On the W126, the power locks operate off an electric vac pump in the trunk, under the plastic cover in front of the spare tire, I think. Smaller than a football.

Check the connections, and check for a bad fuse, too -- no electricity, no pump. A broken line in any door will cause them to fail, as well, as this system uses a single line -- vac to lock, pressure to unlock on the same hose. If the pump runs when you lock a front door, there is a leak or the pump itself leaks -- check with a MitVac. If the pump does not run, and the fuse it good and there is power on the main line to the pump, the pump is shot.

They are NOT operated off the engine pump nor manifold vac.

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!