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POWERTRAIN CONTROL MODULEThe Powertrain Control Module (PCM) is a digital computer containing a microprocessor.
The PCM receives input signals from various A switches and sensors that are referred to as PCM Inputs. Based on these inputs, the PCM adjusts various engine and vehicle operations through devices that are referred to as PCM Outputs.
NOTE: PCM Inputs:
- Air Conditioning Controls
- Battery Voltage
- Battery Temperature Sensor
- Brake Switch
- Camshaft Position Sensor
- Crankshaft Position Sensor
- CCD Bus
- Engine Coolant Temperature Sensor
- Fuel Level Sensor
- Ignition Switch
- Intake Air Temperature Sensor
- Knock Sensor
- Leak Detection Pump
- Manifold Absolute Pressure (MAP) Sensor
- Oxygen Sensors
- Power Steering Pressure Switch
- SCI Receive
- Speed Control Switches
- Throttle Position Sensor
- Transmission Park/Neutral Switch (automatic transmission)
- Vehicle Speed Sensor
NOTE: PCM Outputs:
- Air Conditioning Clutch Relay
- Auto Shutdown (ASD) Relay
- Charging Indicator Lamp
- CCD Bus
- SCI Transmit
- Proportional Purge Solenoid
- EGR Solenoid
- Fuel Injectors
- Fuel Pump Relay
- Generator Field
- Idle Air Control Motor
- Ignition Coils
- Malfunction Indicator (Check Engine) Lamp
- Radiator Fan Relays
- Speed Control Solenoids
Based on inputs it receives, the PCM adjusts fuel injector pulse width, idle speed, ignition spark advance, ignition coil dwell and EVAP canister purge operation. The PCM regulates the cooling fan, air conditioning and speed control systems. The PCM changes generator charge rate by adjusting the generator field. The PCM also performs diagnostics.
The PCM adjusts injector pulse width (air-fuel ratio) based on the following inputs.
- Battery voltage
- Coolant temperature
- Exhaust gas content (oxygen sensor)
- Engine speed (crankshaft position sensor)
- Intake air temperature
- Manifold absolute pressure
- Throttle position
The PCM adjusts ignition timing based on the following inputs.
- Coolant temperature
- Engine speed (crankshaft position sensor)
- Knock sensor
- Manifold absolute pressure
- Throttle position
- Transmission gear selection (park/neutral switch)
- Intake air temperature
The PCM also adjusts engine idle speed through the idle air control motor based on the following inputs.
- Air conditioning sense
- Battery voltage
- Battery temperature
- Brake switch
- Coolant temperature
- Engine speed (crankshaft position sensor)
- Engine run time
- Manifold absolute pressure
- Power steering pressure switch
- Throttle position
- Transmission gear selection (park/neutral switch)
- Vehicle distance (speed)
The Auto Shutdown (ASD) and fuel pump relays are located in the Power Distribution Center (PDC).
The camshaft position sensor and crankshaft position sensor signals are sent to the PCM. If the PCM does not receive the signal within approximately 1 second of engine cranking, it deactivates the ASD relay and fuel pump relay. When these relays are deactivated, power is shut off from the fuel injectors, ignition coils, oxygen sensor heating elements and fuel pump.
The PCM contains a voltage converter that changes battery voltage to a regulated 9 volts direct current to power the camshaft position sensor, crankshaft position sensor and vehicle speed sensor. The PCM also provides a 5 volt direct current supply for the manifold absolute pressure sensor, throttle position sensor, and A/C pressure switch.
PCM GROUND
Ground is provided through mu1tiple 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.
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
8-VOLT SUPPLY-PCM OUTPUT
The PCM supplies 8 volts to the crankshaft position sensor, camshaft position sensor.
AUTOMATIC SHUTDOWN (ASD) SENSE-PCM INPUT
It is an input to the Powertrain Control Module from the rely in the Power Distribution Center, refer to the cover for relay location.
The ASD sense circuit informs the PCM when the ASD relay energizes. A 12 volt signal at this input indicates to the PCM that the ASD has been activated. This input is used only to sense that the ASD relay is energized.
When energized, the ASD relay provides power to operate the injectors, ignition coil, generator field, O2 sensor heaters (both upstream and downstream), and also provides a sense circuit to the PCM for diagnostic purposes. The PCM energizes the ASD any time there is a Crankshaft Position sensor signal that exceeds a predetermined value. The ASD relay can also be energized after the engine has been turned off to perform an O2 sensor heater test, if vehicle is equipped with OBD II diagnostics.
