If you have some sort of mechanical idle control, then you need to figure out how to adjust it. An idle relay won't do you any good without the idle air valve and its plumbing. You also need wiring from the temp sensor and ignition unit, AC switch, etc. I believe this is all integral to the engine/body wiring harness.

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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