. I verified that with the driver's lock down, the other doors would lock when vacuum was applied, and with the lock up, they would open. This demonstrated to me that the hard lines, Y-connectors, and valve in the driver's door were all healthy (thank god).



Onto the trunk, where more problems revealed themselves. First, the gas door actuator was held shut with a press-fit brass ring. The only way to get it off was to destroy it, and I hoped Permatex (once again) would be enough to resecure the two halves.



Second, the three trunk diaphragms were all somewhat SMALLER than the new diaphragms, in contrast with the OEM door diaphragms, which were bigger. Some test-fitting showed the new diaphragms could again be shoehorned in, but again not with a perfect fit. However, the bigger problem was that these diaphragms all had "special hardware" sprouting from their tops. A rod, a special screw, etc. The new diaphragms simply could not be used unless I spent some serious time fabricating ways to transform or move that special hardware.



So I decided to re-use the OEM diaphragms. Luckily, they were salvageable. I could see creases where cracks would eventually happen. Some people just slather them with grease and call it a day, but I knew that would not be sufficient. Again, the Permatex came out, and I slathered it EVERYWHERE, essentially entombing the diaphragms, which held vacuum decently now. And as I hoped, it was also sufficient to hold the trunk door pod together. (Another plus about using Permatex -- it is removable. Not easily, but it does not permanently bond to plastic or metal and can be scraped off. This would NOT be the case with superglue, epoxy, or the like.)

Now the trunk and gas actuators went back in and I returned to the hood for a another test. Everything worked as expected with the Mityvac. As before I was just testing with the vacuum line that came into the firewall, I didn't put the fender tank into the loop. It is clear that the system still leaked slowly; without the tank I estimate it could hold for an hour or two However, the tank holds a TREMENDOUS amount of vacuum. It seems leak-free, and going by how long it takes to evacuate I estimate it has probably 100x the volume of the rest of the system. So with the tank it's probable the system would hold for a few days (although I have not tested this yet). It will never be good enough to hold for weeks like they apparently did from the factory, but I knew that was a pipe dream given my skills and desired time investment. Already this took many days and many more hours than I wanted to spend. But overall I met my goal of ressurrecting a cool antique feature of the car.