With SBEC III, the ASD relay's electromagnet is fed battery voltage, not ignition voltage. The PCM still provides the ground. As mentioned earlier, the PCM energizes the ASD relay during an O2 sensor heater test. This test is performed only after the engine has been shut off. The PCM still operates internally to perform several checks, including monitoring the O2 sensor heaters. This and other DTC tests are explained in detail in the On-Board Diagnostic Student Reference Book.
BATTERY VOLTAGE-PCM INPUT
The direct battery feed to the PCM is used as a reference point to sense battery voltage.
In order for the PCM to operate, it must be supplied with battery voltage and ground. The PCM monitors the direct battery feed input to determine battery charging rate and to control the injector initial opening point. It also has back-up RAM memory used to store Diagnostic Trouble Codes (supply working DTCs). Direct battery feed is also used to perform key-OFF diagnostics and to supply working voltage to the controller for OBDII.
If battery voltage is low the PCM will increase injector pulse width (period of time that the injector is energized).
Effect on Fuel Injectors
Fuel injectors are rated for operation at a specific voltage. If the voltage increases, the plunger will open faster and conversely, if voltage is low the injector will be slow to open. Therefore, if sensed battery voltage drops, the PCM increases injector pulse-width to maintain the same volume of fuel through the injector.
Charging
The PCM uses sensed battery voltage to verify that target charging voltage (determined by Battery Temperature Sensor) is being reached. To maintain the target charging voltage, the PCM will full field the generator to 0.5 volt above target then turn OFF to 0.5 volt below target. This will continue to occur up to a 100 Hz frequency, 100 times per second.
Fig.1 Clutch Interlock/Upstop Switch:
Fig.2 Clutch/Brake Pedal Bracket Assembly:
CLUTCH INTERLOCK/UPSTOP SWITCH
The clutch interlock/upstop switch is an assembly consisting of two switches: an engine starter inhibit switch (interlock) and a clutch pedal upstop switch. The switch assembly is located in the clutch/brake pedal bracket assembly, each switch being fastened by four plastic wing tabs.
Clutch Interlock Switch
The clutch interlock switch prevents engine starter operation and inadvertent vehicle movement with the clutch engaged and the transaxle in gear.
The switch is open while the clutch pedal is at rest. When the clutch pedal is fully depressed, the pedal blade contacts and closes the switch, completing the signal circuit from the PCM and closing the ground path, allowing engine starter operation. The interlock switch is not adjustable.
Clutch Pedal Upstop Switch
With the clutch pedal at rest, the clutch pedal upstop switch is closed, allowing speed control operation. When the clutch pedal is depressed, the upstop switch opens and signals the PCM to cancel speed control operation, and enter a modified engine calibration schedule to improve driveability during gear-to-gear shifts. The upstop switch is not adjustable.
Fig.3 Engine Coolant Temperature Sensor:
ENGINE COOLANT TEMPERATURE SENSOR-PCM INPUT
The coolant sensor threads into the front of the cylinder head near the radiator fill tube. New sensors have sealant applied to the threads.
The ECT Sensor is a Negative Thermal Coefficient (NTC), dual range Sensor. The resistance of the ECT Sensor changes as coolant temperature changes. This results in different input voltages to the PCM. The PCM also uses the ECT Sensor input to operate the low and high speed radiator cooling fans.
The PCM sends 5 volts to the sensor and is grounded through the sensor return line. As temperature increases, resistance in the sensor decreases.
As coolant temperature varies, the coolant temperature sensor resistance changes resulting in a different voltage value at the PCM engine coolant sense circuit.
When the engine is cold, the PCM will provide slightly richer air-fuel mixtures and higher idle speeds until normal operating temperatures are reached.
The combination coolant temperature sensor has two elements. One element supplies coolant temperature signal to the PCM. The other element supplies coolant temperature signal to the instrument panel gauge cluster. The PCM determines engine coolant temperature from the coolant temperature sensor.
As coolant temperature varies the coolant temperature sensors resistance changes resulting in a different input voltage to the PCM and the instrument panel gauge cluster.
When the engine is cold, the PCM will provide slightly richer air-fuel mixtures and higher idle speeds until normal operating temperatures are reached.
The PCM has a dual temperature range program for better sensor accuracy at cold temperatures. At key-ON the PCM sends a regulated five volt signal through a 10,000 ohm resistor to the sensor. When the sensed voltage reaches approximately 1.25 volts the PCM turns on the transistor. The transistor connects a 1,000 ohm resistor in parallel with the 10,000 ohm resistor. With this drop in resistance the PCM recognizes an increase in voltage on the input circuit.
Fig.4 Oxygen Sensor 1/1 Upstream And 1/2 Downstream:
HEATED OXYGEN SENSORS-PCM INPUT
The upstream oxygen sensor threads into the outlet flange of the exhaust manifold. The downstream oxygen sensor threads into the side of the catalytic converter.
