CONTROL UNIT
The Powertrain Control Module (PCM) is a digital computer that controls air/fuel ratio, canister purge, charging system, cooling fan, emission control devices, fuel injector pulse width, idle speed, ignition coil dwell, and spark advance.
PCM has a voltage converter that converts battery voltage to regulated 5-volt or 8-volt outputs. Regulated 5-volt output is used to power Manifold Absolute Pressure (MAP) sensor, Throttle Position Sensor (TPS) and logic circuits. Regulated 8-volt output is used to power distributor on all engines except models equipped with Direct Ignition System (DIS). On models with DIS, an 8-volt output is used to power camshaft and crankshaft sensors.
Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES, which control or produce voltage signals that are monitored by the PCM. The second category covers OUTPUT SIGNALS, which are components controlled by the PCM.
INPUT DEVICES
Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input device used on a specific model, see appropriate wiring diagram in WIRING DIAGRAMS section. The available input signals include the following
A/C Control Module (ACCM)
When A/C function is selected, ACCM sends a request signal to PCM about .5 second before A/C clutch is energized. PCM increases engine idle speed to compensate for increased engine load. About .5 second before ACCM de-energizes A/C compressor clutch, ACCM stops sending request signal and PCM reduces idle speed to compensate for decrease in engine load.
Brake Switch
Brake switch is mounted on brake pedal support bracket. Brake switch is used as an input to PCM during deceleration and full stop conditions. Together with other inputs, brake switch is used to maintain engine idle speed by controlling Idle Speed Control (ISC) motor position.
Camshaft Position (CMP) Sensor
CMP sensor reads slots in cam timing sprocket and provides PCM with fuel injection synchronization and cylinder identification. PCM uses this information along with crankshaft position sensor signal to determine if fuel injectors and ignition coils are properly adjusted for correct cylinders. CMP sensor is located on front of timing case cover.
Coolant Temperature Sensor (CTS)
CTS monitors engine coolant temperature. This sensor is mounted in thermostat housing. Coolant temperature information is used by PCM to slightly enrich or lean air/fuel mixture, adjust idle speed and control cooling fans as necessary, according to engine temperature.
Crankshaft Position (CKP) Sensor
CKP sensor is mounted on transaxle bellhousing. Sensor reads slots (4 per cylinder) on torque converter drive plate. PCM uses this information to determine crankshaft position.
Hall Effect Switch
Used only on 4-cylinder engines, Hall Effect switch is located inside distributor. This switch supplies PCM with engine RPM data and ignition timing information. PCM uses this information to advance or retard ignition timing as necessary.
Knock Sensor (3.5L)
Knock sensors are mounted in top of cylinder block below cylinder heads. Sensors detect knock in any cylinder. PCM uses this information to adjust spark advance.
Manifold Absolute Pressure (MAP) Sensor
MAP sensor monitors manifold vacuum (engine load). This sensor transmits information on manifold vacuum and barometric pressure to PCM. MAP sensor information is used in conjunction with other sensors to adjust air/fuel mixture.
Optical Distributor (3.0L)
Optical distributor provides engine speed and crankshaft position signals to PCM. (Scheme 1) PCM uses this information to control fuel injection, ignition timing and idle speed.
Scheme 1
Oxygen (O2) Sensor
O2 sensor produces a small electrical voltage (.1-.9 volt) when exposed to oxygen in exhaust gas flow. O2 sensor is electrically heated for faster switching.
When a large amount of oxygen is present in exhaust gas, O2 sensor will produce a low voltage (.1 volt). With a small amount of oxygen present in exhaust gas, O2 sensor will produce a high voltage (.9 volt). O2 sensor informs PCM of oxygen content in exhaust gas. PCM uses this information to adjust air/fuel ratio.
Park/Neutral (P/N) Switch
P/N switch is located on transaxle housing. P/N switch informs PCM of automatic transmission position. Input is used to determine idle speed (varied with gear selection), fuel injector pulse width and ignition timing.
Throttle Body Temperature Sensor
This sensor is mounted in throttle body. Sensor monitors throttle body temperature so PCM can enrich air/fuel mixture for a hot restart condition.
Throttle Position Sensor (TPS)
TPS is mounted on throttle body and monitors opening angle of throttle valve. TPS input voltage to PCM varies from about .5 volt at minimum throttle opening (idle) to about 4.5 volts at Wide Open Throttle (WOT). PCM uses this information along with other sensor inputs to adjust air/fuel ratio.
Vehicle Speed Sensor (VSS)
VSS is located in transaxle extension housing. VSS generates 8 pulses per axle shaft revolution. PCM will interpret VSS input along with TPS closed throttle input.
