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PCM Operating Modes

As input signals to powertrain control module (PCM) change, PCM adjusts its response to output devices. For example, PCM must calculate a different injector pulse width and ignition timing for idle than it does for wide open throttle. There are several different modes of operation that determine how PCM responds to various input signals.

There are two types of engine control operation: open loop and closed loop. In open loop operation, PCM receives input signals and responds according to preset programming. Inputs from heated oxygen sensors are not monitored.

In closed loop operation, PCM monitors inputs from heated oxygen sensors. This input indicates to PCM whether or not calculated injector pulse width results in ideal air-fuel ratio of 14.7 parts air to 1 part fuel. By monitoring exhaust oxygen content through oxygen sensor, PCM can fine tune injector pulse width. Fine tuning injector pulse width allows PCM to achieve optimum fuel economy combined with low emissions.

The engine start-up (crank), engine warm-up, and wide open throttle modes are open loop modes. Under most operating conditions, acceleration, deceleration, and cruise modes, with engine at operating temperature, are closed loop modes.

Ignition Switch On (Engine Off) Mode  - When ignition switch activates fuel injection system, following actions occur:

  1. The PCM determines atmospheric air pressure from MAP sensor input to determine basic fuel strategy.
  2. The PCM monitors engine coolant temperature sensor and throttle position sensor input. The PCM modifies fuel strategy based on this input.

When key is in "on" position and engine is not running (zero RPM), auto shutdown relay and fuel pump relay are not energized. Therefore, voltage is not supplied to fuel pump, ignition coil, and fuel injectors.

Engine Start-Up Mode  - This is an open loop mode. The following actions occur when starter motor is engaged:

  1. The auto shutdown and fuel pump relays are energized. If PCM does not receive camshaft and crankshaft signals within approximately one second, these relays are de-energized.
  2. The PCM energizes all fuel injectors until it determines crankshaft position from camshaft and crankshaft signals. The PCM determines crankshaft position within one engine revolution. After crankshaft position has been determined, PCM energizes fuel injectors in sequence. The PCM adjusts injector pulse width and synchronizes fuel injectors by controlling fuel injectors' ground paths.
  3. Once engine idles within 64 RPM of its target engine speed, PCM compares current MAP sensor value with value received during ignition switch on (zero RPM) mode. A diagnostic trouble code is written to PCM memory if a minimum difference between two values is not found.

Once auto shutdown and fuel pump relays have been energized, PCM determines fuel injector pulse width based on following:

The PCM determines spark advance based on following:

Engine Warm-Up Mode  - This is an open loop mode. The PCM adjusts injector pulse width and controls injector synchronization by controlling fuel injectors' ground paths. The PCM adjusts ignition timing and engine idle speed. The PCM adjusts idle speed by controlling idle air control motor.

Cruise or Idle Mode  - When engine is at normal operating temperature, this is a closed loop mode. During certain idle conditions, PCM may enter into a variable idle speed strategy. At this time, PCM adjusts engine speed based on following inputs:

Acceleration Mode  - This is a closed loop mode. The PCM recognizes an increase in throttle position and MAP pressure as engine output demand increases. In response, PCM increases injector pulse width to meet increased output demand.

Deceleration Mode  - This is a closed loop mode. The PCM recognizes a decrease in throttle position and MAP pressure as engine output demand decreases. In response, PCM decreases injector pulse width to meet decreased engine output demand.

Wide Open Throttle Mode  - This is an open loop mode. The throttle position sensor notifies PCM of a wide open throttle condition. Once a wide open throttle is sensed, PCM de-energizes AIC compressor clutch relay. The PCM adjusts injector pulse width to supply a predetermined amount of additional fuel.

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