Modes of Operation

As input signals to the Powertrain Control Module (PCM) change, the PCM adjusts its response to output devices. For example, the 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 the PCM responds to the various input signals.

There are two types of engine control operation: open loop and closed loop.

In open loop operation, the PCM receives input signals and responds according to preset programming. Inputs from the heated oxygen sensors are not monitored.

In closed loop operation, the PCM monitors the inputs from the heated oxygen sensors. This input indicates to the PCM whether or not the calculated injector pulse width results in the ideal air/fuel ratio of 14.7 parts air to 1 part fuel. By monitoring the exhaust oxygen content through the oxygen sensor, the PCM can tune injector pulse width. Fine tuning injector pulse width allows the 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, the acceleration, deceleration, and cruise modes, with the engine at operating temperature, are closed loop modes.

IGNITION SWITCH ON (ENGINE OFF) MODE
When the ignition switch activates the fuel injection system, the following actions occur:

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

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

ENGINE START-UP MODE
This is an open loop mode. The following actions occur when the starter motor is engaged:

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

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

- engine coolant temperature
- manifold absolute pressure
- intake air temperature
- engine speed
- battery voltage

The PCM determines the spark advance based on the following:

- engine coolant temperature
- engine speed
- intake air temperature
- manifold absolute pressure
- throttle position

ENGINE WARM-UP MODE
This is an open loop mode. The PCM adjusts injector pulse width and controls injector synchronization by controlling the fuel injectors' ground paths. The PCM adjusts ignition timing and engine idle speed. The PCM adjusts the idle speed by controlling the idle air control motor.

CRUISE OR IDLE MODE
When the engine is at normal operating temperature, this is a closed loop mode. During certain idle conditions, the PCM may enter into a variable idle speed strategy. At this time, the PCM adjusts engine speed based on the following inputs:

- throttle position
- engine coolant temperature

ACCELERATION MODE
This is a closed loop mode. The PCM recognizes an increase in throttle position and a decrease in MAP as engine load increases. In response, the PCM increases the injector pulse width to meet the increased load.

DECELERATION MODE
This is a closed loop mode. The PCM recognizes a decrease in throttle position and a increase in MAP as engine load decreases. In response, the PCM decreases the injector pulse width to meet the decreased engine load.

WIDE OPEN THROTTLE MODE
This is an open loop mode. The throttle position sensor notifies the PCM of a wide open throttle condition. Once a wide open throttle is sensed, the PCM de-energizes the A/C compressor clutch relay. The PCM adjusts injector pulse width to supply a predetermined amount of additional fuel.