xander,

The aux pump knocking out the PB unit is so common that the rebuilders of the PB units won't warranty their unit when you don't replace the aux pump. The aux pump is easy to check by checking current draw. I'm not sure about a 190, but the one on a 300E should not draw more 1.3 Amps.

Good luck,

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Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles