Engine Controls - Theory & Operation Buick Century V

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.

2.2L 4-Cyl Component Locations. Scheme 1

3.1L V6 Component Locations. Scheme 2

TERMINOLOGY

Due to Federal government requirements, manufacturers may use names and acronyms for systems and components different than those used in previous years. The following table will help eliminate confusion when dealing with these components and systems. Only relevant components and systems whose names have changed from current General Motors Corp. terminology have been listed.

SAE TERMINOLOGY

Mass Airflow (MAF)

Sensor measures flow of air entering the engine in grams per second. This measurement of airflow is a reflection of engine load (throttle opening and air volume), similar to the relationship of engine load to MAP or vacuum sensor signal. Mass Airflow (MAF) signal should remain relatively constant at cruise, gradually changing with throttle angle and rapidly changing on sudden acceleration. The PCM uses MAF information to control fuel delivery. Sensor produces a frequency signal which cannot be easily measured in testing (32-150 Hertz). This varying signal is proportional to airflow.

Speed Density

On models equipped with MAP and MAT sensors, the speed density method is used to compute the airflow rate. Manifold pressure and temperature are used to calculate the airflow rate to the PCM. The MAP sensor responds to manifold vacuum changes due to engine load and speed changes.

The PCM sends a voltage signal to MAP sensor. Manifold pressure changes result in resistance changes in MAP sensor. By monitoring MAP sensor output voltage, PCM determines manifold pressure. If MAP sensor fails, PCM will supply a fixed MAP value and use the TP sensor to control fuel.

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 (ECM/PCM), input devices (sensors and switches) and output signals.

POWERTRAIN CONTROL MODULE (PCM)

Note. Most models use a Powertrain Control Module (PCM) instead of an Electronic Control Module (ECM). The only difference between an ECM and PCM is the PCM controls electronic transmission internals and cruise control system in addition to electronic engine controls. Unless specifically stated, references to ECM in text and diagnostic flow charts also apply to PCM-equipped models.

On most vehicles, PCM is located in passenger compartment. For location of PCM, see ECM/PCM LOCATION in articles or COMPONENT LOCATIONS in appropriate G - TEST W/ CODES and I - SYS/COMP TESTS articles in the ENGINE PERFORMANCE section listed below. The PCM contains the Arithmetic Logic Unit (ALU), Central Processing Unit (CPU), power supply and system memories.

1. «TESTS W/CODES - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-tests-wcodes-31l)
2. «TESTS W/CODES - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-tests-wcodes-22l)
3. «SYSTEM/COMPONENT TESTS - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l)
4. «SYSTEM/COMPONENT TESTS - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-22l)

The PCM has a "learning" ability which allows it to make minor corrections for fuel system variations. If battery power to PCM is interrupted, a vehicle performance change may be noticed. This will correct itself and normal performance will return if vehicle is allowed to "relearn" optimum control conditions. This is accomplished by driving vehicle at normal operating temperature, under part throttle, moderate acceleration and idle conditions.

Arithmetic Logic Unit (ALU)

This internal component of the PCM converts electrical signals, received by PCM from various engine sensors, into digital signals for use by the CPU.

Central Processing Unit (CPU)

Digital signals received by CPU are used to perform all mathematical computations and logic functions necessary to deliver proper air/fuel mixture. CPU also calculates spark timing and idle speed. The CPU commands operation of emission control, "closed loop" fuel control and diagnostic system.

Power Supply

Power for PCM reference output signals (5 volts) and control devices (12 volts) is received from the battery (through ignition circuit when ignition switch is in ON position). Keep alive memory power is received directly from the battery.

Memories

PCM uses 5 types of memories: Read Only Memory (ROM), Random Access Memory (RAM), Programmable Read Only Memory (PROM), fuel system Calibration Package (CAL-PAC) and Memory Calibration unit (MEM-CAL).

1. Read Only Memory (ROM)

ROM is programmed information that can only be read by ECM. The ROM program cannot be changed. If battery voltage is removed, ROM information will be retained.

