Engine Controls - Theory & Operation Buick Century VI

INTRODUCTION

This article covers basic description and operation of engine performance-related systems and components. Read this article before diagnosing vehicles or systems with which you are not completely familiar.

AIR INDUCTION SYSTEMS

The primary function of the Air Intake System is to provide filtered air to the engine. The system uses a cleaner element mounted in a housing. The cleaner housing is remotely mounted and uses intake ducts to route the incoming air into the throttle body. The secondary function of the Air Intake System is to muffle air induction noise. This is achieved through the use of resonators attached to the air intake ducts. The resonators are tuned to the specific powertrain. The Mass Air Flow (MAF) sensor is used to measure the air entering the engine.

Mass Airflow Sensor

The Mass Airflow (MAF) sensor is located in the air cleaner duct. (Scheme 1) MAF is an air flow meter that measures flow of air entering the engine in grams per second. The Powertrain Control Module (PCM) uses the MAF sensor signal to provide the correct fuel delivery for all engine speeds and loads. A small quantity of air entering the engine indicates a deceleration or idle condition. A large quantity of air entering the engine indicates an acceleration or high load condition. The MAF sensor has an ignition 1 voltage circuit, a ground circuit and a signal circuit. The PCM applies a voltage to the sensor on the signal circuit. The sensor uses the voltage to produce a frequency based on the inlet air flow through the sensor bore. The frequency varies within a range of near 2000 Hertz at idle to near 10,000 Hertz at maximum engine load.

The PCM uses the following sensor inputs to calculate a predicted MAF value

1. The Barometric Pressure (BARO) at key ON.
2. The Manifold Absolute Pressure (MAP) sensor.
3. The Intake Air Temperature (IAT) sensor.
4. The Engine Coolant Temperature (ECT) sensor.
5. The Throttle Position (TP) sensor.
6. The engine speed (RPM).

The PCM compares the actual MAF sensor frequency signal to the predicted MAF value. This comparison will determine if the signal is stuck based on a lack of variation, or is too low or too high for a given operating condition.

Scheme 1

COMPUTERIZED ENGINE CONTROLS

The computerized engine control system monitors and controls a variety of engine/vehicle functions. The computerized engine control system is primarily an emission control system which is designed to maintain a 14.7:1 air/fuel ratio under most operating conditions. When the ideal air/fuel ratio is maintained, the 3-way catalytic converter can control Oxides Of Nitrogen (NOx), Hydrocarbon (HC) and Carbon Monoxide (CO) emissions.

The computerized engine control system consists of the master controller (PCM or ECM), input devices (sensors and switches) and output signals.

POWERTRAIN CONTROL MODULE

On Century and Grand Prix, PCM is located left side of engine compartment, inside air cleaner assembly. On Malibu, PCM is located under Instrument Panel (I/P), next to Data Link Connector (DLC). (Scheme 2)and (Scheme 3).

The powertrain has electronic controls to reduce exhaust emissions while maintaining excellent driveability and fuel economy. The Powertrain Control Module (PCM) is the control center of this system. The PCM monitors numerous engine and vehicle functions. The PCM constantly looks at the information from various sensors and other inputs, and controls the systems that affect vehicle performance and emissions. The PCM also performs the diagnostic tests on various parts of the system. The PCM can recognize operational problems and alert the driver via the Malfunction Indicator Light (MIL). When the PCM detects a malfunction, the PCM stores a Diagnostic Trouble Code (DTC). The problem area is identified by the particular DTC that is set. The PCM supplies a buffered voltage to various sensors and switches. Review the components and wiring diagrams to determine which systems are controlled by the PCM. The following are some of the functions that the PCM controls

1. The engine fueling.
2. The Ignition Control (IC).
3. Transmission shifting.
4. The Knock Sensor (KS) system.
5. The Evaporative Emissions (EVAP) system.
6. The Secondary Air Injection (AIR) system (if equipped).
7. The Exhaust Gas Recirculation (EGR) system.
8. The automatic transmission functions.
9. The generator.
10. The A/C clutch control.
11. The cooling fan control.

The PCM constantly looks at the information from various sensors and other inputs and controls systems that affect vehicle performance and emissions. The PCM also performs diagnostic tests on various parts of the system. The PCM can recognize operational problems and alert the driver via the MIL. When the PCM detects a malfunction, the PCM stores a DTC. The problem area is identified by the particular DTC that is set. The PCM supplies a buffered voltage to various sensors and switches. The input and output devices in the PCM include analog-to-digital converters, signal buffers, counters, and output drivers. The output drivers are electronic switches that complete a ground or voltage circuit when turned on. Most PCM controlled components are operated via output drivers. The PCM monitors these driver circuits for proper operation and, in most cases, can set a DTC corresponding to the controlled device if a problem is detected.

Scheme 2

Scheme 3

INPUT DEVICES

Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES, which control or produce voltage signals monitored by the control unit. The second category covers OUTPUT SIGNALS, which are components controlled by the control unit.

Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input devices used on a specific model, see ENGINE PERFORMANCE in appropriate SYSTEM WIRING DIAGRAMS article in ELECTRICAL. Also see COMPONENT LOCATIONS in SELF-DIAGNOSTICS - 3.1L CENTURY, GRAND PRIX & MALIBU article. The available input signals include the following

A/C Request Signal

The A/C system can be engaged when the VENT switch is pressed or the HVAC control module is in FRONT DEFROST, MIX-BLEND mode or during automatic operation. The HVAC control module sends a Class 2 A/C request message to the Powertrain Control Module (PCM). The following conditions must be met in order for the PCM to turn on the compressor clutch

1. HVAC Control Module - Ambient temperature is more than 40°F (4°C).
2. PCM - Engine Coolant Temperature (ECT) is less than 255°F (124°C), Engine speed is less than 5000 RPM and A/C Pressure is 410-30 psi (2826-207 kPa).

Once engaged, the compressor clutch will be disengaged for the following conditions

1. Throttle position is 100 percent.
2. A/C Pressure is more than 410 psi (2826 kPa).
3. A/C Pressure is less than 30 psi (207 kPa).
4. ECT is more than 255°F (124°C).
5. Engine speed is more than 5000 RPM.
6. Transmission shift.
7. PCM detects excessive torque load.
8. PCM detects insufficient idle quality.
9. PCM detects a hard launch condition.

When the compressor clutch disengages, the compressor clutch diode protects the electrical system from a voltage spike.

A/C Pressure Sensor

The A/C pressure sensor is a 3 wire piezoelectric pressure transducer located on left side of the engine compartment mounted on the A/C line near the accumulator. (Scheme 4) A 5 volt reference, low reference, and signal circuits enable the sensor to operate. The A/C pressure signal can be between 0-5 volts. When the A/C pressure is low, the signal value is near zero volts. When the A/C pressure is high, the signal value is near 5 volts. The A/C pressure sensor protects the A/C system from operating when an excessively high or low pressure condition exists. The PCM disables the compressor clutch under the following conditions

1. A/C pressure is more than 432 psi (2979 kPa). The clutch will be enabled after the pressure decreases to less than 219 psi (1510 kPa).
2. A/C pressure is less than 27 psi (186 kPa). The clutch will be enabled after the pressure increases to more than 30 psi (207 kPa).

Scheme 4

Automatic Transmission Input Shaft Speed Sensor

The Automatic Transmission Input Shaft Speed Sensor (AT ISS) is a magnetic inductive pickup that relays information about the transmission input speed to the PCM. (Scheme 5)or (Scheme 6). The PCM uses this information to control the line pressure, TCC apply and release, and the transmission shift patterns. This information is also used to calculate the appropriate operating gear ratios and TCC slippage. The AT ISS mounts on the transmission case under the channel plate next to the drive sprocket. An air gap of 0.010-0.114" (0.26-2.90 mm) is maintained between the sensor and the teeth of the drive sprocket. The sensor consists of a permanent magnet surrounded by a coil of wire. As the drive sprocket is driven by the turbine shaft, an AC signal is induced in the AT ISS. Higher engine speeds induce a higher frequency and voltage measurement at the sensor. Sensor resistance should be 625-725 ohms when measured at 68°F (20°C). Output voltage will vary with speed from a minimum of 0.5 volts AC at 550 RPM, to 200 volts AC at 7000 RPM.

