Electronic Control Module (ECM)
The ECM reads and computes the signal from the oxygen sensor. In response, it sends a proportionate signal to the mixture control solenoid (in carburetor), which creates either a lean or rich mixture, as required. The signal which is sent from the ECM to the mixture control solenoid is also modified by inputs from throttle position vacuum switches and coolant temperature sensor (CTS).
The ECM contains a replaceable integrated circuit "chip" which has stored data unique to each vehicle (i.e., axle ratio, body style, etc.). This is called a Programmable Read Only Memory (PROM). This PROM's memory system stores an average set of operating conditions with the ideal air/fuel ratio for these conditions. If any settings change from these stored conditions (as determined by engine sensors), the ECM will make necessary adjustments to bring engine back to the "ideal" set of conditions.
Before the ECM can begin in governing the air/fuel ratio, it is important to note that a minimum of 10 seconds must have elapsed after startup before any C-4 operation begins, engine coolant temperature must be above 150° F, and the oxygen sensor must be sufficiently hot (600°F) and putting out sufficient voltage before the ECM can react. During cold engine startup, the ECM is off-line and a fixed carburetor condition is maintained until proper warm-up occurs.