My highly simplified understanding is that the during warm-up the coolant temp sensor is sending a resistance value to the control unit, decreasing as the engine warms. The control unit considers that along with air flow and throttle position, and tells the EHA how much to enrich. When the coolant temp hits 80 C - "cutover", the resistance has fallen to the point where the control unit is programmed to accept input from the O2 sensor and uses that to fine-tune the mixture.

Cutover might be sooner if you have a heated O2 sensor.

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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