Scheme 5

Scheme 6

Battery Voltage

The PCM monitors the system voltage to make sure that the voltage stays within the proper range. Damage to components, and incorrect data input can occur when the voltage is out of range. The PCM monitors the system voltage over an extended length of time. If the PCM detects a system voltage outside an expected range for the calibrated length of time, DTC P0560 will set.

Cruise Control Release/Brake Light Switch (Century & Grand Prix)

On Century and Grand Prix, the cruise control release switch and brake light switch are used to disengage the cruise control. A release switch assembly and a brake light switch assembly are mounted on the brake pedal bracket. (Scheme 7) To disengage the system the driver presses the brake pedal. The speed of the vehicle at brake actuation will be stored in the memory of the cruise module.

Scheme 7

TCC/Brake Light Switch (Malibu)

On Malibu, the TCC/Brake switch and brake light switch control the cruise control release signal and the brake light signal. These signal circuits are used to disengage the cruise control. The switch assemblies are mounted on the brake pedal bracket. (Scheme 8) To disengage the system electrically the driver presses the brake pedal. The speed of the vehicle at brake actuation will be stored in the memory of the cruise module.

Scheme 8

Camshaft Position Sensor

A 3-wire Camshaft Position (CMP) sensor is located at top of timing cover, behind water pump. (Scheme 9) The CMP sensor signal is a digital ON/OFF pulse, output once per revolution of the camshaft. The CMP sensor does not directly affect the operation of the ignition system. The CMP sensor information is used by the Powertrain Control Module (PCM) to determine the position of the valve train relative to the crankshaft position. By monitoring the CMP and Crankshaft Position (CKP) signals the PCM can accurately time the operation of the fuel injectors. The PCM supplies the sensor with a 12-volt reference, low reference, and signal circuit.

During cranking, the Ignition Control Module (ICM) monitors the 7X CKP sensor signal. Once the engine starts the ICM determines spark synchronization, by the CMP sensor pulses. The PCM constantly monitors the number of pulses on the CMP signal circuit and compares the number of CMP pulses to the number of 24X reference pulses and the number of 3X reference pulses being received.

Scheme 9

Crankshaft (3X/7X & 24X) Sensor

The Crankshaft Position (CKP) sensor "A" contains a Hall Effect switch located on the front side of the motor above and forward of the oil filter. (Scheme 10)and (Scheme 11). A Hall Effect switch is a solid state switching device that produces a digital ON/OFF pulse when a rotating element passes between the sensor tip and a magnet. This rotating element is called an interrupter ring or blade. In this case the interrupter ring has 24 evenly spaced blades and windows and is part of the crankshaft damper assembly. This sensor provides the Powertrain Control Module (PCM) with 24X signals, or 24 identical pulses per crankshaft revolution. The 24X signal is used for enhanced smoothness and idle stability at a lower calibrated RPM. The PCM supplies the sensor with a 12-volt reference, low reference, and signal circuit.

The CKP sensor "B" is a variable reluctance sensor located behind the crankshaft pulley. (Scheme 12) The magnetic field of the sensor is altered by a crankshaft mounted reluctor wheel that has seven machined slots, six of which are equally spaced 60 degrees apart. The seventh slot is spaced 10 degrees after one of the 60 degree slots. This sensor provides the Ignition Control Module (ICM) with 7X signals, or seven pulses for each revolution of the crankshaft. The pulse from the 10 degree slot is known as the sync pulse. Both of the sensor circuits are connected to the ICM. A signal converter within the ICM produces digital 3X output pulse to the PCM, the 3X reference is known as the low resolution engine speed signal.

Scheme 10

Scheme 11

Scheme 12

Engine Coolant Temperature Sensor

The Engine Coolant Temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. ECT sensor is located on the top left of the engine. (Scheme 13) The Powertrain Control Module (PCM) supplies 5 volts to the signal circuit and a ground for the ECT low reference circuit. When the engine coolant temperature is low, the sensor resistance is high. When the engine coolant temperature is high, the sensor resistance is low. The PCM uses this High Side Coolant Rationality test to determine if the ECT input is skewed high. The internal clock of the PCM will record the amount of time the ignition is OFF. At restart the PCM will compare the temperature difference between the ECT and the Intake Air Temperature (IAT). Before failing this test, the PCM will perform a calculation to determine the presence of an engine block heater.

Engine coolant temperature signal is used in the control of most systems the PCM controls (i.e., fuel delivery, ignition timing, idle speed, emission control devices). After a vehicle has been parked overnight, ECT and IAT sensor signals (resistance and temperature) should be close to same reading. An ECT sensor which is out of calibration will not set a Diagnostic Trouble Code (DTC) but will cause fuel delivery and driveability problems. Failure in ECT sensor circuit (open or short to ground) will cause monitored voltage to swing high or low and should set a related DTC.

Scheme 13

Fuel Level Sensor

The fuel level sensor is part of the fuel sender assembly consisting of the fuel level sensor, Fuel Tank Pressure (FTP) sensor, fuel pump and fuel strainer. (Scheme 14) The fuel level sensor consists of a float, a wire float arm, and a ceramic resistor card. The position of the float arm indicates the fuel level. The fuel level sensor contains a variable resistor which changes resistance in correspondence with the amount of fuel in the fuel tank. The PCM sends the fuel level information via the Class II circuit to the Instrument Panel (IP) cluster. This information is used for the IP fuel gauge and the low fuel warning indicator, if applicable. The PCM also monitors the fuel level input for various diagnostics.

Scheme 14

Fuel Tank Pressure Sensor

The Fuel Tank Pressure (FTP) sensor measures the difference between the air pressure, or vacuum, in the fuel tank and the outside air pressure. The sensor mounts at the top of the fuel sender assembly. (Scheme 14) The PCM supplies a 5-volt reference voltage and ground to the sensor. The sensor provides a signal voltage between 0.1-4.9 volts to the PCM. When the air pressure in the fuel tank is equal to the outside air pressure, such as when the fuel fill cap is removed, the output voltage of the sensor will measure 1.3-1.7 volts. When the air pressure in the tank is 4.5 in. H2O, the sensor output voltage should measure 0.3-0.7 volt. The sensor voltage increases to approximately 4.5 volts at 14 in. H2O. As the FTP increases, FTP sensor voltage decreases, high pressure = low voltage. As the FTP decreases, FTP voltage increases, low pressure or vacuum = high voltage.

Heated Oxygen Sensors

Heated Oxygen Sensors (HO2S) are used for fuel control and post catalyst monitoring. (Scheme 15) Each HO2S compares the oxygen content of the surrounding air with the oxygen content of the exhaust stream. When the vehicle is first started, the Powertrain Control Module (PCM) operates in an Open Loop mode, ignoring the HO2S signal voltage when calculating the air-to-fuel ratio. The PCM supplies the HO2S with a reference, or bias, voltage of about 450 mV. The HO2S generates a voltage within a range of 0-1000 mV that fluctuates above and below bias voltage once in Closed Loop. A high HO2S voltage output indicates a rich fuel mixture. A low HO2S voltage output indicates a lean mixture. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature, and to provide an accurate voltage signal. If the PCM detects that the HO2S voltage did not switch enough times during a calibrated time period, voltage average response time is too slow, loop status is open too long, voltage that stays above or below a specified value, voltage remains at or near the bias voltage amount, heater low control circuit current exceeds a calibrated amount, heater low control circuit current level is not within the calibrated range, heater takes too long to heat or that the calculated transition time ratio is incorrect, a DTC will set.

Each HO2S has the following circuits

1. HO2S high signal.
2. HO2S low reference.
3. HO2S heater ignition voltage.
4. HO2S heater low control.
5. Low reference loop.

Scheme 15

Ignition Control Module/Ignition Coils

Three dual tower ignition coils are mounted to the Ignition Control Module (ICM), and are serviced individually. (Scheme 12) The ICM performs the following functions

1. The ICM receives and processes the signals from the CKP sensor "B".
2. The ICM determines the correct direction of the crankshaft rotation, and cuts spark and fuel delivery to prevent damage from backfiring if reverse rotation is detected.
3. The ICM determines the correct coil triggering sequence, based on the 7X CKP signal. This coil sequencing occurs at start-up, and is remembered by the ICM. After the engine is running, the ICM will continue to trigger the coils in the correct sequence.
4. The ICM produces and inputs 3X reference signals to the Powertrain Control Module (PCM).
5. The ICM contains the coil driver circuits that command the coils to operate.

The PCM is responsible for maintaining proper spark and fuel injection timing for all driving conditions. Ignition Control (IC) spark timing is the method the PCM uses to control spark advance. To provide optimum driveability and emissions, the PCM monitors input signals from the following components in calculating ignition spark timing

1. The ICM.
2. The Throttle Position (TP) sensor.
3. The Engine Coolant Temperature (ECT) sensor.
4. The Mass Air Flow (MAF) sensor.
5. The Intake Air Temperature (IAT) sensor.
6. The Vehicle Speed Sensor (VSS).
7. The transmission gear position or range information sensors.
8. The engine Knock Sensors (KS).

The following describes the PCM to ICM circuits

1. Low Resolution Engine Speed, 3X Reference - PCM Input, From The ICM - The PCM uses this signal to calculate engine RPM and CKP. The PCM also uses the pulses on this circuit to initiate injector operation.
2. Low Reference - PCM Input - this is a ground circuit for the digital RPM counter inside the PCM, but the wire is connected to engine ground only through the IC module. This circuit creates a common ground plane and assures there is no ground drop between the PCM and IC module.
3. IC Timing Signal --PCM Output - ICM controls spark timing while the engine is cranking, this is called bypass mode. Once the PCM receives 3X reference signals from the ICM, the PCM applies 5 volts to the IC timing signal circuit allowing the ICM to switch spark advance to PCM control.
4. IC Timing Control - PCM Output - The IC output circuitry of the PCM sends out timing signals to the ICM on this circuit. When in the Bypass Mode, the ICM grounds these signals. When in the IC Mode, the signals are sent to the ICM to control spark timing.

Ignition/Crank Signal

The PCM monitors initial cranking (RPM) signal to determine when the engine is being started. This information is used for starting enrichment. If this signal is intermittent or not available, hard starting or a no-start condition will result.

Intake Air Temperature Sensor

The Intake Air Temperature (IAT) sensor is a variable resistor located left of the engine, on the air intake duct. (Scheme 1) The IAT sensor has a signal circuit and a low reference circuit. The IAT sensor measures the temperature of the air entering the engine. The Powertrain Control Module (PCM) supplies 5 volts to the IAT signal circuit and a ground for the IAT low reference circuit. When the IAT sensor is cold, the sensor resistance is high. When the air temperature increases, the sensor resistance decreases. With high sensor resistance, the PCM detects a high voltage on the IAT signal circuit. With lower sensor resistance, the PCM detects a lower voltage on the IAT signal circuit. If the PCM detects an excessively high or low IAT signal voltage, indicating a low or high temperature, a DTC sets.

Knock Sensor

The Knock Sensor (KS) system enables the Powertrain Control Module (PCM) to control the ignition timing advance for the best possible performance while protecting the engine from potentially damaging levels of detonation. The sensors in the KS system are used by the PCM as microphones to listen for abnormal engine noise that may indicate pre-ignition/detonation. Knock sensor is located in the lower rear of the engine block, below the exhaust manifold. (Scheme 10)

There are 2 types of KS currently being used

1. The broadband single wire sensor.
2. The flat response 2-wire sensor.

Both sensors use piezo-electric crystal technology to produce and send signals to the PCM. The amplitude and frequency of this signal will vary constantly depending on the vibration level within the engine. Flat response and broadband KS signals are processed differently by the PCM. The major differences are outlined below

1. All broadband sensors use a single wire circuit. Some types of controllers will output a bias voltage on the KS signal wire. The bias voltage creates a voltage drop the PCM monitors and uses to help diagnose KS faults. The KS noise signal rides along this bias voltage, and due to the constantly fluctuating frequency and amplitude of the signal, will always be outside the bias voltage parameters. Another way to use the KS signals is for the PCM to learn the average normal noise output from the KS. The PCM uses this noise channel, and KS signal that rides along the noise channel, in much the same way as the bias voltage type does. Both systems will constantly monitor the KS system for a signal that is not present or falls within the noise channel.
2. The flat response KS uses a 2-wire circuit. The KS signal rides within a noise channel which is learned and output by the PCM. This noise channel is based upon the normal noise input from the KS and is known as background noise. As engine speed and load change, the noise channel upper and lower parameters will change to accommodate the KS signal, keeping the signal within the channel. If there is knock, the signal will range outside the noise channel and the PCM will reduce spark advance until the knock is reduced. These sensors are monitored in much the same way as the broadband sensors, except that an abnormal signal will stay outside of the noise channel or will not be present.

KS diagnostics can be calibrated to detect faults with the KS diagnostic inside the PCM, the KS wiring, the sensor output, or constant knocking from an outside influence such as a loose or damaged component. In order to determine which cylinders are knocking, the PCM uses KS signal information when the cylinders are near Top Dead Center (TDC) of the firing stroke.

Manifold Absolute Pressure Sensor

The Manifold Absolute Pressure (MAP) sensor, located at the top rear of the engine, on the intake plenum near the ignition coil assembly, responds to pressure changes in the intake manifold. (Scheme 16) The pressure changes occur based on the engine load. The MAP sensor has the following circuits

1. 5-volt reference circuit.
2. Low reference circuit.
3. MAP sensor signal circuit.

The Powertrain Control Module (PCM) supplies 5 volts to the MAP sensor on the 5-volt reference circuit. The PCM also provides a ground on the low reference circuit. The MAP sensor provides a signal to the PCM on the MAP sensor signal circuit which is relative to the pressure changes in the manifold. The PCM should detect a low signal voltage at a low MAP, such as during an idle or a deceleration. The PCM should detect a high signal voltage at a high MAP, such as the ignition in RUN position, engine OFF, or at a Wide Open Throttle (WOT). The MAP sensor is also used to determine the Barometric Pressure (BARO). This occurs when the ignition switch is turned to RUN position, engine OFF. The BARO reading may also be updated whenever the engine is operated at WOT. The PCM monitors the MAP sensor signal for voltage outside of the normal range.

Scheme 16

The Mass Airflow (MAF) sensor, located in left front of the engine, in the air cleaner duct, is an air flow meter that measures the amount of air entering the engine. (Scheme 1) The Powertrain Control Module (PCM) uses the MAF sensor frequency signal to provide the correct fuel delivery for a wide range of engine speeds and loads. A small quantity of air entering the engine indicates a deceleration or idle. A large quantity of air entering the engine indicates an acceleration or high load condition. The MAF sensor has the following

1. An ignition 1 voltage circuit.
2. A ground circuit.
3. A signal circuit.

The PCM applies a voltage to the sensor on the signal circuit. The sensor uses the voltage to produce a frequency based on inlet air flow through the sensor bore. The frequency varies within a range of around 2000 Hertz at idle to about 10,000 Hertz at maximum engine load. DTC sets if the PCM detects a frequency signal higher or lower than the possible range of a properly operating sensor.

RPM Reference Signal

The RPM is monitored by PCM through tach/pulse signals produced by either the ignition control module or crankshaft position sensor (Hall Effect signal, PM generator signal on DIS and IDI). These signals are used by PCM for determining control of timing, fuel delivery, EGR function and idle speed.

Throttle Position Sensor

The Throttle Position (TP) sensor, located on top of the engine, on the lower front of the throttle body, is used by the Powertrain Control Module (PCM) to determine the throttle plate angle for various engine management systems. (Scheme 17) The TP sensor is a potentiometer type sensor with 3 circuits

1. A 5-volt reference circuit.
2. A low reference circuit.
3. A signal circuit.

The PCM provides the TP sensor with a 5-volt reference circuit and a low reference circuit. Rotation of the TP sensor rotor from the closed throttle position to the Wide Open Throttle (WOT) position provides the PCM with a signal voltage from less than one volt to more than 4 volts through the TP sensor signal circuit. If the PCM detects an signal voltage out of predicted range a DTC will set.

Scheme 17

Output Shaft Speed Sensor

The Output Shaft Speed (OSS) Sensor is a magnetic inductive pickup that relays information relative to vehicle speed to the Powertrain Control Module (PCM). Vehicle speed information is used by the PCM to control shift timing, line pressure, and TCC apply and release. The OSS sensor mounts in the case at the speed sensor rotor which is pressed onto the differential. (Scheme 5)or (Scheme 6). An air gap of 0.011-0.062" (0.27-1.57 mm) is maintained between the sensor and the teeth on the speed sensor rotor. The sensor consists of a permanent magnet surrounded by a coil of wire. As the differential rotates, an AC signal is induced in the OSS sensor. Higher vehicle speeds induce a higher frequency and voltage measurement at the sensor. Sensor resistance should be 1500-1650 ohms when measured at 68°F (20°C). Output voltage will vary with speed from a minimum of 0.5 volts AC at 25 RPM to 200 volts AC at 1728 RPM.

Input Shaft Speed Sensor

The Input Shaft Speed (ISS) sensor is a magnetic inductive pickup that relays turbine shaft speed information to the Powertrain Control Module (PCM). The PCM uses ISS sensor information in order to control line pressure, transmission shift patterns and TCC apply and release. This information is also used in order to calculate the appropriate operating gear ratios and TCC slippage. The AT ISS sensor mounts in the case cover, next to the automatic transmission input shaft speed sensor reluctor wheel assembly. (Scheme 5)or (Scheme 6). An air gap of 0.0032-0.0834" (0.08-2.12 mm) occurs between the sensor and the teeth on the speed sensor reluctor wheel as the drive sprocket rotates. The speed sensor reluctor wheel is secured to and turns with the drive sprocket by the tangs on the drive sprocket forward thrust washer.

The sensor consists of a permanent magnet surrounded by a coil of wire. As the turbine shaft rotates the speed sensor reluctor wheel and the drive sprocket, an AC signal is produced by the AT ISS sensor. This AC signal consists of a voltage and frequency that changes based on vehicle speed. The PCM uses the frequency portion of this signal to determine input shaft speed. Higher input shaft speeds induce a higher frequency and a higher voltage measurement at the sensor. The voltage portion of the signal is used in diagnostic procedures. Sensor resistance should measure between 820-1020 ohms at 68°F (20°C). Output voltage will vary with the vehicle speed from a minimum of 0.5 volts AC at 300 RPM to 200 volts at 6000 RPM.

OUTPUT SIGNALS

Note. Vehicles are equipped with different combinations of PCM-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 Compressor Clutch

See A/C COMPRESSOR CLUTCH under MISCELLANEOUS CONTROLS.

Air Injection System

See SECONDARY AIR INJECTION SYSTEM (GRAND PRIX) under EMISSION SYSTEMS & SUB-SYSTEMS.

Canister Purge Control Solenoid

See EVAP CANISTER PURGE VALVE under EVAPORATIVE EMISSION CONTROL under EMISSION SYSTEMS & SUB-SYSTEMS.

Cooling Fan Relay

See ELECTRIC COOLING FAN under MISCELLANEOUS CONTROLS.

Electronic Ignition

See IGNITION SYSTEMS .

Fuel Injectors

See FUEL INJECTORS under FUEL CONTROL under FUEL SYSTEMS.

Fuel Pump

See FUEL PUMP under FUEL DELIVERY under FUEL SYSTEMS.

Fuel Pump Relay

See FUEL PUMP RELAY under FUEL DELIVERY under FUEL SYSTEMS.

Coolant TEMP Warning Indicator

See COOLANT TEMP WARNING INDICATOR under MISCELLANEOUS CONTROLS.

Idle Air Control Valve

See IDLE AIR CONTROL VALVE IDLE SPEED under FUEL SYSTEMS.

Linear EGR Valve

See LINEAR EGR SYSTEM under EXHAUST GAS RECIRCULATION under EMISSION SYSTEMS & SUB-SYSTEMS.

Malfunction Indicator Light

See MALFUNCTION INDICATOR LIGHT under SELF-DIAGNOSTIC SYSTEMS.

Self-Diagnostics

See SELF-DIAGNOSTIC SYSTEMS .

Serial Data

See SERIAL DATA under SELF-DIAGNOSTIC SYSTEMS.

Shift Solenoid Valves

See 1-2 AND 2-3 SHIFT SOLENOID VALVES under TRANSMISSION under MISCELLANEOUS CONTROLS.

Torque Converter Clutch

See TORQUE CONVERTER CLUTCH PULSE WIDTH MODULATION SOLENOID VALVE under TRANSMISSION under MISCELLANEOUS CONTROLS.

FUEL DELIVERY

The fuel tank stores the fuel supply. An electric fuel pump, located in the fuel tank with the fuel sender assembly, pumps fuel through an in-line fuel filter to the fuel rail assembly. The pump provides fuel at a pressure more than is needed by the injectors. The fuel pressure regulator, part of the fuel rail assembly, keeps fuel available to the injectors at a regulated pressure. A separate pipe returns unused fuel to the fuel tank.

Fuel Tank

The fuel tank stores the fuel supply. The fuel tank is located in the rear of the vehicle. The fuel tank is held in place by 2 metal straps that attach to the frame. The fuel tank is molded from high density polyethylene.

The fuel level sensor consists of a float, a wire float arm, and a ceramic resistor card. (Scheme 14) The position of the float arm indicates the fuel level. The fuel level sensor contains a variable resistor which changes resistance in correspondence with the amount of fuel in the fuel tank. The PCM sends the fuel level information via the Class II circuit to the Instrument Panel (IP) cluster. This information is used for the IP fuel gauge and the low fuel warning indicator, if applicable. The PCM also monitors the fuel level input for various diagnostics.

The fuel pump is mounted in the fuel sender assembly reservoir. The fuel pump is an electric high pressure pump. Fuel is pumped to the fuel rail at a specified flow and pressure. Excess fuel from the fuel rail assembly returns to the fuel tank through the fuel return pipe. The fuel pump delivers a constant flow of fuel to the engine even during low fuel conditions and aggressive vehicle maneuvers. The PCM controls the electric fuel pump operation through a fuel pump relay. The fuel pump flex pipe acts to dampen the fuel pulses and noise generated by the fuel pump.

Fuel Pressure Regulator

The fuel pressure regulator is a vacuum operated diaphragm relief valve mounted at one end of the fuel rail. (Scheme 18) The diaphragm has fuel pressure on one side and regulator spring pressure and intake manifold vacuum on the other side. The fuel pressure regulator maintains a constant pressure differential across the fuel injectors at all times. The pressure regulator compensates for engine load by increasing fuel pressure as the engine vacuum drops.

Scheme 18

When the ignition switch is turned to RUN position, PCM will turn on electric fuel pump by energizing the fuel pump relay, located in the underhood junction block. PCM will keep relay energized if engine is running or cranking (PCM is receiving reference pulses from the ignition control module). If no reference pulses exist, PCM turns pump off within 2 seconds after key on.

Fuel Rail Assembly

The fuel rail assembly attaches to the engine intake manifold. (Scheme 18) The fuel rail assembly performs the following functions

1. Positions the injectors in the intake manifold.
2. Distributes fuel evenly to the injectors.
3. Integrates the fuel pressure regulator with the fuel metering system.

FUEL CONTROL

The Powertrain Control Module (PCM) monitors voltages from several sensors in order to determine how much fuel to give the engine. The fuel is delivered under one of several conditions called modes. The PCM controls all modes.

Starting Mode

With the ignition switch in the RUN position, before engaging the starter, the PCM energizes the fuel pump relay for 2 seconds allowing the fuel pump to build up pressure. The PCM first tests speed density, then switches to the Mass Air Flow (MAF) sensor. The PCM also uses the Engine Coolant Temperature (ECT), the Throttle Position (TP), and the Manifold Absolute Pressure (MAP) sensors to determine the proper air/fuel ratio for starting. The PCM controls the amount of fuel delivered in the starting mode by changing the pulse width of the injectors. This is done by pulsing the injectors for very short times.

