My power steering (on the 97 E420) does not work well. it gets hard, better, hard again (very hard, no ps at all). Fluid is full and no noise.
How to know whether it is the rack or the pump??

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2008 S550
2010 GLK350
1997 E420 (retired)
2005 CLK500 (retired)

This may seem like a dumb question, but have you checked to see if the belt is slipping?

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Also check the condition of the belt tensioner and idler pulley.