Description And Operation: Operation

The Powertrain Control Module (PCM) is a pre-programmed microprocessor digital computer. The PCM operates the fuel system, regulates ignition timing, air-fuel ratio, emission control devices, charging system, certain transmission features, speed control, air conditioning compressor clutch engagement (if equipped) and idle speed. The PCM can adapt its programming to meet changing operating conditions. It also receives input signals from various switches and sensors. Based on these inputs, the PCM regulates various engine and vehicle operations through different system components. These components are referred to as PCM outputs.

The PCM adjusts ignition timing based upon inputs it receives from sensors that react to engine RPM, manifold absolute pressure, engine coolant temperature, throttle position, transmission gear selection (automatic transmission), vehicle speed and the brake switch. It also adjusts idle speed based on inputs it receives from sensors that react to throttle position, vehicle speed, transmission gear selection, engine coolant temperature and from inputs it receives from the air conditioning clutch switch and brake switch. As input signals change, the PCM adjusts its response to the output devices. For example, the PCM must calculate different injector pulse width, ignition timing and camshaft settings for idle as compared to Wide Open Throttle (WOT). It also adjusts the generator charge rate through control of the generator field and provides speed control operation (depending on vehicle build).

The PCM will operate in two different modes, open loop and closed loop. During open loop operation, the PCM still receives all input signals but responds only to a limited amount of inputs based on the algorithms for output control. Inputs from the Oxygen (O2) sensors and other sensors (depending on the PCM programming) are not monitored during open loop operation. During closed loop operation, the PCM will monitor all inputs and outputs. The inputs indicate to the PCM whether or not the calculated output settings result in an ideal 14.7:1 air-fuel ratio. By monitoring the exhaust oxygen content through the O2 sensor and all other inputs, the PCM has the ability to fine tune the air handling, fuel control, ignition timing and spark advance settings. This is done to achieve optimum fuel economy combined with low emission engine performance.