The rollercoaster cam is the part inside the engine that operates the mechanism in the vacuum pump. If you peek in the hole that the vac pump attaches to, you'll see it on the sprocket that drives the injection pump.

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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!)