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Old 07-03-2003, 05:02 PM
JHZR2's Avatar
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Join Date: Jan 2002
Location: New Jersey
Posts: 4,239
vacuum element diaphragm test


can someone explain to me how to test a vacuum element on the benchtop? Does the opposite connection that is not attached to the vacuum/pressure pump have to be closed off? if not, should air be able to pass through from side to side? Id think not, as there ought to be a diaphragm inside.
I think I just dont quite understand the concept of testing... I tried applying vacuum the the main line going to the locks, and couldnt get them to lock. yet, after I isolated the trunk, and closed the system up, it seems that vac holds a bit longer.
so, is it just the following:

-pull vac on the yellow/red line and the element should lock
-put pressure on the yellow/green line, and a locked element should unlock
-have a disconnected element, and it ought to do the same without the opposite connection isolated off... this will prove that the diaphrams is still sealed.
-pull vac on the main line going from the 3-way check valve under the hood, and closed doors, in unlocked position should lock.
-putting vac/pressure on either side of the element ought to hold pressure or vacuum, even if it doesnt actuate the mechanism, i.e. the diaphragm is sealed

Is this correct? Thanks very much,


Current Diesels:
1981 240D (73K)
1982 300CD (162k)
1991 350SD (113k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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Old 07-03-2003, 10:20 PM
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Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
Door lock systems from that era work strictly on vac -- there is a switch in the drivers door that applies vac to either the red stripe OR the green stripe lines, all the time.

Both sides must hold vac indefinitely. If they leak, you can buy replacement diaphrams for everthing but the gas filler element, it's different. Performance Analysis has them, I've bought a set, they work great, $5.00 each. Buy all of them, replace all of them, save yourself the hassle of doing them one at a time.

W123 chassis has dual diaphram elements, so that's 4 for each door, probably two for the gas filler door, but I don't know for sure there.

The upper side ones (lock) are usually worse than the lower ones, they are more exposed when the door seals get bad.


1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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