Engine Control Module: Description and Operation
POWERTRAIN CONTROL MODULEFig. 5 Powertrain Control Module (PCM):
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, transmission, and vehicle operations through devices referred to as Powertrain Control Module Outputs.
NOTE: PCM Inputs:
- Air Conditioning Pressure Transducer
- ASD Relay
- Battery Voltage
- Brake Switch Status
- Camshaft Position Sensor
- Crankshaft Position Sensor
- Distance Sensor
- EGR Position Feedback
- Engine Coolant Temperature Sensor
- Heated Oxygen Sensors
- Ignition sense
- Intake Air Temperature Sensor
- Knock Sensor
- Manifold Absolute Pressure (MAP) Sensor
- Natural Vacuum Leak Detection
- PCI Bus
- Power Steering Pressure Switch
- Proportional Purge Sense
- SCI Receive
- Speed Control
- Throttle Position Sensor
- Torque Management Input (Integral to PCM)
- Transmission Control Relay (Switched B+)
- Transmission Range Sensor (TRS)
- Transmission Pressure Switches (L/R, 2/4, OD)
- Transmission Temperature Sensor (Integral to TRS)
- Transmission Input Shaft Speed Sensor
- Transmission Output Shaft Speed Sensor
- TRS Hall Effect Switch (Autostick)
- Vehicle Speed (From Transmission)
NOTE: PCM Outputs:
- Air Conditioning Clutch Relay
- Automatic Shut Down (ASD) and Fuel Pump Relays
- Data Link Connector (PCI and SCI Transmit)
- Double Start Override
- EGR Solenoid
- Fuel Injectors
- Generator Field
- High Speed Fan Relay
- Idle Air Control Motor
- Ignition Coils
- Low Speed Fan Relay
- MTV Actuator
- Natural Vacuum Leak Detection
- Proportional Purge Solenoid
- SRV Valve
- Speed Control Relay
- Speed Control Vent Relay
- Speed Control Vacuum Relay
- Transmission Autostick Display (To Cluster)
- Transmission Control Relay
- Transmission PRNDL Position (To Cluster)
- Transmission Solenoids (L/R, 2/4, OD and UD)
- Transmission Torque Reduction Request (Internal to PCM)
- Transmission Temperature (Internal to PCM)
- Vehicle Speed (Internal to PCM)
- 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 also determines the appropriate transmission shift schedule and shift points, depending on the present operating conditions and driver demand. 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
- Intake Air Temperature Sensor
- EGR valve position (if equipped)
- Engine Coolant Temperature
- Engine Speed (crankshaft position sensor)
- EVAP Purge Valve Position
- Exhaust Gas Oxygen Content (heated oxygen sensors)
- Manifold Absolute Pressure
- Throttle Position
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)
- Park/Neutral (transmission gear selection)
- Transaxle Gear Engagement
- Throttle Position
- Vehicle Speed (from Transmission)
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
In addition to monitoring inputs and controlling outputs, the PCM has other important transmission related responsibilities and functions:
- Storing and maintaining Clutch Volume Indices (CVI)
- Storing and selecting appropriate Shift Schedules
- System self-diagnostics
- Diagnostic capabilities (with DRB III scan tool)
The automatic shut down (ASD) and fuel pump relays are mounted externally, but turned ON and OFF by the powertrain control module.
The camshaft and crankshaft signals are sent to the powertrain control module. If the PCM does not receive both signals within approximately two 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 85.0 volts. The 5.0 volts power the camshaft position sensor, crankshaft position sensor, vehicle speed sensor, engine coolant temperature sensor, intake air temperature sensor, manifold absolute pressure sensor, 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.
TRANSMISSION CONTROL - CLUTCH VOLUME INDEX (CVI)
An important function of the PCM is to monitor Clutch Volume Index (CVI). CVIs represent the volume of fluid needed to compress a clutch pack.
The PCM monitors gear ratio changes by monitoring the Input and Output Speed Sensors. The Input, or Turbine Speed Sensor sends an electrical signal to the PCM that represents input shaft rpm. The Output Speed Sensor provides the PCM with output shaft speed information.
Fig. 6 Example Of CVI Calculation:
By comparing the two inputs, the PCM can determine transaxle gear position. This is important to the CVI calculation because the PCM determines CVIs by monitoring how long it takes for a gear change to occur.
Gear ratios can be determined by using the DRB Scan Tool and reading the Input/Output Speed Sensor values in the "Monitors" display. Gear ratio can be obtained by dividing the Input Speed Sensor value by the Output Speed Sensor value.
For example, if the input shaft is rotating at 1000 rpm and the output shaft is rotating at 500 rpm, then the PCM can determine that the gear ratio is 2:1. In direct drive (3rd gear), the gear ratio changes to 1:1. The gear ratio changes as clutches are applied and released. By monitoring the length of time it takes for the gear ratio to change following a shift request, the PCM can determine the volume of fluid used to apply or release a friction element.
The volume of transmission fluid needed to apply the friction elements are continuously updated for adaptive controls. As friction material wears, the volume of fluid need to apply the element increases.
Clutch Volumes:
Certain mechanical problems within the input clutch assembly (broken return springs, out of position snap rings, excessive clutch pack clearance, improper assembly, etc.) can cause inadequate or out-of-range element volumes. Also, defective Input/Output Speed Sensors and wiring can cause these conditions. The chart identifies the appropriate clutch volumes and when they are monitored/updated:
TRANSMISSION CONTROL - SHIFT SCHEDULES
As mentioned earlier, the PCM has programming that allows it to select a variety of shift schedules. Shift schedule selection is dependent on the following:
- Shift lever position
- Throttle position
- Engine load
- Fluid temperature
- Software level
As driving conditions change, the PCM appropriately adjusts the shift schedule. Refer to the chart to determine the appropriate operation expected, depending on driving conditions.
DATA BUS COMMUNICATION RECEIVE - PCM INPUT
The PCM uses the SCI communication bus to perform engine diagnostics and flash operation. The transmission side of the PCM uses the SCI communications bus to the flash new software. However, diagnostics is performed via the vehicles J1850 bus for the transmission side of the PCM
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. There five different ground pins. There are engine power grounds and trans power grounds.
The power grounds are used to control the ground side of any relay, solenoid, ignition coil or injector.
The NGC 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. For EMI and RFI protection the case is also grounded separately from the ground pins.
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
- Crankshaft position sensor
- Camshaft position sensor