Engine Control Module: Description and Operation

Fig. 3 Powertrain Control Module (PCM):

POWERTRAIN CONTROL MODULE
The Powertrain Control Module (PCM) is a digital computer containing a microprocessor. The PCM receives input signals from various switches and sensors referred to as Powertrain Control Module Inputs. Based on these inputs, the PCM adjusts various engine and vehicle operations through devices referred to as Powertrain Control Module Outputs.

NOTE: PCM Inputs:
- Air Conditioning Pressure Transducer
- ASD Sense
- Battery Temperature Sensor
- Battery Voltage Brake Switch
- Camshaft Position Sensor
- Crankshaft Position Sensor
- Distance Sensor (From TCM)
- EGR Position Feedback
- Engine Coolant Temperature Sensor
- Heated Oxygen Sensors
- Ignition sense
- Intake Air Temperature Sensor
- Knock Sensors
- Leak Detection Switch
- Manifold Absolute Pressure (MAP) Sensor
- Park/Neutral (From TCM)
- PCI Bus
- Power Steering Switch (if equipped)
- Proportional Purge Sense
- SCI Receive
- Speed Control
- Torque Management Input (From TCM)
- Transaxle Control Module (TCM)
- Transaxle Gear Engagement (From TCM)
- Throttle Position Sensor
- Trans Temperature
- Vehicle Speed (From TCM)

NOTE: PCM Outputs:
- Air Conditioning Clutch Relay
- Automatic Shut Down (ASD) and Fuel Pump Relays
- Data Link Connector (SCI Transmit)
- Double Start Override
- EGR Solenoid
- EATX RPM Output
- Fuel Injectors
- Generator Field
- High Speed Fan Relay
- Idle Air Control Motor
- Ignition Coils
- Leak Detection Pump
- Low Speed Fan Relay
- MTV Actuator
- Proportional Purge Solenoid
- SRV Valve
- Speed Control Vent Relay
- Speed Control Vacuum Relay
- Service Engine Soon
- Tach
- Transmission Fan
- 8 Volt Output
- 5 Volt Output

Based on inputs it receives, the Powertrain Control Module (PCM) adjusts fuel injector pulse width, idle speed, ignition timing, and canister purge operation. The PCM regulates the cooling fans, air conditioning and speed control systems. The PCM changes generator charge rate by adjusting the generator field.

The PCM adjusts injector pulse width (air-fuel ratio) based on the following inputs.
- Battery Voltage
- Engine Coolant Temperature
- Engine Speed (crankshaft position sensor)
- Exhaust Gas Oxygen Content (heated oxygen sensors)
- Manifold Absolute Pressure
- Throttle Position
- Intake Air Temperature Sensor

The PCM adjusts engine idle speed through the idle air control motor based on the following inputs.
- Brake Switch
- Engine Coolant Temperature
- Engine Speed (crankshaft position sensor)
- Perk/Neutral (transmission gear selection)
- Transaxle Gear Engagement Throttle Position
- Vehicle Speed (From TCM)

The PCM adjusts ignition timing based on the following inputs.
- Intake Air Temperature
- Engine Coolant Temperature
- Engine Speed (crankshaft position sensor)
- Knock Sensor
- Manifold Absolute Pressure
- Park/Neutral (transmission gear selection)
- Transaxle Gear Engagement
- Throttle Position

The Automatic Shut Down (ASD) and fuel pump relays are in the PDC, but not turned ON or OFF by the PCM through the same circuit.

The camshaft and crankshaft signals are sent to the powertrain control module. If the PCM does not receive both signals within approximately one second of engine cranking, it deactivates the ASD and fuel pump relays. When these relays are deactivated, power is shut OFF to the fuel injectors, ignition coils, fuel pump and the heating element in each oxygen sensor.

The PCM contains a voltage converter that changes battery voltage to a regulated 8.0 volts. The 8.0 volts power the camshaft position sensor, crankshaft position sensor. The PCM also provides a 5.0 volt supply for the engine manifold absolute pressure sensor, a/c pressure transducer, and throttle position sensor.

The PCM engine control strategy prevents reduced idle speeds until after the engine operates for 320 km (200 miles). If the PCM is replaced after 320 km (200 miles) of usage, update the mileage in new PCM. Use the DRB scan tool to change the mileage in the PCM. Refer to the appropriate Powertrain Diagnostic and the DRB scan tool.

SENSOR RETURN - PCM INPUT
The sensor return circuit provides a low electrical noise ground reference for all of the systems sensors. The sensor return circuit connects to internal ground circuits within the Powertrain Control Module (PCM).

SCI RECEIVE - PCM INPUT
SCI Receive is the serial data communication receive circuit for the DRB scan tool. The Powertrain Control Module (PCM) receives data from the DRB through the SCI Receive circuit.

IGNITION SENSE - PCM INPUT
The ignition sense input informs the Powertrain Control Module (PCM) that the ignition switch is in the crank or run position.

PCM GROUND
Ground is provided through multiple pins of the PCM connector. Depending on the vehicle there may be as many as three different ground pins. There are power grounds and sensor grounds.

The power grounds are used to control the ground side of any relay, solenoid, ignition coil or injector. The signal ground is used for any input that uses sensor return for ground, and the ground side of any internal processing component.

The SBEC III case is shielded to prevent RFI and EMI. The PCM case is grounded and must be firmly attached to a good, clean body ground.

Internally all grounds are connected together, however there is noise suppression on the sensor ground. For EMI and RFI protection the case is also grounded separately from the ground pins.

8-VOLT SUPPLY - PCM OUTPUT
The PCM supplies 8 volts to the crankshaft position sensor, camshaft position sensor.

5 VOLT SUPPLY - PCM OUTPUT
The PCM supplies 5 volts to the following sensors:
- A/C pressure transducer
- Engine coolant temperature sensor
- Manifold absolute pressure sensor
- Throttle position sensor
- Linear EGR solenoid
- Battery temperature Knock sensor