Clear Flood Mode

If the engine floods, clear the engine by pressing the accelerator pedal down to the floor and then crank the engine. When the Throttle Position (TP) sensor is at wide open throttle, the PCM reduces the injector pulse width in order to increase the air to fuel ratio. The PCM holds this injector rate as long as the throttle stays wide open and the engine speed is below a predetermined RPM. If the throttle is not held wide open, the PCM returns to the starting mode.

Run Mode

The run mode has 2 conditions called Open Loop and Closed Loop. When the engine is first started and the engine speed is above a predetermined RPM, the system begins Open Loop operation. The PCM ignores the signal from the Heated Oxygen Sensor (HO2S) and calculates the air/fuel ratio based on inputs from the ECT, MAF, MAP, and TP sensors. The system stays in Open Loop until meeting the following conditions

1. Both HO2S have varying voltage output, showing that they are hot enough to operate properly. This depends upon the engine temperature.
2. The ECT sensor is above a specified temperature.
3. A specific amount of time has elapsed after starting the engine.

Specific values for the above conditions exist for each different engine, and are stored in the Electrically Erasable Programmable Read-Only Memory (EEPROM). The system begins Closed Loop operation after reaching these values. In Closed Loop, the PCM calculates the air/fuel ratio, injector ON time, based upon the signal from various sensors, but mainly from the HO2S. This allows the air/fuel ratio to stay very close to 14.7:1.

Acceleration Mode

When the driver pushes on the accelerator pedal, air flow into the cylinders increases rapidly, while fuel flow tends to lag behind. To prevent possible hesitation, the PCM increases the pulse width to the injectors to provide extra fuel during acceleration. The PCM determines the amount of fuel required based upon the TP, the coolant temperature, the MAP, the MAF, and the engine speed.

Deceleration Mode

When the driver releases the accelerator pedal, air flow into the engine is reduced. The PCM reads the corresponding changes in TP, MAP, and MAF. The PCM shuts OFF fuel completely if the deceleration is very rapid, or for long periods, such as long, closed-throttle coast-down. The fuel shuts OFF in order to protect the catalytic converters.

Battery Voltage Correction Mode

When the battery voltage is low, the PCM compensates for the weak spark delivered by the ignition system in the following ways

1. Increasing the amount of fuel delivered.
2. Increasing the idle RPM.
3. Increasing the ignition dwell time.

Fuel Cut-Off Mode

The PCM cuts OFF fuel from the fuel injectors when the following conditions are met in order to protect the powertrain from damage and improve driveability

1. The ignition is OFF. This prevents engine run-on.
2. The ignition is ON but there is no ignition reference signal. This prevents flooding or backfiring.
3. The engine speed is too high, above Redline.
4. The vehicle speed is too high, above rated tire speed.
5. During an extended, high speed, closed throttle coast down. This reduces emissions and increases engine braking.
6. During extended deceleration, in order to protect the catalytic converters.

Short Term Fuel Trim

The short term fuel trim values change rapidly in response to the HO2S signal voltages. These changes "fine tune" the engine fueling. The ideal fuel trim values are around zero percent. A positive fuel trim value indicates that the PCM is adding fuel to compensate for a lean condition. A negative fuel trim value indicates that the PCM is reducing the amount of fuel to compensate for a rich condition. When the PCM determines that the short term fuel trim is out of the operating range, the following DTCs will set

1. DTC P0171: FUEL TRIM SYSTEM LEAN
2. DTC P0172: FUEL TRIM SYSTEM RICH

Long Term Fuel Trim

The long term fuel trim is a matrix of cells arranged by RPM and Manifold Absolute Pressure (MAP). As the engine operating conditions change, the PCM will switch from cell to cell to determine what long term fuel trim factor to use in the base pulse width equation.

While in any given cell, the PCM also monitors the short term fuel trim. If the short term fuel trim is far enough from zero percent, the PCM will change the long term fuel trim value. Once the long term fuel trim value is changed, the long term fuel trim should force the short term fuel trim back toward zero percent. If the mixture is still not correct, the short term fuel trim will continue to have a large deviation from the ideal zero percent. In this case, the long term fuel trim value will continue to change until the short term fuel trim becomes balanced. Both the short term fuel trim and long term fuel trim have limits which vary by calibration. If the mixture is off enough so that long term fuel trim reaches the limit of long term fuel trim control and still cannot correct the condition, the short term fuel trim would also go to the short term fuel trim limit of control in the same direction. If the mixture is still not corrected by both short term fuel trim and long term fuel trim at short term fuel trim and long term fuel trim extreme values, a fuel trim Diagnostic Trouble Code (DTC) will likely result. When the PCM determines that the long term fuel trim is out of the operating range, the following DTCs will set

1. DTC P0171: FUEL TRIM SYSTEM LEAN
2. DTC P0172: FUEL TRIM SYSTEM RICH

Under the conditions of power enrichment, the PCM sets the short term fuel trim to zero percent until power enrichment is no longer in effect. This is done so the Closed Loop factor and the long term fuel trim will not try to correct for the power enrichment condition.

The Multec 2 fuel injector assembly is a solenoid operated device, controlled by the PCM, that meters pressurized fuel to a single engine cylinder. The PCM energizes the high-impedance (12.0 ohms) injector solenoid to open a normally closed ball valve. This allows fuel to flow into the top of the injector, past the ball valve, and through a director plate at the injector outlet. The director plate has four machined holes that control the fuel flow, generating a spray of finely atomized fuel at the injector tip. Fuel from the injector tip is directed at the intake valve, causing it to become further atomized and vaporized before entering the combustion chamber. An injector stuck partly open can cause a loss of pressure after engine shutdown. Consequently, long cranking times would be noticed on some engines.

IDLE SPEED

PCM controls engine idle speed based upon engine operating conditions. The PCM senses engine operating conditions and determines the best idle speed.

The engine idle speed is controlled by the Idle Air Control (IAC) valve. The IAC valve is on the throttle body. (Scheme 17) The IAC valve pintle moves in and out of an idle air passage bore to control air flow around the throttle plate. The IAC valve consists of a movable pintle, driven by a gear attached to an electric motor called a stepper motor. The stepper motor is capable of highly accurate rotation, or of movement, called steps. The stepper motor has 2 separate windings that are called coils. Each coil is supplied current by two circuits from the Powertrain Control Module (PCM). When the PCM changes polarity of a coil, the stepper motor moves one step. The PCM uses a predetermined number of counts to determine the IAC pintle position. Observe IAC counts with a scan tool. The IAC counts will increment up or down as the PCM attempts to change the IAC valve pintle position. An IAC Reset will occur when the ignition key is turned OFF. First, the PCM will seat the IAC pintle in the idle air passage bore. Second, the PCM will retract the pintle a predetermined number of counts to allow for efficient engine start-up. If the engine idle speed is out of range for a calibrated period of time, an idle speed Diagnostic Trouble Code (DTC) sets.

The Electronic Ignition (EI) system is responsible for producing and controlling a high energy secondary spark. This spark is used to ignite the compressed air/fuel mixture at precisely the correct time. This provides optimal performance, fuel economy, and control of exhaust emissions. This ignition system uses an individual coil for each cylinder. The ignition coils and Ignition Control Module (ICM) are contained within two assemblies, one for each cylinder bank. The assemblies are mounted in the center of each camshaft cover, with short boots connecting the coils to the spark plugs. The driver modules within each ICM are commanded ON/OFF by the Powertrain Control Module (PCM). The PCM primarily uses engine speed and position information from the Crankshaft Position (CKP) and Camshaft Position (CMP) sensors to control the sequence, dwell, and timing of the spark. The EI system consists of the following components

1. CMP sensor.
2. Camshaft reluctor wheel.
3. CKP sensor.
4. Crankshaft reluctor wheel.
5. Ignition Coil/ICM assembly.
6. PCM.

