I can't think of anything off-hand that would cause the sensor plate to not move with the engine running if it is nice and easy with the engine off. Since the car has not run for a while, I suspect that you have some crud in the FD that is preventing the movement. You could try jumping 30 to 87 on the fuel pump relay or running 12v directly to the pump and loosen the fuel line fittings and see what comes out.

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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