Powertrain Management: Description and Operation
DESCRIPTIONThe Powertrain Control Module (PCM) will be referred to as a control module. The control module is designed to maintain exhaust emission levels to Federal or California standards, while providing excellent driveability and fuel efficiency. Review the Description and Operation articles and wiring diagrams to determine which systems are controlled by each specific control module. The control module monitors numerous engine and vehicle functions and controls the following operations:
^ Fuel control
^ Ignition Control (IC)
^ Knock Sensor (KS) system
^ Automatic transmission shifting
^ Cruise Control Enable
^ Emission Systems
^ A/C Clutch Control
^ Cooling Fan Control
OPERATION
The control module supplies a buffered voltage to various sensors and switches. This is accomplished through resistance in the control module which is so high in value that a test light may not illuminate when connected to the circuit. An ordinary shop voltmeter may not give an accurate reading because the voltmeter input impedance is too low. Use a 10 megohm input impedance digital voltmeter J 39200 or equivalent to assure accurate voltage readings.
The input/output devices in the control module include analog to digital converters, signal buffers, counters, and special drivers. The PCM controls most components with electronic switches which complete a ground circuit when turned "ON." These switches are arranged in groups of 4 and 7, called either a surface mounted Quad Driver Module (QDSM), which can independently control up to 4 outputs (PCM) terminals or Output driver Modules (ODMs) which can independently control up to 7 outputs. Not all outputs are always used.
FUEL CONTROL
The main control sensor is the Heated Oxygen Sensor (H02S) located in the exhaust manifold. The HO2S tells the PCM how much oxygen is in the exhaust gas and the PCM changes the air/fuel ratio to the engine by controlling the fuel injectors. The best mixture to minimize exhaust emissions is 14.7 to 1 which allows the catalytic converter to operate most efficiently. Because of the constant measuring and adjusting of the air/fuel ratio, the fuel injection system is called a "Closed Loop" system.
IGNITION CONTROL
The Electronic Ignition (EI) System does not use the conventional distributor and coil. The ignition system consists of three ignition coils, an electronic ignition module, a dual Hall-effect crankshaft position sensor, an engine crankshaft balancer with interrupter rings attached to the rear, related connecting wires, and the Ignition Control (IC) and fuel metering portion of the PCM. To provide optimum engine performance, fuel economy, and control of exhaust emissions, the PCM controls spark advance of the ignition system.
KNOCK SENSOR
To control spark knock, a Knock Sensor (KS) system is used. This system is designed to retard spark timing up to 15 degrees to reduce spark knock in the engine. This allows the engine to use maximum spark advance to improve driveability and fuel economy. The knock sensor detects abnormal vibration (spark knocking) in the engine. The sensor is mounted in the engine block near the cylinders. The sensor produces an AC output voltage which increases with the severity of the knock. This signal voltage inputs to the Powertrain Control Module (PCM). The PCM then adjusts Ignition Control (IC) timing to reduce spark knock.
AUTOMATIC TRANSMISSION SHIFTING
The PCM monitors inputs to control various transmission functions including shift quality and transmission diagnostics. The PCM receives input information from sensors, switches, and components for process for use within its' control program. Based on this input information, the PCM controls various transmission output functions and devices.
CRUISE CONTROL ENABLE
The stepper motor cruise control (SMCC) module sends the cruise status input to the PCM to indicate when cruise control is engaged. The PCM monitors the cruise status signal while commanding cruise to be disengaged via the cruise inhibit circuit. Any of the following conditions may cause the PCM to inhibit cruise control operation:
^ Engine not running long enough for cruise control operation.
^ Transmission range inputs indicate park, neutral, low, or reverse gear selected.
^ Engine speed is too high or too low.
^ Vehicle speed is too high or too low.
^ ABS is active for longer than 2 seconds.
^ Vehicle acceleration or deceleration rate is too high.
EMISSION SYSTEMS
Evaporative Emission (EVAP) Control System
The PCM operates a normally closed solenoid valve, which controls vacuum to the purge valve in the charcoal canister.
The PCM turns ON the PWM signal to the solenoid valve to control purge when all conditions for allowing purge are met.
The PCM operates a normally closed solenoid valve, which controls vacuum to the purge valve in the charcoal canister. The PCM turns ON the PWM signal to the solenoid valve to control purge when all conditions for allowing purge are met.
Exhaust Gas Recirculation (EGR) System
The main element of the system is the linear EGR valve. The EGR valve feeds small amounts of exhaust gas back into the combustion chamber. With the fuel/air mixture thus diluted, combustion temperatures are reduced.
The valve controls EGR flow from the exhaust to the intake manifold through an orifice with a PCM controlled pintle. During operation, the PCM controls pintle position by monitoring the pintle position feedback signal. If a problem with the EGR system will not allow the PCM to control pintle position properly, DTC P1406 should set. The PCM also tests for EGR flow; if incorrect flow is detected, DTC P0401 should set.
A/C CLUTCH CONTROL
The A/C compressor operation is controlled by the PCM in order to improve idle quality, wide open throttle performance, and to protect the compressor in the event of incorrect refrigerant pressures.
COOLING FAN CONTROL
The PCM controls the operation of the high and low speed cooling fans in order to regulate the engine and A/C refrigerant temperatures. When certain DTC's are present, the PCM may command the cooling fans to run all the time.