The O2 sensors produce voltages from 0 to 1 volt, depending upon the oxygen content of the exhaust gas in the exhaust manifold. When a large amount of oxygen is present (caused by a lean air/fuel mixture), the sensors produce a voltage below 0.45 volts. When there is a lesser amount present (rich air/fuel mixture) it produces a voltage above 0.45 volts. By monitoring the oxygen content and converting it to electrical voltage, the sensors act as a rich-lean switch.
The oxygen sensors are equipped with a heating element that keeps the sensors at proper operating temperature during all operating modes. Maintaining correct sensor temperature at all times allows the system to enter into closed loop operation sooner.
Also, it allows the system to remain in closed loop operation during periods of extended idle. Upstream O2S (California emission equipped) are Pulse Width Modulated (PWM). They have a start-up delay to remove moisture from the heater element on cold start. This start-up delay may cause the DRB O2 Heater actuator test to be denied. The ground circuit is controlled by the PCM. This allows the PCM to duty cycle the heating element for the upstream O2S
In Closed Loop operation the PCM monitors the O2 sensor input (along with other inputs) and adjusts the injector pulse width accordingly. During Open Loop operation the PCM ignores the O2 sensor input. The PCM adjusts injector pulse width based on preprogrammed (fixed) values and inputs from other sensors.
The Automatic Shutdown (ASD) relay supplies battery voltage to both the upstream and downstream heated oxygen sensors, The oxygen sensors are equipped with a heating element. The heating elements reduce the time required for the sensors to reach operating temperature.
Upstream Oxygen Sensor 1/1
The input from the upstream heated oxygen sensor tells the PCM the oxygen content of the exhaust gas. Based on this input, the PCM fine tunes the air-fuel ratio by adjusting injector pulse width.
The sensor input switches from 0 to 1 volt, depending upon the oxygen content of the exhaust gas in the exhaust manifold. When a large amount of oxygen is present (caused by a lean air-fuel mixture), the sensor produces voltage as low as 0.1 volt. When there is a lesser amount of oxygen present (rich air-fuel mixture) the sensor produces a voltage as high as 1.0 volt. By monitoring the oxygen content and converting it to electrical voltage, the sensor acts as a rich-lean switch.
The heating element in the sensor provides heat to the sensor ceramic element. Heating the sensor allows the system to enter into closed loop operation sooner. Also, it allows the system to remain in closed loop operation during periods of extended idle.
In Closed Loop, the PCM adjusts injector pulse width based on the upstream heated oxygen sensor input along with other inputs. In Open Loop, the PCM adjusts injector pulse width based on preprogrammed (fixed) values and inputs from other sensors.
Downstream Oxygen Sensor 1/2
The Downstream O2 Sensor has two functions. One function is measuring catalyst efficiency. This is an OBD II requirement. The oxygen content of the exhaust gasses has significantly less fluctuation than at the inlet if the converter is working properly. The PCM compares upstream and Downstream 02 Sensor switch rates under specific operating conditions to determine if the catalyst is functioning proper~.
The other function is a downstream fuel control. The upstream 02 goal varies within the window of operation of the O2 Sensor. In the past the goal was a preprogrammed fixed value based upon where it believed the catalyst operated most efficiently.
While the Upstream O2 Sensor input is used to maintain the 14.7:1 air/fuel ratio, variations in engines, exhaust systems and catalytic converters may cause this ratio to not be the most ideal for a particular catalyst and engine. To help maintain the catalyst operating at maximum efficiency, the PCM will fine tune the air/fuel ratio entering the catalyst based upon the oxygen content leaving the catalyst. This is accomplished by modifying the Upstream O2 Sensor voltage goal.
If the exhaust leaving the catalyst has too much oxygen (lean) the PCM increases the upstream 02 goal which increases fuel in the mixture causing less oxygen to be left over. Conversely, if the oxygen con- tent leaving the catalyst has is too little oxygen (rich) the PCM decreases the upstream O2 goal down which removes fuel from the mixture causing more oxygen to be left over. This function only occurs during downstream closed loop mode operation.
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.
Fig.5 Inlet Air Temperature Sensor:
INLET/INTAKE AIR TEMPERATURE SENSOR-PCM INPUT
The IAT sensor attaches to the intake air duct.
The IAT Sensor is a Negative Temperature Coefficient (NTC) Sensor that provides information to the PCM regarding the temperature of the air entering the intake manifold. The PCM sends 5 volts to the sensor and is grounded through the sensor return line. As temperature increases, resistance in the sensor decreases.