Circuit/System Description

The engine control module (ECM) supplies 5 V reference voltage, through the 5 V reference voltage circuit, to the fuel economy mode switch. The 5 V reference voltage causes current to flow through a series of resistors in the switch to the switch ground circuit. The ECM also supplies a signal circuit to detect voltage within the resistor network. When the fuel economy mode switch button is momentarily pressed, the switch contacts allow the current to by-pass one of the resistors. The ECM detects the resulting change in voltage drop as a fuel economy mode request and sends a GMLAN message to the transmission control module (TCM) indicating the request. When the TCM receives the request and if operating conditions are correct, the transmission shift points are modified to optimize fuel efficiency. The TCM sends a confirmation message to the ECM and the instrument panel cluster (IPC) also receives the fuel economy mode request message from the TCM and illuminates the fuel economy mode indicator lamp.