POWERTRAIN CONTROL MODULE
The Powertrain Control Module (PCM) is the central computer which receives data from numerous sensors and switches. The PCM processes information received and then generates output signals to control various relays, solenoids and actuators. PCM is located behind top right side of instrument panel, behind glove compartment.
Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES, which control or produce signals monitored by the PCM. The second category covers OUTPUT SIGNALS, which are components controlled by the PCM.
Brake On-Off (BOO) Switch
BOO switch is mounted on the brake pedal. Switch is closed when brake pedal is depressed. Lock-up torque converter uses this signal and is a factor in air/fuel ratio adjustment.
Camshaft Position (CMP) Sensor
The PCM uses CMP input to determine injector and spark timing. The CMP sensor is located in the distributor. The CMP sensor uses a photo sensor and slotted disc, mounted inside the distributor.
The slotted disc rotates with the distributor shaft. The slotted disc has 360 slots for one degree signal and 6 slots for 120 degree signals.
When the slot passes through the photo sensor, an input signal is generated and sent to the PCM. Using this information, the PCM calculates camshaft position and engine speed. This signal affects air/fuel ratio, ignition timing and emissions.
Data Link Connector
The OBD-II Data Link Connector (OBD-II DLC) is located below left side of instrument panel. The DLC is the main point of connection for scan tools when diagnosing engine performance and emission system problems.
Engine Coolant Temperature (ECT) Sensor
Sensor signals coolant temperature to PCM. The ECT sensor changes resistance in response to coolant temperature. The ECT sensor resistance decreases as coolant temperature increases. The PCM uses ECT sensor signal to adjust ignition timing, EGR flow, air/fuel ratio, idle speed, A/C and cooling fan operation.
EGR Temperature Sensor
Sensor is located in EGR passage downstream of EGR valve. EGR temperature sensor operates by sensing temperature increase when exhaust gases pass through EGR passage. If PCM commands EGR solenoid on and EGR temperature sensor does not sense temperature increase for 2 occurrences, DTC will set and MIL will illuminate.
Evaporative (EVAP) System Pressure Sensor
Sensor is located on EVAP purge hose and senses pressure changes in purge line. Sensor is only used for on-board diagnosis of EVAP system.
Fuel Temperature Sensor
This temperature sensor is used in California emissions vehicles only. Sensor is located in fuel tank. PCM uses signal for EVAP system operation.
Heated Oxygen Sensors (HO2S)
Heated Oxygen Sensors (HO2S) monitor oxygen content of exhaust gases. Heated oxygen sensors are mounted before and after catalytic converter in exhaust stream. When HO2S is at operating temperature, a voltage signal is produced, which varies according to oxygen content of exhaust gases. HO2S uses a built-in heating circuit. The heating circuit is used to bring the HO2S up to operating temperature, enabling faster conversion to closed-loop operation. Front HO2S monitors fuel mixture and is used by PCM to constantly adjust fuel mixture rich or lean. Rear HO2S is used to monitor catalyst efficiency. Signals between front and rear HO2S are compared and if sufficient difference is not produced, a DTC will set and MIL will illuminate.
Idle Switch
Idle is mounted on throttle body to monitor closed throttle position. PCM uses this signal to control idle speed. PCM sends signal to Transmission Control Module (TCM) to disengage Torque Converter Clutch (TCC).
Knock Sensor
The Knock Sensor (KS) detects vibrations (pre-ignition) and converts them into a voltage signal. The PCM monitors signal from KS and adjusts ignition timing if pre-ignition occurs. The KS is threaded into the engine block, between cylinder banks.
Manifold Absolute Pressure/Barometric Pressure (MAP/BARO) Sensor & Solenoid
This combination sensor alternately senses engine vacuum and ambient air pressure. When solenoid is on, MAP/BARO sensor is sensing Barometric Pressure (BARO). When solenoid is off, MAP/BARO sensor is sensing Manifold Air Pressure (MAP). PCM uses these signals to control EVAP operation.
Mass Airflow Sensor (MAF)
MAF sensor is located in throttle body. Sensor is a resistor with voltage applied to maintain resistor at a constant high temperature. When intake air flows past resistor, resistor cools off and additional voltage is required to keep resistor at high temperature. Measured voltage is used to determine quantity of air flowing in to engine. PCM uses this signal to control fuel mixture, ignition timing and other outputs.
