I have a slow leak in the locking system of the '80 300SD/ W116:
the locks work after running a while, but the vacuum leaks down after about 45 min. when the car is off.
Can anyone tell me where (or if there are any) the Y connectors are like the 123s in the floorboard. On the 116 the lines go into metal channels under the seats; do I have to remove seats? (Might as well remove each door panel and check each actuator.) It's easy on the 123. Any help? (I have searched)
Kerry, any work in this area?
thanks,
Howell

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'85 300TD W123
'83 300D W123, Daughter's
'80 300SD W116

Haven't done any vacuum work on the door system on the W116. I'll be curious to know what you find out. I'm also anxious to know how to fix the doors so the outside handles work correctly. Driver's side rear door opens easily from the inside but won't open from the outside on mine.

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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

They all work the same way. Most likely one of the diaphrams has developed a pinhole leak. I use a mityvac on each until I find it. Also, since you have the door panel off even if the one you test is OK it might make sense to replace the rubber diaphragms anyway to ensure you won't be facing trouble later.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

Quote: "Also, since you have the door panel off even if the one you test is OK it might make sense to replace the rubber diaphragms anyway to ensure you won't be facing trouble later"
I don't have the panels off yet...I was hoping I could find where the lines connect: going to the driver door/the Y going to passenger doors and the back on the lock and unlock sides. The lines look like they are under a protective layer I hate to tear into. It's different from the w123. If I get the panel off, I will change out some diaphragms.
I am losing vacuum on the lock side of the vacuum switch.

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'85 300TD W123
'83 300D W123, Daughter's
'80 300SD W116

You can easily find the lines which go to the trunk lock actuator and fuel door usually as those are in the trunk, right near the actuators themselves...so start with those. If your leak is on the lock side then it is possible one of them is the source - you might just get lucky. If you don't have a mity-vac you can simply plug the lines to those two actuators and see if that fixes it...if so reconnect one at a time to eliminate it.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

Had the same issue 5 years ago and got it answered 1st time in this very Forum.
yes it probably one of the diaphrams. Usually the main offender is the drivers door controller, so start with this one.
Once they start to perish you find all the diaphrams tend to give up at close intervals.
Also check the Y connectors under the bonnet where the lines go into the firewall. Common for one of these to perish also.