These inputs allow PCM to differentiate between closed throttle deceleration and closed throttle idle (vehicle stopped) conditions. During deceleration, PCM controls Idle Speed Control (ISC) motor to maintain a desired manifold pressure value. During idle (vehicle stopped), PCM controls ISC motor to maintain a desired idle speed.
OUTPUT SIGNALS
Note. Vehicles are equipped with different combinations of computer-controlled components. Not all components listed below are used on every vehicle. For theory and operation on each output component, refer to system indicated after component.
A/C Clutch Relay
See MISCELLANEOUS CONTROLS.
Auto Shutdown (ASD) Relay
See FUEL DELIVERY under FUEL SYSTEM.
Electric Exhaust Gas Recirculation Transducer (EET)
See EMISSION SYSTEMS.
Exhaust Gas Recirculation Solenoid
See EMISSION SYSTEMS.
Fuel Injector(s)
See FUEL CONTROL under FUEL SYSTEM.
Idle Speed Control (ISC) Motor
See IDLE SPEED under FUEL SYSTEM.
In-Tank Fuel Pump
See FUEL DELIVERY under FUEL SYSTEM.
Lock-Up Torque Converter Solenoid
See TRANSMISSION CONTROL under MISCELLANEOUS CONTROLS.
Malfunction Indicator Light
See SELF-DIAGNOSTIC SYSTEM.
Purge Solenoid
See EMISSION SYSTEMS.
Radiator Fan Relay
See MISCELLANEOUS CONTROLS.
Speed Control Servo
See MISCELLANEOUS CONTROLS.
Automatic Shutdown (ASD) Relay
ASD relay is energized when ignition is on. If PCM does not receive a distributor signal (camshaft or crankshaft sensor signal on DIS models), PCM will de-energize ASD relay. When ASD relay is de-energized, power to fuel pump, fuel injectors, ignition coil and O2 sensor heater element is interrupted.
Fuel Pressure Regulator
Fuel pressure regulator is a mechanical device located on top of throttle body on Throttle Body Injection (TBI) models or on fuel rail on 3.3L and 3.5L Multiport Fuel Injection (MFI) models. (Scheme 2) On all models, regulator is controlled by manifold vacuum. Its purpose is to maintain correct fuel pressure at fuel injector. A spring-loaded, vacuum-assisted diaphragm is located inside pressure regulator.
When fuel pump is energized, fuel flows past fuel injector into fuel pressure regulator. Regulator restricts fuel from flowing any further until proper fuel pressure is reached.
When proper fuel pressure is reached, pressure pushes on a spring behind diaphragm. As pressure moves spring and diaphragm, a return line to fuel tank is uncovered. This allows excess fuel to return to fuel tank, keeping fuel pressure constant across injector.
During acceleration, vacuum to diaphragm drops off. Without vacuum assist, spring pressure only acts upon diaphragm, resulting in a slight increase in fuel pressure before return port is opened.
Scheme 2
Fuel Pump
Fuel pump is a positive displacement, immersible gerotor pump with a permanent magnet motor. Fuel pump incorporates a sock (strainer) attached to pump pick-up.
Fuel pump contains 3 check valves. First check valve is used to relieve internal pump pressure and regulate maximum fuel pump output. Second check valve, located in pump outlet, is used to restrict fuel movement in either direction when pump is not operational. Third check valve, in fuel return circuit, prevents fuel tank leakage if fuel line is damaged in an accident. Voltage to operate pump is supplied through ASD relay.
Fuel Injectors
Fuel injector is an electronic solenoid. PCM determines pulse width (on time) for injector. When injector is energized, a spring-loaded check ball is lifted from its seat. Fuel then flows in a cone-shaped spray pattern into air stream on TBI models.
On MFI models, armature and pintle move a short distance against a spring, opening a small orifice. Since fuel is under high pressure, a fine spray is developed and injected directly behind intake valve.
ISC motor adjusts idle speed to compensate for engine load and ambient temperature. ISC motor varies amount of air by-pass through throttle body.
PCM uses coolant temperature, VSS, TPS and various switch input operations to adjust ISC to obtain optimum idle conditions. To prevent deceleration stall, airflow is increased when throttle is suddenly closed.
3.3L & 3.5L
Crankshaft Position (CKP) sensor detects slots (4 per cylinder, 20 degrees apart) around an extension of drive plate. Basic timing is preset by CKP sensor position and is not adjustable. By using a CKP sensor, spark scatter is eliminated.
Camshaft Position (CMP) sensor supplies fuel injection synchronization and cylinder identification information by sensing slots located on camshaft sprocket. CMP sensor is located on front of timing case cover.
