Usually, there are two terminals on the solenoid. One takes the 12V from the Start position of the ignition switch. It should be labeled "50." It looks like the other one "16" goes to the switching unit via the terminal on the resistor and provides 12V when the starter is cranking, the purpose being a hotter spark when starting. When you release the key, the flow the the switching unit goes back through the resistor.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe