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Old 07-15-1999, 10:24 AM
Posts: n/a
Due to the number of requests I've received for this information, I'm posting it here for all to view and comment. If you've got further experience, please feel free to add and reply.

Diagnosis and repair of vacuum leaks in 240D door and trunk locks.
Written by Nathan Carter, summer 1999. <or>

<disclaimers: I'm not a certified mechanic; however, I have owned two
240D's, which have been my primary cars for the past eight years. All
the following information is more or less accurate for my 240D and may
or may not be the same for other models. Also, I'm writing this from my
desk and not from inside the car; it's as accurate as I can remember
without looking at the car.>

I know this is long, but if you've got the door lock problem you'll
want to read the whole thing, even print it out and stick it with your
service manuals. The most valuable information is near the end, but go
ahead and read the whole thing

Everything in the car is controlled by the vacuum system - brakes, engine
shut-off, air vents, door/trunk locks, etc. There's a vacuum pump in the
engine, vacuum reservoir in the trunk, and about four thousand feet of
vacuum lines running all over the car.

Also, for some unknown reason, most of the vacuum lines throughout the
car are made out of a semi-hard yellow plastic. After a number of years,
the plastic becomes brittle and cracks or outright breaks, especially in
the corners where the lines feed into the doors (and therefore have to
flex every time the door opens or closes).

The mechanisms in the doors that cause the doors to lock and unlock
are prone to failing. They each have two (or four?) little rubber
diaphragms that will eventually rot and split, causing loss of vacuum.
I've found that these seem to fail all at once, or thereabouts. I've
replaced three of the four of mine over the past three years, and have a
spare ready for when the fourth fails.

If the lock in your doors only work intermittently, or only work while
the car is running, or will only work once, immediately after you shut
the car off and get out, then you've got a vacuum leak. Generally, it's
one of the following: a leak in one of the vacuum lines (semi-likely),
a leak in one of the vacuum-powered door lock mechanisms (very likely),
a leak in the reservoir itself (very UNlikely), a failing vacuum pump
(very UNlikely, and knock on wood that this ain't it).

While the car's running, you probably won't have any vacuum problems. The
vacuum pump is powerful enough to generate enough vacuum to overcome small
leaks in the system; when the pump is not running, the vacuum pressure
will (relatively) quickly dissipate because of the leak, and you'll have
problems such as doors not locking.

If you're the type to want to fix this yourself, you'd be well-advised to
invest in a few tools to aid your battle against the vacuum system:
- A small air vacuum/pressure meter
- a hand-held vacuum pump
- some rubber tubing of the appropriate size
(Note: a kit containing all of the above can be had from IMPCO for $60)
- some wooden golf tees. The golf tees are for sticking into the end of
a piece of rubber tubing to seal off that section of the vacuum line
from the rest of the system. It works!

The first thing to check, and generally the most likely cause of the
problem, are the vacuum-powered door lock or trunk lock mechanisms.
Here are some diagnosis steps (do all three):

A. Sit in the car with the doors unlocked and the car running. Do the
doors lock and unlock properly? Does the trunk lock and unlock properly?
If there is one door (or trunk, or fuel-fill cover lock) that does not
work at all, even with the car running, then you've got a huge leak in
that area and should be able to find it relatively easily. If it's a big
enough leak, you may even be able to hear it leaking. If this is the case,
then you should do a little happy-dance because it's rare that finding a
vacuum leak is that easy.

B. Sit in the car with the doors unlocked and the car running. Shut off
the car and wait for a minute or two, then lock the driver's door. Do the
rest of the doors/trunk lock? If the other doors don't lock, then the
leak MAY be in one of your many unlock lines, or in the unlock side of one
of the locking mechanisms. This test alone doesn't tell you much.

C. Sit in the car with the doors locked and the car running. Shut off
the car and wait for a minute or two, then UNlock the driver's door. Do
the rest of the doors/trunk unlock? If the other doors don't lock, then
the leak MAY be in one of your many lock lines, or in the lock side of one
of the locking mechanisms. Again, this test alone doesn't tell you much.

Now, with the data you got from tests B and C, you should be able to
figure out roughly where your leak is.

Case 1:
If both tests B and C failed (i.e. no locking OR unlocking after a few
minutes of the car being off) then it's most likely that the vacuum leak
is somewhere else in the system besides the door locks. There are other
systems in the car that are also controlled by the vacuum. Engine shut-off,
brakes, climate control ductwork, probably more. I've never had a problem
with these systems, though, so can't really be of any help here. (okay,
well, I DID have a problem once where, ahile doing an oil change, I
accidentally clamped some vacuum lines under the cover for the oil filter
canister, but that's an unrelated story)

Case 2:
Test B fails, but Test C succeeds. After shutting off the engine and
waiting for a few minutes (doors locked), you're able to unlock the doors,
but then can't lock them again; after shutting off the engine and waiting
for a few minutes (doors unlocked) you're not able to lock the doors.

While sitting there waiting with the doors unlocked, there is constant
vacuum to the UNlock lines (and the UNlock side of the locking
mechanisms); your vacuum reservoir quickly empties due to a leak in one of
the unlock lines and you're unable to lock the doors because you've got no

While sitting there waiting with the doors locked, there's constant vacuum
to the lock lines; since there is no leak, your vacuum is reserved. When
you unlock the doors, as soon as there is vacuum to the unlock lines it is
expended by unlocking the doors and leaking out the leak. You can't lock
the doors again because there is no vacuum in your reservoir.

Case 3:
The exact opposite of Case 2. Doors stay locked; you can't unlock them
because there is a leak in the lock lines and no vacuum pressure.

Now, for actually finding the leak! I realize this is a long, long
process, but the vacuum lines are really obnoxious to work on. I'm going
to assume that you did all the tests and came up with Case 2 as your
problem. You need to find and fix a leak in the unlock lines.

If there is one door (or the trunk) that seems to unlock more slowly than
the rest, start checking there first. You should find two vacuum lines
running to each locking mechanism, one for vacuum-lock and one for
vacuum-unlock. In my car they are yellow with red stripe and yellow with
green stripe, but I can't remember which is which, sorry.

***Method to narrow down your search:
This is correct for my 240D, but may be different for other models.

You can find the vacuum lines for various parts of the car running
underneath four floormats. Pull out the floormats, then the sub-mats, and
you'll be staring at the metal bottom of the car. There are plastic covers
that protect the vacuum lines (and some electrical lines? can't remember).
These can be pulled aside pretty easily and you'll find the yellow vacuum
lines. Search around until you find a rubber connector in the vacuum
line; there are probably sev