PCM fires one coil at a time. This one coil in turn fires 2 spark plugs simultaneously. (Scheme 3) One cylinder is on compression stroke, and other cylinder is on exhaust stroke. A low primary resistance allows PCM to fully charge ignition coils for each firing.
Scheme 3
2.2L & 2.5L TBI
A Hall Effect switch is located in distributor assembly. Pick-up supplies RPM, fuel injection synchronization (2.5L TBI) and ignition timing data to PCM. PCM determines ignition spark timing and injector pulse width to maintain optimum driveability.
3.0L
Timing member is a thin disk, mounted on distributor shaft and driven at 1/2 crankshaft speed. Disk has 2 sets of slots on its surface. (Scheme 1) Outer (high data rate) set of slots occurs at intervals of 2 degrees of crankshaft rotation. It is used for ignition timing at engine speeds up to 1200 RPM to increase timing accuracy.
During cranking and idle, engine speed changes with firing pulse of each cylinder. High data rate signal is used to trigger ignition at correct crankshaft position regardless of these speed changes.
Inner (low data rate) set contains 6 slots, which are correlated to piston Top Dead Center (TDC) for each cylinder. This set is used to trigger fuel injection system and operation at speeds greater than 1200 RPM, where speed changes due to small individual firing pulses. This set of slots is also used for ignition timing. Light Emitting Diodes (LED) and photo diodes are mounted in facing positions on opposite sides of disk, in line with slots.
Masks over LED and photo diodes focus light beams onto photo diodes. As each slot passes between diodes, light beam is turned on and off. This creates an alternating voltage in each photo diode, which is converted into on-off pulses by an integrated circuit within distributor. These pulses are transmitted to PCM.
Powertrain Control Module (PCM)
PCM controls ignition system. During a crank/start mode, PCM will set a fixed amount of spark advance for an efficient engine start.
Ignition Timing Advance Control
Amount of spark advance or retard is determined by inputs that PCM receives from coolant temperature, engine vacuum and engine RPM sensors. During engine operation, PCM can supply an infinite number of advance curves to ensure proper engine operation.
AIR ASPIRATOR SYSTEM
Some 2.2L & 2.5L TBI engines use an air aspirator system. (Scheme 4) System includes a valve that uses exhaust pressure pulsation to draw fresh air through air cleaner assembly into exhaust system. Fresh air introduced into exhaust system helps reduce carbon monoxide (CO) and hydrocarbon (HC) emissions. Aspirator valve works most efficiently at idle and slightly off idle when exhaust pulsations are strongest. Aspirator valve is closed at higher engine speeds.
Scheme 4
EVAPORATIVE EMISSION (EVAP) SYSTEM
System stores fuel vapors from fuel tank, preventing vapors from escaping to atmosphere. As fuel evaporates inside fuel tank, vapors are routed through vent hoses to charcoal canister, located in wheelwell area, where they are stored until engine is started.
Charcoal canister purging is controlled by PCM through a canister purge solenoid. During engine warm-up and for a short period after hot restarts, PCM energizes canister purge solenoid, preventing vacuum signal from reaching charcoal canister.
After engine reaches a predetermined operating temperature and a timer has run out, PCM de-energizes canister purge solenoid, allowing engine vacuum to purge charcoal canister. Canister purge solenoid is also de-energized during certain idle conditions so PCM can update fuel delivery calibration.
EXHAUST GAS RECIRCULATION (EGR) SYSTEM
Note. Models equipped with 2.5L flexible fuel engines are not equipped with EGR system.
EGR system allows a predetermined amount of exhaust gas to enter cylinder with air/fuel mixture. This dilution of cylinder air/fuel volume reduces peak temperatures inside combustion chamber, which reduces oxides of nitrogen (NOx) and helps prevent spark knock.
EGR system uses an Electric Exhaust Gas Recirculation Transducer (EET). (Scheme 5) EET is a backpressure transducer and electric vacuum solenoid combined into a single unit.
Vacuum solenoid portion of EET is controlled by PCM and is used to regulate vacuum to transducer portion of EET. Backpressure transducer measures amount of exhaust gas backpressure on exhaust side of EGR valve. Backpressure transducer then varies amount of vacuum applied to EGR valve. This system allows backpressure transducer to provide proper vacuum signal to EGR valve for all engine operating conditions.
Scheme 5
POSITIVE CRANKCASE VENTILATION (PCV) SYSTEM
Crankcase and piston blow-by gases are removed from crankcase with manifold vacuum. These gases are introduced into incoming air/fuel mixture and become part of calibrated mixture.
