Modes Of Operation

As input signals to PCM change, PCM adjusts its commands to output devices. There are 2 modes of operation; open loop and closed loop. In open loop mode, PCM is not using input from HO2S and is responding to preset programming to determine injector pulse width and ignition timing. In closed loop mode, PCM adjusts ignition timing and uses input from HO2S to fine tune injector pulse width. The following inputs may be used to determine PCM mode:

• A/C Control Positions
• A/C Switch
• Battery Voltage
• Brake Switch
• Camshaft Position (CMP) Sensor
• Crankshaft Position (CKP) Sensor
• Engine Coolant Temperature (ECT) Sensor
• Engine Speed (RPM)
• Heated Oxygen Sensor (HO2S)
• Intake Air Temperature (IAT) Sensor
• Manifold Absolute Pressure (MAP) Sensor
• Park/Neutral (P/N) Switch
• Starter Relay
• Throttle Position (TP) Sensor
• Vehicle Speed Sensor (VSS)

From these inputs, PCM determines which mode vehicle is in and responds appropriately. Not all inputs are used in all modes or by all models. Modes of operation are:

• Ignition Switch On (Engine Not Running)
  
  This is an open loop mode. PCM pre-positions IAC motor based on ECT sensor input. PCM determines atmospheric pressure from MAP sensor and determines basic fuel strategy. PCM modifies fuel strategy according to IAT sensor, ECT sensor and TP sensor inputs. PCM activates ASD relay, which in turn activates fuel pump for only 2 seconds unless engine is cranked. PCM also energizes HO2S heater element for approximately 2 seconds, unless engine is cranked.
• Engine Start-Up
  
  This is an open loop mode. When starter is engaged, PCM receives input from ignition switch, battery voltage, CKP sensor, CMP sensor, ECT sensor, IAT sensor, MAP sensor and TP sensor. Based on these inputs, voltage is applied to fuel injectors with PCM controlling injection sequence, rate, and pulse width. PCM provides ground for injectors to fire in proper order.

  PCM determines proper ignition timing according to input received from CKP sensor. If PCM does not receive CKP sensor signal within 3 seconds after engine begins cranking, fuel injection system is shut down and a Diagnostic Trouble Code (DTC) is set in PCM memory.

• Engine Warm-Up
  
  This is an open loop mode. PCM determines injector pulse width using input information from battery voltage, CKP sensor, CMP sensor, ECT sensor, IAT sensor, MAP sensor and TP sensor. PCM also monitors A/C request and P/N switch (A/T only) for fuel calculation. PCM controls engine idle speed through IAC motor and timing. PCM controls ignition timing based on CKP sensor input, modified using ECT, CMP, IAT, MAP and TP sensor inputs.

  PCM also operates A/C compressor clutch (if A/C is requested) through A/C clutch relay. When engine reaches operating temperature, vehicle will go into idle mode and PCM will begin monitoring HO2S input and go into closed loop operation.

• Idle
  
  This is a closed loop mode. In idle mode, PCM now adds HO2S signal to array of inputs used in ENGINE WARM-UP mode. PCM maintains correct air/fuel ratio by adjusting injector pulse width and ignition timing. PCM also controls A/C clutch operation (if A/C is requested).
• Cruise
  
  When engine is at operating temperature, this is a closed loop mode. Using information from A/C switch, battery voltage, CKP sensor, ECT sensor, IAT sensor, MAP sensor and CMP sensor. PCM also monitors A/C request and P/N switch (A/T only), TP sensor and VSS signals for fuel calculation. PCM monitors HO2S and adjusts air/fuel ratio as needed. PCM controls engine idle speed through IAC motor. PCM controls spark advance as necessary.
• Acceleration
  
  This is a closed loop mode when engine is at operating temperature and under acceleration. When PCM recognizes an abrupt increase in throttle position or manifold pressure as a demand for increased engine output, it increases injector pulse width in response to increased fuel demand.
• Deceleration
  
  This is a closed loop mode when engine is at operating temperature and under deceleration. When PCM receives inputs signaling a closed throttle and an abrupt decrease in manifold pressure, it reduces injector pulse width to lean air/fuel mixture. Under certain RPM and closed throttle position conditions, HO2S signals are ignored and PCM cuts off fuel injection until idle speed is reached. PCM also drives IAC motor for smooth transition to idle mode.
• Wide Open Throttle
  
  This is an open loop mode. When PCM senses wide open throttle, it grounds fuel injectors in sequence, it ignores HO2S input and it controls pulse width to supply a pre-determined amount of additional fuel. PCM also adjusts spark advance and disengages A/C clutch for approximately 15 seconds.
• Ignition Switch Off
  
  This is an open loop mode. PCM drives IAC motor into position in anticipation of next start-up. All outputs are turned off, no inputs are monitored and PCM shuts down.