Peter had a good point about possible weak vacuum pump....always when diagnosing vacuum problems, first pull off a hose on the main vacuum line from the vacuum pump to the brake booster and check to see how much vacuum you have...ideally you should have a maximum vacuum of about 15 to 18 inches of mercury.

A leak in the ACC system can cause failure of door lock system because you may not be able to build enough pressure to "fill" the door lock vacuum reservoir.

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)