Many of the 1992 W140's have(had) this problem, Benz modified the pump 2/93. Try removing the #9 fuse in the TRUNK & see if it works a few times.

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MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS DEALER M.B. Shopforeman
190E 2.3 ITS RACECAR
1986 190E 16V