Modes of Operation

Both CKP sensors provide identical pulses, although one signal is shifted several degrees of crankshaft rotation to the other. This amount depends on whether the sensors are separate or integrated with each other. The two CKP signals allow the PCM to perform an angle-based decode operation. This is considered a self-clocked system, where one sensor acts as a clock and the other is a data signal. The advantage of angle-based decoding is the increased accuracy and consistency of signals, even during engine acceleration and deceleration. If one sensor is not operating correctly, the PCM uses a time-based decode operation. This mode will read the pulse width of the remaining signal, and thereby provide a means of back-up with the minimum of performance loss. Diagnostic trouble codes are available to accurately diagnose the ignition system with a scan tool.

EMISSION SYSTEMS & SUB-SYSTEMS

Note. To determine emission systems usage, see appropriate EMISSION APPLICATIONS article.

Three-Way Catalytic Converter

The catalytic converter is an emission control device added to the engine exhaust system to reduce Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides Of Nitrogen (NOx) pollutants from the exhaust gas.

The catalytic converter is comprised of a ceramic monolith substrate, supported in insulation and housed within a sheet metal shell. The substrate may be wash coated with 3 noble metals, Platium (Pt), Palladium (Pd) and Rhodium (Rh). The catalyst in the converter is not serviceable.

Resonator

Some exhaust systems are equipped with a resonator. The resonator, located either before or after the muffler, allows the use of mufflers with less back pressure. Resonators are used when vehicle characteristics require specific exhaust tuning.

EXHAUST GAS RECIRCULATION

The Exhaust Gas Recirculation (EGR) system is used to reduce the amount of Nitrogen Oxide (NOx) emission levels caused by combustion temperatures exceeding 1500°F (816°C). It does this by introducing small amounts of exhaust gas back into the combustion chamber using a EGR valve mounted on the top rear of the engine. (Scheme 17) The exhaust gas absorbs a portion of the thermal energy produced by the combustion process and thus decreases combustion temperature. The EGR system will only operate under specific temperature, barometric pressure and engine load conditions in order to prevent driveability concerns and to increase engine performance.

Linear EGR System

The linear EGR valve consists of the following

1. The EGR valve position sensor.
2. The EGR valve position sensor cap.
3. The bobbin and coil assembly.
4. The valve pintle.
5. The primary pole piece.
6. The armature sleeve.
7. The armature and base assembly.
8. The exhaust gas inlet port.
9. The exhaust gas outlet port.

The linear EGR valve is controlled by a high side driver within the PCM. The high side driver provides 12 volts that is Pulse Width Modulated (PWM) by a duty cycle via the high control circuit of the EGR valve. The ground path is provided by the low control circuit of the EGR valve. The PCM calculates the amount of EGR needed based on the following inputs

1. The Engine Coolant Temperature (ECT) sensor.
2. The Intake Air Temperature (IAT) sensor.
3. The Barometric Pressure (BARO).
4. The Manifold Absolute Pressure (MAP) sensor.
5. The Throttle Position (TP) sensor.
6. The Mass Air Flow (MAF) sensor.

The PCM tests the EGR flow during deceleration by momentarily commanding the EGR valve to open while monitoring the signal of the MAP sensor. When the EGR valve is opened, the PCM will expect to see a predetermined increase in MAP. If the expected increase in MAP is not detected, the PCM records the amount of MAP difference that was detected and adjusts a calibrated fail counter towards a calibrated fail threshold level. When the fail counter exceeds the fail threshold level, the PCM will set a DTC.

Normally, the PCM will only allow one EGR Flow Test Count during an ignition cycle. To aid in verifying a repair, the PCM allows twelve EGR Flow Test Counts during the first ignition cycle following a code clear or a battery disconnect. Between 9 and 12 EGR Flow Test Counts should be sufficient for the PCM to determine adequate EGR flow and pass the EGR flow test. If the PCM detects an EGR flow error, a DTC will set.

The PCM monitors the position of the EGR valve pintle via the EGR position sensor. If the PCM detects a calibrated variance between the Desired EGR Position parameter and the EGR Position Sensor parameter, actual position, for a calibrated amount of time a DTC will set.

The PCM also monitors the EGR solenoid high control circuit EGR solenoid low control circuit for electrical faults. If an EGR control circuit fault is detected for a calibrated amount of time a DTC will set.

EVAPORATIVE EMISSION CONTROL

The Evaporative Emission (EVAP) control system limits the fuel vapors from escaping into the atmosphere. The EVAP transfers the fuel vapors from the sealed fuel tank to an activated carbon, or charcoal storage device the EVAP canister. The EVAP canister stores the vapors until the engine is able to use the extra fuel vapor. When the engine is able to use the extra fuel vapor, the intake air flow purges the fuel vapor from the carbon element and then the normal combustion process consumes the fuel vapor. The system is required in order to detect the evaporative fuel system leaks as small as 0.04" (1.0 mm) between the fuel filler cap and the EVAP canister purge valve. The system can test the evaporative system integrity by applying a vacuum signal, ported or manifold, to the fuel tank in order to create a small vacuum.

The Powertrain Control Module (PCM) then monitors the ability of the system to maintain the vacuum. If the vacuum remains for a specified period of time, then there are no evaporative leaks, and a PASS is reported by the PCM. If there is a leak, the system either will not achieve a vacuum, or a vacuum cannot be maintained. Usually a fault can only be detected after a cold start with a trip of sufficient length and driving conditions to run the needed tests. The enhanced evaporative system diagnostic conducts sub-tests to detect the fault conditions. If the diagnostic fails a sub-test, the PCM stores a Diagnostic Trouble Code (DTC) to indicate the type of fault detected.

EVAP Canister

The canister is filled with carbon pellets used to absorb and store fuel vapors. (Scheme 19) Fuel vapor is stored in the canister until the PCM determines that the vapor can be consumed in the normal combustion process.

Scheme 19

EVAP Canister Purge Valve

The EVAP canister purge valve, located on the lower front of the engine, controls the flow of vapors from the EVAP system to the intake manifold. (Scheme 20) This normally closed solenoid is Pulse Width Modulated (PWM) by the PCM to precisely control the flow of fuel vapor to the engine. The valve will also be opened during some portions of the EVAP testing, allowing engine vacuum to enter the EVAP system.

EVAP Vent Solenoid Valve

The EVAP vent valve, located under the vehicle, controls fresh airflow into the EVAP canister. (Scheme 21)or (Scheme 22). The valve is normally open. The PCM and the solenoid closed during some EVAP tests, allowing the system to be tested for leaks.

The Fuel Tank Pressure (FTP) sensor mounted on top of the fuel sender assembly, measures the difference between the pressure or vacuum in the fuel tank and outside air pressure. (Scheme 14) The PCM provides a 5-volt reference and a ground to the FTP sensor. The FTP sensor provides a signal voltage back to the control module that can vary between 0.1-4.9 volts. As FTP increases, FTP sensor voltage decreases, high pressure - low voltage. As FTP decreases, FTP voltage increases, low pressure or vacuum - high voltage.

EVAP Service Port

The EVAP service port is located in the EVAP purge pipe between the EVAP purge solenoid and the EVAP canister. The service port is identified by a Green colored cap.

Scheme 20

Scheme 21

Scheme 22

POSITIVE CRANKCASE VENTILATION

The Positive Crankcase Ventilation (PCV) system is used to consume crankcase vapors in the combustion process instead of venting them to atmosphere. Fresh air from the throttle body is supplied to the crankcase, mixed with blow by gases and then passed through a PCV valve into the intake manifold. (Scheme 23)

The primary control is through the PCV valve which meters the flow at a rate depending on inlet vacuum. To maintain idle quality, the PCV valve restricts the flow when inlet vacuum is high. If abnormal operating conditions arise, the system is designed to allow excessive amounts of blow by gases to back flow through the crankcase vent into the throttle body to be consumed by normal combustion.

Scheme 23

SECONDARY AIR INJECTION SYSTEM (GRAND PRIX)

The secondary Air Injection (AIR) system helps reduce exhaust emissions (if equipped). The system forces fresh filtered air into the exhaust stream in order to accelerate the catalyst operation. The system includes the following components

AIR Pump (Electric)

The AIR pump supplies filtered air through the secondary air injection system into the exhaust stream. The control module provides ground for the pump relay, then the battery voltage is applied to the pump. The filter is the only serviceable part of the pump.

