My bad.

After finding the right diagram (P 120/2 on the 124 ETM). It looks like it gets power from the OVP relay, and that the control unit provides ground. The coolant temp sensor is shown in the loop inside the control unit. So another scenario is a bad coolant sensor - the control unit is thinks the coolant is warm and is shutting the cold start valve down.

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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