Powertrain Control Module
The Powertrain Control Module (PCM) is a digital computer that controls ignition timing, air/fuel ratio, fuel injector pulse width, ignition coil(s), spark advance, emission control devices, cooling fan, charging system, idle speed, cruise control (if equipped), fuel pump and tachometer. See Fig 1 , Fig 2 or Fig 3 . PCM uses data from various input sources to control output devices in order to achieve optimum engine performance for all operating conditions.
PCM has voltage converters that convert battery voltage to regulated 5-volt output. The 5-volt output powers battery temperature sensor, camshaft position sensor on models equipped with Distributorless Ignition System (DIS) or distributor on models without DIS, crankshaft position sensor, engine coolant temperature sensor, intake air temperature sensor, logic circuits, manifold absolute pressure sensor and throttle position sensor.
As input signals to the Powertrain Control Module (PCM) change, the PCM adjusts its response to the output devices. For example, the PCM must calculate different injector pulse width and ignition timing for idle than it does for Wide Open Throttle (WOT). The PCM will operate in two different modes: Open Loop and Closed Loop. During Open Loop modes, the PCM receives input signals and responds only according to preset PCM programming. Input from the oxygen (O2S) sensors is not monitored during Open Loop modes. During Closed Loop modes, the PCM will monitor the oxygen (O2S) sensor's input. This input indicates to the PCM whether or not the calculated injector pulse width results in the ideal air/fuel ratio. This ratio is 14.7 parts air to 1 part fuel. By monitoring the exhaust oxygen content through the O2S sensor, the PCM can fine tune the injector pulse width. This is done to achieve optimum fuel economy combined with low emission engine performance.
Components are grouped into 2 categories. The first category, INPUT DEVICES , includes components that control or produce voltage signals monitored by the PCM. The second category, OUTPUT SIGNALS , includes components controlled by the PCM (this is accomplished by the PCM grounding individual circuits).