The vacuum swith on the key switch can be bad, but if you are also having trouble with the door locks, I'd guess you have a more general vacuum problem.

You will need a MitiVac or similar hand pump with guage.

Connect it to the main vacuum line from the pump to the booster and run the engine at idle -- you must have about 11" or so of vacuum. If not, you have either a leak or a bad pump.

To check the shutoff, connect it to the brown plastic line that runs from the main vacuum line and pump to see if it holds vacuum. If it does, switch the key on and off while operating the pump (or just switch the key then go back under the hood and pump) -- if the stop lever goes down and stays down and holds vacuum on the hand pump, you have a vacuum supply problem.

Same thing with the door locks -- the yellow plastic line that goes into the dash is the vacuum supply. Hook the pump up to that and pump -- it will take a while, there is a vacuum tank you will have to evacuate -- and see it the locks work. If you can't pull a vacuum, you will need to pull door panels and see what is leaking. The locks operate off a vacuum actuator with a yellow with red and a yellow with green stripe hose. The yellow line from the engine goes to the vacumm switch in the driver's door (no actuator in that door). A broken line or bad diaphram in one of the doors, the trunk, or the filler cap, will cause the locks not to work properly.

More details under various postings for door locks and vacuum shutoff. Fairly easy to work on with the hand pump, other than removing and replacing the door panels.

Peter

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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!