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I don't know if your 97 S420 is similar to my 93 400E, but I ran into the same problem when I first tried to take my pump and bracket off. It turns out that there are two 13 mm mount bolts that are accessed from the back of the pump bracket (see pictures). I had to take the left front exhaust elbow off (from the top with 1/4 drive and 1.5" extension) to snake my hand up there from the bottom (with 3/8 drive and 1" extension). This was a horrible idea for Mercedes to design a bracket with bolts that were so difficult to access and I can remember uttering a litany of colorful adjectives while doing this job.
On my car there are a total of six bolts in the front (2 mount bolts to the left and 4 bolts around the front of the pump--see the fourth picture) that will enable me to pull the front half of the pump off. Most of the time, the radial seal leaks and this is accessible by just pulling the front off.
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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles
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