The W123 Vacuum Lock system was a cinch to troubleshoot and repair--usually the problem was the door diaphragms rotting out or not enough vacuum because of a vacuum leak somewhere like on the Transmission modulator. On my 85 300D I got the thing fixed so well, the system will operate even two or three cycles even after the car has been sitting for a month.

The W124 is a different animal. It uses electrical pressure sensors and has its own vacuum pump I think. What is WIERD is mine actually came to LIFE for a two weeks and worked well, then stopped working.

Any Hints?

I would start by checking the electrical connection at the vacuum pump. Maybe there is a loose ground wire in the circuit causing the intermittent operation of the door locks.

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2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed
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'92 300D 2.5 Turbodiesel - sold
'83 300D Turbodiesel - 4 speed manual/2.88 diff - sold
'87 300D Turbodiesel - sold
'82 300D Turbodiesel - sold