Kurt:

The usual reason that the vac locks aren't working well on a benz as old as yours is that the little rubber connectors for the hard plastic lines have deteriorated to the point the vac leaks off.

In the engine compartment you will find three vac lines branching off the mail hose to the brake booster. The brown line is the supply for the vacuum powered fuel shutoff, the green lines are for the AC/heater system, and the yellow lines are for the door locks.

There are two vac reservoirs -- the one for the locks is under the left front fender or inside the font bumper, and the one for the AC is inside the rear bumper. The hoses "T" by the firewall and one hose goes to the tank, the other to the locks or AC. There is a check valve before the T.

The door locks operate off a mechanical switch in the driver's door, supplied by the yellow plastic line. When the door is locked, vac is ported to a set of yellow with red stripe lines to the other three doors, the trunk lock, and the filler door lock. Unlocking is performed by the yellow with green line going to the other side of the servos.

The best way to troubleshoot vac systems is to get a MitiVac hand pump ($30-40 at the local parts store). This handy little device will allow you to "make" vac on demand, and has a nice gauge on it, too. Also has the bits to do brake bleeding, etc.

You can unplug the vac lines and attach them to the MitiVac and see if the entire system holds vac and operates properly, you can test individual parts, and you can check engine supplied vac.

I'd get some vacuum hose of the correct size and replace all the deteriorated connectors first, the check to make sure you have good vac by hooking up the MitiVac with the engine running. You should be getting 18" vac or so. If not, you have a leaking main line or a bad vac pump (not impossible in a 22 year old car!).

If you have vac and still have door lock trouble, you are going to have to test the individual vac elements in the doors. When you get the system vac tight again (also look for cracked hard plastic lines), the locks, AC, and shutoff will work properly!

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!