Power Steering Pressure (PSP) Switch
The PSP switch monitors power steering pressure. When power steering fluid pressure exceeds the preset limit, the PSP switch sends an input signal to the PCM. The PCM then adjusts idle speed. PSP switch is located in power steering pump.
Throttle Position (TP) Sensor
TP sensor monitors throttle plate opening. TP signal to PCM is proportional to opening angle. TP sensor is mounted on throttle body, at throttle plate shaft. If Mass Air Flow (MAF) sensor fails, PCM will use TP sensor to provide airflow information. This is called fail safe mode.
Transmission Range (TR) Sensor
The TR sensor is used on vehicles with A/T. TR sensor monitors shift lever positions, and then inputs signal to the PCM and Transaxle Control Module (TCM). This signal affects starter engagement, air/fuel ratio, idle speed and ignition timing.
Vehicle Speed Sensor (VSS)
Vehicle Speed Sensor (VSS) is mounted in transaxle. Transaxle output shaft turns a cable that is connected to the instrument cluster speedometer and transferred into a vehicle signal. VSS sends a constant pulse signal to PCM when vehicle is in motion.
OUTPUT SIGNALS
Note. For theory and operation of each output component, refer to the system indicated after component.
Cooling Fan
See MISCELLANEOUS CONTROLS .
EGR System
See EGR SYSTEM COMPONENTS under EMISSION SYSTEMS.
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
Fuel Pressure Regulator
See FUEL DELIVERY under FUEL SYSTEM.
Idle Air Control Valve
See IDLE SPEED under FUEL SYSTEM.
FUEL SYSTEM
The fuel system is controlled by PCM. The PCM monitors engine operating conditions based on input received from engine switches and sensors. Control of output actuators determines fuel mixture and idle speed.
Fuel Pump
Fuel is supplied by in-tank electric fuel pump. Pump delivers fuel from fuel tank, through fuel filter, to fuel rail.
Fuel Pump Relay
Fuel pump relay is activated while cranking. After cranking, a switch ground closes (if engine is running) and completes ground circuit to relay. Relay ground circuit is controlled by the PCM. Fuel pump relay is located in left front engine compartment relay panel. (Scheme 1)
Scheme 1
Fuel pressure regulator is attached to fuel rail downstream of fuel injectors. Regulator is controlled by manifold vacuum signal and maintains fuel pressure at 43 psi (294 kPa).
Inertia Fuel Shutoff (IFS) Switch
In the event of a collision or vehicle rollover, mechanical action within the inertia switch opens fuel pump circuit. If IFS switch trips, vehicle will not restart unless the switch is reset. A reset button is located on the switch assembly. IFS switch is located in left front engine compartment relay panel. (Scheme 2)
Scheme 2
| WARNING | DO NOT reset IFS switch until complete fuel system has been inspected for leaks. |
The PCM controls fuel injector ON time (pulse width) to meter fuel quantity into intake ports. The PCM receives inputs from engine sensors to compute fuel flow necessary to maintain correct air/fuel ratio throughout engine operating range.
Each cylinder has a solenoid-operated injector that sprays fuel toward each intake valve. Each injector receives battery voltage through an ignition switch circuit. The PCM-controlled ground circuit is used to complete the circuit and energize the injector. Each fuel injector fires sequentially (in engine firing order) once per complete cycle.
Injectors consist of solenoid-actuated pintle and needle valve assembly. Amount of fuel delivered to engine is regulated by length of time solenoid is energized. This period is known as pulse width. Atomized spray pattern is obtained by shape of pintle.
Idle Air Control (IAC) Valve
IAC valve is an electromechanical device controlled by the PCM. The IAC valve is mounted on the throttle body and allows air to by-pass the throttle plate. The amount of air allowed to by-pass the throttle plate is determined by PCM and controlled by a duty signal.
See INPUT DEVICES under COMPUTERIZED ENGINE CONTROLS.
DISTRIBUTOR IGNITION (DI)
The DI system uses a power transistor, Camshaft Position (CMP) sensor and the PCM to regulate ignition timing. The CMP sensor is mounted in the distributor and sends a signal to the PCM.