AIR Shut Off Valve

The AIR shut off valve is vacuum operated. When the secondary air injection system is enabled, vacuum is applied to the valve. The vacuum opens the valve, and allows air from the AIR Pump to flow to the check valves.

AIR Vacuum Control Solenoid

The AIR vacuum control solenoid controls the AIR Shut Off Valve. When the secondary air injection system is enabled, the control module provides a ground to the solenoid. Enabling the solenoid allows the engine vacuum to be applied to the AIR shut off valve.

Check Valves

The check valves prevent back flow of the exhaust gases into the secondary air injection system. An inoperative AIR pump that had shown indications of exhaust gases in the outlet port would indicate a check valve failure.

Plumbing

The plumbing carries the air from the pump to the exhaust stream. The plumbing includes the hoses, the pipes, and the clamps. You can test the plumbing for leaks using a soapy water solution. With the AIR pump running, the bubbles with form if a leak exists.

DESCRIPTION

The PCM is programmed with test routines that test the operation of the various systems the PCM controls. Some tests monitor internal PCM functions. Many tests are run continuously. Other tests run only under specific conditions, referred to as Conditions for Running the DTC. When the vehicle is operating within the conditions for running a particular test, the PCM monitors certain parameters and determines if the values are within an expected range. The parameters and values considered outside the range of normal operation are listed as Conditions for Setting the DTC. When the Conditions for Setting the DTC occur, the PCM executes the Action Taken When the DTC Sets. Some DTCs alert the driver via the MIL or a message. Other DTCs do not trigger a driver warning, but are stored in memory. The PCM also saves data and input parameters when most DTCs are set.

The DTCs are categorized by type. The DTC type is determined by the MIL operation and the manner in which the fault data is stored when a particular DTC fails. In some cases there may be exceptions to this structure. Therefore, when diagnosing the system it is important to read the Action Taken When the DTC Sets and the Conditions for Clearing the DTC in the supporting text.

There are different types of DTCs and different actions taken when the DTCs set. This data is stored in the Freeze Frame and/or Failure Records. The 3 code types are defined as follows

1. Type "A" Emission related faults that illuminate MIL at first occurrence of a fail condition.
2. Type "B" Emission related faults that illuminate MIL if a fault occurs in 2 consecutive ignition cycles.
3. Type "C" Non-emission related faults that do not illuminate MIL. Driver Information Center (DIC) may display a message.

Emission related DTCs (type "A" or "B") cause MIL to illuminate and remain on until the malfunction is repaired. If MIL illuminates and remains on during vehicle operation, cause of malfunction must be determined using affected DTC located in SELF-DIAGNOSTICS - 3.1L CENTURY, GRAND PRIX & MALIBU article. PCM records and updates failure records/operating conditions to Freeze Frame. If a sensor fails, PCM will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional but impaired driveability is likely. Use a scan tool to clear codes and turn off MIL.

Non-emission related DTCs (type "C") do not illuminate the MIL, but may display a message on the Driver Information Center (DIC). The corresponding DTC, however, will be retained in PCM memory. PCM records operating conditions at time of failure into memory. If related fault does not reoccur within 40 warm-up cycles, related DTC will be erased from PCM memory. Intermittent failures may be caused by a sensor, connector or wiring related problem. See TROUBLE SHOOTING - NO CODES - 3.1L CENTURY, GRAND PRIX & MALIBU article.

DTC Status

When the scan tool displays a DTC, the status of the DTC is also displayed. The following DTC statuses are indicated only when they apply to the DTC that is set.

1. Fail This Ign. (Fail This Ignition) - Indicates that this DTC failed during the present ignition cycle.
2. Last Test Fail - Indicates that this DTC failed the last time the test ran. The last test may have run during a previous ignition cycle if an A or B type DTC is displayed. For type C DTCs, the last failure must have occurred during the current ignition cycle to appear as Last Test Fail.
3. MIL Request - Indicates that this DTC is currently requesting the MIL. This selection will report type "B" DTCs only when they have requested the MIL (failed twice).
4. Test Fail SCC (Test Failed Since Code Clear) - Indicates that this DTC that has reported a failure since the last time DTCs were cleared.
5. History - Indicates that the DTC is stored in the PCM History memory. Type "B" DTCs will not appear in History until they have requested the MIL (failed twice). History will be displayed for all type "A" DTCs and type "B" DTCs (which have requested the MIL) that have failed within the last 40 warm-up cycles. Type "C" DTCs that have failed within the last 40 warm-up cycles will also appear in History.
6. Not Run SCC (Not Run Since Code Clear) - DTCs will be listed in this category if the diagnostic has not run since DTCs were last cleared. This status is not included with the DTC display since the DTC can not be set if the diagnostic has not run. This information is displayed when DTC Info is requested using the scan tool.

The Malfunction Indicator Light (MIL) is located in the Instrument Panel Cluster (IPC). The MIL will display as either SERVICE ENGINE SOON or as an Engine icon. As a bulb and system check, the MIL will illuminate when ignition switch is turned to RUN position and engine is not running. When engine is started, MIL should go out. If MIL does not go out, a malfunction has been detected in the computerized engine control system or MIL circuit is faulty.

The MIL indicates that an emissions related fault has occurred and vehicle service is required. The following is a list of the modes of operation for the MIL

1. The MIL illuminates when the ignition switch is turned to RUN position, with the engine OFF. This is a bulb test to ensure the MIL is able to illuminate.
2. The MIL turns OFF after the engine is started if a diagnostic fault is not present.
3. The MIL remains illuminated after the engine is started if the Powertrain Control Module (PCM) detects a fault. A Diagnostic Trouble Code (DTC) is stored any time the PCM illuminates the MIL due to an emissions related fault. The MIL turns OFF after three consecutive ignition cycles in which a Test Passed has been reported for the diagnostic test that originally caused the MIL to illuminate.
4. The MIL flashes if the PCM detects a misfire condition which could damage the catalytic converter.
5. When the MIL is illuminated and the engine stalls, the MIL will remain illuminated as long as the ignition switch is in RUN position.
6. When the MIL is not illuminated and the engine stalls, the MIL will not illuminate until the ignition is cycled OFF and then ON.

To access codes, see SELF-DIAGNOSTICS - 3.1L CENTURY, GRAND PRIX & MALIBU article.

PCM is equipped with a serial data line. Serial data is a stream of electrical impulses which can be interpreted by special testers or other control modules. When the ignition is turned on, each module sends a state of health message to the other modules using the serial data line. Ensuring the modules are working properly. If a module stops communicating, other control modules expecting information from that module will set a DTC. Serial data and codes must be accessed using a scan tool connected to the Data Link Connector (DLC). Update intervals and information contained within the data stream vary with model application.

MISCELLANEOUS CONTROLS

Note. Although not considered true engine performance-related systems, some controlled devices may affect driveability if they malfunction.

On many models, PCM regulates operation of A/C compressor clutch through a PCM-controlled A/C compressor clutch relay. This allows the PCM to disengage the A/C compressor when compressor load on engine may cause driveability problems (i.e., during hot restart, idle, low speed steering maneuvers and wide open throttle operation), or if A/C refrigerant pressure drops or rises beyond normal operating levels.

Refrigerant pressure sensing may be accomplished by monitoring high and low pressure switches or a pressure sensor which will register either high or low pressure levels. Power steering load is monitored through a power steering pressure switch. Hot restart is monitored through the coolant temperature sensor. For component application and related wiring, see appropriate A/C COMPRESSOR CLUTCH CONTROLS article in AIR CONDITIONING & HEATING.

See A/C PRESSURE SENSOR under INPUT DEVICES.

