While I am waiting for the new vac pump to arrive for the door locks... I have opened the channel at the passenger front side and inspected the yellow tubing and the rubber connector. There are three tubes to the rear and one tube in the front which must go to the defroster, etc flaps (which do not work).

Question - if I plug the single tube to the flaps, should the new vac pump (if it works) give me working door locks - or do all the tubes need to be hooked up?

81 300SD

Vacuum Diagrams

Print out the one that applies to your car and have at it. Also, I suggest you buy a mighty Vac so you can test and isolate your leaks.

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1981 300TD "The Green Lantern"
1980 300TD
1983 300D Euro "China Cat"

Yellow lines are all door/fuel/trunk lines. Climate control flaps use a different color.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I already have the diagrams, purchased a mityvac, and thoroughly tested the system. Severe leak in the door channel, going forward on the single tube. All four tubes (three heading rearward and one headed to the front) are yellow; maybe the one to the front does not go to the flaps - maybe it goes someplace else like the driver's door. Need to look at the diagram.

Therefore it looks like I need to follow the forward leading tube - it does not hold vacuum and maybe that is why I cannot get the mityvac to pull down the locks.

I believe the line to which you refer is the main vacuum line going to the passenger side from the driver's side of the car. If it is, the source of your problem is likely on the driver's side.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Think you are right. The single yellow tube disappears under the dash and is hard to trace. Wonder why MB did not route it straight across to the driver's side. So far I have replaced the actuator at driver's side rear (was broken) and thereafter the locks only worked once and a while. So I think I will pull apart the driver's side door and take a look. Have a lot of experience taking apart these doors - regulators, locks, now actuators - even cleaned the points on the seat control switch.

Sorry to tell you, but on the 123 the line from the driver's door does go straight across the hump to the passenger side.
If the SD locks have a similar circuit as the D/TD locks, then testing the line coming from the drivers door (at the passenger door) won't tell you very much. From the passenger door to the drivers door, it goes to the master switch (a 3-way valve) and is either routed to the vacuum reservoir (in which case you'll have to do a lot of pumping before you register any vacuum) or vented to the atmosphere.
You didn't say if the line you're tracing has a red or green stripe...there should be one of each leading from the drivers door to the passenger door, and each should branch off into 3 lines at a connector.
It would be best to start at the drivers door. To test the master switch, you'll have to plug the vacuum supply port, lock the door, and test the yellow/red line. Then do it again for the yellow/green with the door unlocked. If you don't want to take the door apart yet, you can plug the vacuum supply line under the hood and test the lines leading from the switch by pulling up the drivers carpet, but then you also have some rubber connectors in the mix that might be leaky.

Again, I'm familiar with the TD lock system, if the SD is much different, I'm sorry to have wasted your time.

Here's a pretty good guide for troubleshooting the whole system:
http://www.schumanautomotive.com/forums/showthread.php?t=2755

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1985 300TD-euro 352,000 mi
1974 240D (1?)52,000 mi - has a new home now

Just received a used air supply pump from Sun Valley Mercedes Dismantlers : http://www.mercedesdismantlers.com; they are really good folks. I highly recommend the company.

Plugged it in last night and immediately had door locks again.

So here is the entire story: took the car to the indy because the driver's side rear door gave a big clunk upon closing. Indy said he greased the closer and I had no more problem. About a year later that same door stopped locking. When I took it apart, I found that the indy had unhooked the door check (which instantly stopped the clunking) and left it in the door. Finally it got entangled with the rod to the door actuator and destroyed the actuator. I replaced the door check (very easy $20 fix) and the actuator and the locks worked again - except the drivers door was slow. Then I made a mistake. I took apart the air supply pump in the trunk and obviously destroyed it - not sure how because I carefully put it back together.

The passenger front door lock does not work perfectly but I an inclined to leave it alone.

Thanks to all in the forum; info was very helpful.