THERMOSTATIC AIR CLEANER (TAC)
Used only on TBI engines, this system controls incoming air temperature to throttle body when ambient temperature is low. Heated air allows leaner throttle body calibration, provides better cold driveability and minimizes throttle body icing.
When ambient temperature is at least 15°F (9°C) greater than control temperature, airflow will be through outside outlet. When ambient temperature is less than control temperature, air flows through both air inlets after engine has been started.
With a colder ambient temperature, more air flows past exhaust manifold to heat incoming air. With a warmer ambient temperature, more air flows through air cleaner snorkel, by-passing exhaust manifold.
Temperature of incoming air charge is controlled by intake manifold vacuum, temperature sensor (inside air cleaner assembly) and a vacuum diaphragm which operates door inside air cleaner snorkel.
Vacuum diaphragm is opposed by a spring which closes air door when vacuum signal is removed. Temperature control occurs during road load conditions or when intake manifold vacuum is above operating vacuum of vacuum diaphragm.
MALFUNCTION INDICATOR LIGHT (MIL)
MIL illuminates each time ignition switch is turned to ON position. Light stays on for 3 seconds as a bulb test. MIL is illuminated if PCM receives no signal or an incorrect signal from any of the following: battery voltage input, charging system, Coolant Temperature Sensor (CTS), Manifold Absolute Pressure (MAP) sensor, Throttle Position Sensor (TPS) or an emission related fault. This warns driver that PCM is in limp-in mode and immediate repairs are necessary. MIL can also be used to display fault codes. For additional information, see appropriate G - TESTS W/ CODES article in the ENGINE PERFORMANCE section listed below.
- «TESTS W/CODES - 2.2L»(ref-10830)
- «TESTS W/CODES - 2.5L TBI»(ref-10831)
- «TESTS W/CODES - 2.5L FLEX FUEL»(ref-10825)
- «TESTS W/CODES - 3.0L»(ref-10826)
- «TESTS W/CODES - 3.3L»(ref-10827)
- «TESTS W/CODES - 3.5L»(ref-10832)
MISCELLANEOUS CONTROLS
Note. Although not considered true engine performance-related systems, some controlled devices may affect driveability if they malfunction.
A/C clutch relay is controlled by PCM and A/C switch. A/C clutch relay is powered by radiator fan relay. This relay is energized during engine operation when A/C switch is closed and blower switch is on.
When PCM senses low idle speed or Wide Open Throttle (WOT) through Throttle Position Sensor (TPS), it will de-energize A/C clutch relay, preventing A/C operation.
Except Concorde, Intrepid, LHS, New Yorker & Vision
PCM energizes radiator fan relay during the following conditions
- When A/C clutch is engaged.
- At vehicle speeds greater than 40 MPH, fan relay is switched on at coolant temperature of 230°F (110°C) and off at 220°F (104°C).
- At vehicle speed less than 40 MPH, fan relay is switched on at 210°F (99°C) and off at 199°F (93°C).
- Fan relay is energized for about 3 minutes when ambient temperature is less than 60°F (16°C), coolant temperature is 100-195°F (38-91°C), and engine is at idle. This prevents steaming, a condition where moisture evaporates from radiator core, giving the appearance of engine overheating.
Concorde, Intrepid, LHS, New Yorker & Vision
PCM controls radiator fan relay. Radiator fan will not run during engine crank/start. PCM controls fan relay based on coolant temperature and A/C head pressure. A pressure transducer on A/C compressor discharge line sends a signal to PCM, which energizes fan relay at programmed pressure input. Radiator fan relay will also be energized during conditions outlined in RADIATOR COOLING FAN OPERATIONS table.
| Fan Speed | Switch Position | Temperature | |
|---|---|---|---|
| 3.3L Engine | |||
| Low | On | 210°F (99°C) | |
| Low | Off | 199°F (93°C) | |
| High | On | 230°F (110°C) | |
| High | Off | 219°F (104°C) | |
| 3.5L Engine | |||
| Low | On | 219°F (104°C) | |
| Low | Off | 210°F (99°C) | |
| High | On | 230°F (110°C) | |
| High | Off | 221°F (105°C) | |
RADIATOR COOLING FAN OPERATIONS
This system is electrically actuated and vacuum operated. Controls located on steering wheel consist of following buttons: OFF/ON, RESUME/ACCEL and SET/DECEL. Speed control servo is controlled by PCM. This system will operate at speeds greater than 35 MPH.
TRANSMISSION CONTROL
PCM controls lock-up of torque converter through part-throttle unlock solenoid. Torque converter locks up only when in direct drive mode.