1. The IPC illuminates the TEMP indicator (thermometer icon) when the Powertrain Control Module (PCM) determines that the coolant temperature is more than 262°F (128°C). The Instrument Panel Cluster (IPC) receives a discrete input from the PCM requesting illumination. The IPC performs the displays test at the start of each ignition cycle. The TEMP indicator will illuminate during this test.
2. The vehicle also has a temperature gauge that shows the engine coolant temperature. If the gauge pointer moves into the Red area, the engine is too hot. This reading indicates the same thing as the warning light. The IPC displays the engine coolant temperature as determined by the PCM. The IPC receives a class 2 message from the PCM indicating the engine coolant temperature. The engine coolant temperature gage defaults to "C" (cold) or below if the PCM detects a malfunction in the engine coolant temperature sensor circuit and if the IPC detects a loss of class 2 communications with the PCM.

Cooling Fan Control

The engine cooling fan system consists of 2 electrical cooling fans and three fan relays. The relays are arranged in a series/parallel configuration that allows the Powertrain Control Module (PCM) to operate both fans together at low or high speeds. The cooling fans and fan relays receive battery positive voltage from the underhood accessory wiring junction block. The ground path is provided at ground point G117.

Some models use more than one cooling fan. The second fan may function as an auxiliary cooling device when A/C is engaged or (on models using A/C high and low pressure switches) during periods of engine overheating, or high A/C refrigerant pressures.

During low speed operation, the PCM supplies the ground path for the low speed fan relay through the low speed cooling fan relay control circuit. This energizes the cool fan 1 relay coil, closes the relay contacts, and supplies battery positive voltage from the cool fan 1 fuse through the cooling fan motor supply voltage circuit to the left cooling fan. The ground path for the left cooling fan is through the cool fan relay and the right cooling fan. The result is a series circuit with both fans running at low speed.

During high speed operation the PCM supplies the ground path for the cool fan 1 relay through the low speed cooling fan relay control circuit. After a 3-second delay, the PCM supplies a ground path for the cool fan 2 relay and the cool fan relay through the high speed cooling fan relay control circuit. This energizes the cool fan relay coil, closes the relay contacts, and provides a ground path for the left cooling fan. At the same time the cool fan 2 relay coil is energized closing the relay contacts and provides battery positive voltage from the cool fan 2 fuse on the cooling fan motor supply voltage circuit to the right cooling fan. During high speed fan operation, both engine cooling fans have there own ground path. The result is a parallel circuit with both fans running at high speed.

The PCM commands Low Speed Fans on under the following conditions

1. Engine coolant temperature exceeds approximately 223°F (106°C).
2. When A/C is requested and the ambient temperature is more than 122°F (50°C).
3. A/C refrigerant pressure exceeds 190 psi (1310 kPa).
4. After the vehicle is shut off if the engine coolant temperature at key-off is more than 284°F (140°C) and system voltage is more than 12 volts. The fans will stay on for approximately 3 minutes.

The PCM commands High Speed Fans on when engine coolant temperature reaches 230°F (110°C), A/C refrigerant pressure exceeds 240 psi (1655 kPa) or when certain DTCs set.

Torque Converter Clutch Pulse Width Modulation Solenoid Valve

The Torque Converter Clutch Pulse Width Modulation (TCC PWM) solenoid valve is a normally closed (hydraulically), Pulse Width Modulation (PWM) solenoid which is used to control the apply and release of the converter clutch. (Scheme 5)or (Scheme 6). The PCM operates the solenoid with a negative duty cycle at a fixed frequency of 32 Hz to control the rate of TCC apply/release. The solenoid's ability to ramp the TCC apply and release pressures results in smoother TCC operation.

When the vehicle's operating conditions are appropriate to apply the TCC, the PCM immediately increases the duty cycle to approximately 22 percent. The PCM then ramps the duty cycle up to a maximum of 98 percent to achieve full TCC apply pressure. The rate at which the PCM increases the duty cycle controls the TCC apply. Similarly, the PCM also ramps down the TCC solenoid duty cycle in order to control TCC release.

Some operating conditions prevent or enable TCC apply under various conditions. Also, if the PCM receives a zero voltage signal from the TCC brake switch, signaling that the brake pedal has been depressed, the PCM immediately releases the TCC.

TCC duty cycle for Electronically Controlled Capacity Clutch (ECCC) equipped vehicles is 22 percent for minimum apply pressure and 43 percent for maximum apply pressure. The TCC PWM solenoid valve will typically be 40-60 percent at full apply. Your results may vary. The TCC PWM solenoid valve resistance should measure 10-12 ohms when measured at 68°F (20°C) and 13-15 ohms when measured at 190°F (88°C).

Electronic Transmission

The 4T65-E is a fully automatic front wheel drive electronically controlled transmission. The 4T65-E provides four forward ranges including overdrive. The PCM controls shift points by means of two shift solenoids. (Scheme 5)or (Scheme 6). A vane-type oil pump supplies the oil pressure. The PCM regulates oil pressure by means of a pressure control solenoid valve. All vehicles equipped with a 4T65-E transmission have an Electronically Controlled Capacity Clutch (ECCC) system. In the ECCC system, the pressure plate does not fully lock to the torque converter cover. It is instead, precisely controlled to maintain a small amount of slippage between the engine and the turbine, reducing driveline torsional disturbances.

1. Pressure Control Solenoid Valve The Pressure Control (PC) solenoid valve is a precision electronic pressure regulator that controls transmission line pressure based on current flow through its coil windings. As current flow is increased, the magnetic field which is produced by the coil moves the solenoid's plunger further away from the exhaust port. Opening the exhaust port decreases the output fluid pressure, which is regulated by the PC solenoid valve. This ultimately decreases line pressure. The PCM controls the PC solenoid valve based upon various inputs including throttle position, fluid temperature, MAP sensor, and gear state. The PCM controls the PC solenoid valve on a positive duty cycle at a fixed frequency of 585 Hz (cycles per second). Duty cycle is defined as the percentage of time when current flows through the solenoid coil during each cycle. A higher duty cycle provides a greater current flow through the solenoid. The high (positive) side of the PC solenoid valve electrical circuit at the PCM controls the PC solenoid valve operation. The PCM provides a ground path for the circuit, monitors average current, and continuously varies the PC solenoid valve duty cycle in order to maintain the correct average current flowing through the PC solenoid valve. See «PC SOLENOID VALVE DUTY CYCLE»(/buick/century/vi-1997-2005/remont/theory-operation/#engine-controls-theory-operation) table.

PC SOLENOID VALVE DUTY CYCLE

1-2 and 2-3 Shift Solenoid Valves

The shift solenoid valves are two identical, normally open, electronic exhaust valves that control upshifts and downshifts in all forward gear ranges. These shift solenoid valves work together in a combination of ON and OFF sequences in order to control the positions of the 1-2 and 2-3 shift valve trains. See SHIFT SOLENOID VALVES. The PCM monitors numerous inputs to determine the appropriate solenoid state combination and the transmission gear for the vehicle operating conditions. (Scheme 5)or (Scheme 6).

SHIFT SOLENOID VALVES

The PCM energizes the shift solenoids by providing a ground to the solenoid's electrical circuit. This sends a current through the coil winding of the solenoid, thereby creating a magnetic field. The magnetic field repels the plunger inside the solenoid. This seats the solenoid metering ball against the fluid inlet port. This action prevents the exhaust of fluid through the solenoid and provides an increase in fluid pressure at the end of the shift valves. This fluid pressure initiates an upshift by moving the shift valves. Refer to the oil flow diagrams for a complete description of the hydraulic control of the shift valves for each gear range.

Shift solenoid resistance should measure 19-24 ohms when measured at 68°F (20°C) and between 24-31 ohms when measured at 190°F (88°C). The shift solenoid valves should energize when the voltage is greater than 7.5 volts. The shift solenoid valves should de-energize when the voltage is less than one volt.

See also:
• COMPONENT LOCATIONS
• TROUBLE SHOOTING - NO CODES - 3.1L CENTURY, GRAND PRIX & MALIBU
• A/C COMPRESSOR CLUTCH
• SECONDARY AIR INJECTION SYSTEM (GRAND PRIX)
• EVAP CANISTER PURGE VALVE
• LINEAR EGR SYSTEM
• 1-2 AND 2-3 SHIFT SOLENOID VALVES
• TORQUE CONVERTER CLUTCH PULSE WIDTH MODULATION SOLENOID VALVE
• A/C PRESSURE SENSOR