The CMP signal is used to control fuel injection timing, detection of cylinder position, ignition timing, Idle Air Control (IAC) operation and engine speed detection when the engine is running (except during starting).
Exhaust Gas Recirculation (EGR) Valve
EGR valve recirculates a portion of the exhaust gas back into the engine to lower combustion temperature and reduce the amount of oxides of nitrogen created during combustion. The amount of exhaust gases that are released into the intake manifold are proportional to engine load.
Exhaust Gas Recirculation (EGR) Modulator Valve
EGR modulator valve is used to control EGR. EGR valve is operated by ported vacuum that will normally be vented off at EGR modulator valve. As engine RPM increases, exhaust pressure increases and pushes on diaphragm in EGR modulator valve and closes vacuum vent, allowing vacuum to EGR valve.
Exhaust Gas Recirculation (EGR) Vacuum Regulator Solenoid Valve
EGR vacuum regulator solenoid valve is controlled by Powertrain Control Module (PCM). EGR vacuum regulator solenoid valve controls vacuum to EGR valve. When EGR vacuum regulator solenoid valve is off, vacuum is supplied to EGR valve. When EGR vacuum regulator solenoid valve is on, vacuum is turned off keeping EGR valve closed. PCM will command EGR vacuum regulator valve on at engine start, low engine coolant temperature, high engine coolant temperature, idle, high engine speed, and mass airflow sensor failure.
Evaporative (EVAP) Canister
EVAP canister is used for evaporative fuel control on all vehicles. The evaporative emission control system stores gasoline fumes from fuel tank in a carbon canister until fumes can be drawn from carbon canister into the intake plenum.
Evaporative (EVAP) Canister Pressure Sensor
EVAP canister pressure sensor is an input to Powertrain Control Module (PCM) and is used only for on-board diagnostics of EVAP emission system. Sensor is located in purge line of EVAP canister and senses changes in pressure in EVAP canister purge line.
Evaporative (EVAP) Canister Purge Valve
EVAP canister purge valve is controlled by Powertrain Control Module (PCM). EVAP canister purge valve controls EVAP canister purge. When EVAP purge is on, vacuum is supplied to EVAP canister and vapors in EVAP canister are purged. When EVAP purge valve is off, solenoid is released to prevent purging of fuel vapors from EVAP canister.
Evaporative (EVAP) Canister Vent Control Valve
This valve is only used during EVAP system pressure testing. At all other times, valve is normally open.
Vacuum Cut By-Pass Solenoid
This valve is only used during EVAP system diagnostic testing. At all other times, valve is normally closed.
Vacuum Cut Valve
This is a one-way valve that allows fuel vapors to enter EVAP purge line but will not allow vacuum to be applied to fuel tank.
Water Separator
Water separator is attached to canister vent control valve inlet. When canister vent control valve is open, air is drawn through water separator and water vapor in incoming air is vented to atmosphere.
POSITIVE CRANKCASE VENTILATION (PCV)
The PCV system uses intake manifold vacuum to extract blow-by vapors from crankcase. The mixture is then passed into the combustion chamber and burned. PCV valve provides control by metering flow of blow-by vapors according to manifold vacuum. PCV valve is located in valve cover.
Under conditions where abnormal amounts of blow-by gases are produced (such as worn cylinders or rings), the system is designed to allow excess gases to flow back through crankcase vent hose and into air inlet for consumption during normal combustion.
MISCELLANEOUS CONTROLS
Note. Although not considered true engine performance-related systems, some controlled devices may affect driveability if they malfunction.
The PCM regulates operation of the electric cooling fan through the low fan and high fan control relays. Using information supplied by Cooling Fan Engine Coolant Temperature (ECTF) sensor, Engine Coolant Temperature (ECT) sensor, A/C signal and TP sensor, the PCM controls cooling fan operation by grounding and ungrounding relay circuits.
MALFUNCTION INDICATOR LIGHT (MIL)
The MIL will illuminate when ignition switch is turned to the ON position (bulb check), or when systems related to the EEC system malfunction during normal engine operation. For additional information, see SELF-DIAGNOSTICS - VILLAGER article.