1. Random Access Memory (RAM)

RAM is the scratch pad for the CPU. Data input, diagnostic codes and results of calculations are constantly updated and temporarily stored in RAM. If battery voltage is removed from PCM, all information stored in RAM is lost.

1. Programmable Read Only Memory (PROM)

PROM is factory programmed engine calibration data which "tailors" PCM for specific transmission, engine, emission, vehicle weight and rear axle ratio applications. The PROM can be removed from PCM. If battery voltage is removed, PROM information will be retained. An Electronically Erasable Programmable Read Only Memory (EEPROM) is used on some models. This is the same as a PROM except it can be electronically reprogrammed by the manufacturer using special equipment.

1. Calibration Package (CAL-PAC)

Some models use a PROM and a device called a CAL-PAC. The CAL-PAC provides fuel delivery back-up so engine will run in case of a PROM or PCM failure. Anytime PCM is replaced, PROM and CAL-PAC must both be installed into replacement PCM. If battery voltage is removed, CAL-PAC information will be retained.

1. Memory Calibration Unit (MEM-CAL)

Models may also use another type of PCM containing a Memory Calibration unit (MEM-CAL). This assembly contains functions of PROM and CAL-PAC and, on some models, the ESC control module. If power to PCM is removed, MEM-CAL information will be retained.

1. EEPROM

Some models may use an Electronically Erasable Programmable Read Only Memory (EEPROM). This is the same as a PROM except it can be electronically reprogrammed by the manufacturer using special equipment.

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.

INPUT DEVICES

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 appropriate wiring diagram in WIRING DIAGRAMS section. The available input signals include the following

A/C "On" Switch

The air conditioner "on" switch is mounted in instrument panel. This switch provides a simple "on" or "A/C request" signal which is monitored by the PCM. The PCM uses this signal to determine control of A/C clutch relay (if equipped) and to adjust idle speed when A/C compressor clutch is engaged. On some models, PCM may also activate radiator cooling fan when this signal is present. If this signal is not present on A/C-equipped vehicles, vehicle may idle rough when A/C compressor cycles. To check function of the A/C switch, perform functional check of switch. See appropriate I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section. Refer to the following

1. «SYSTEM/COMPONENT TESTS - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l)
2. «SYSTEM/COMPONENT TESTS - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-22l)

A/C Pressure Sensor

Some models are equipped with an air conditioner pressure sensor which is used to inform PCM of A/C system pressure levels. Low pressure signal will cause PCM to disengage the A/C compressor to prevent system damage. High pressure levels cause PCM to energize high speed fans while A/C compressor clutch is engaged. Extremely high pressure levels will cause PCM to disengage A/C compressor clutch to prevent system damage.

A/C Pressure Switches

A/C high and low pressure switches may be used in the PCM-monitored A/C request circuit. Switches are normally closed, completing the circuit between ignition and PCM. PCM will engage or disengage A/C clutch relay based upon status of this circuit. When system freon pressure increases beyond a certain point, high side switch will open, causing A/C request line voltage to drop. If system freon level decreases, causing freon pressure to drop below normal, low side pressure switch will open, once again causing A/C request line voltage to drop. Switches may be used as normal clutch cycling devices or as safety devices which prevent compressor damage in the event of excessively high or low freon pressure.

A/C Temperature Sensors

Air conditioner high side and low side temperature sensors inform PCM of A/C system temperature levels. Low temperature signal will cause A/C compressor to disengage. High temperature levels help PCM determine control of A/C compressor relative to cooling fans and idle speed.

Battery Voltage

Battery voltage is monitored by PCM. If battery voltage swings low, a weak spark or improper fuel control may result. To compensate for low battery voltage, PCM may increase idle speed, advance ignition timing, increase ignition dwell or enrich the air/fuel mixture. If voltage swings excessively high or low, PCM may set a charging system fault code and turn on Malfunction Indicator Light (MIL).

Brake Switch Feedback

Models equipped with cruise control systems may monitor the brake switch circuit to determine when to engage and disengage cruise control. On vehicles equipped with a Torque Converter Clutch (TCC), one circuit of brake switch is in series with the power supply for the TCC solenoid located in the transmission/transaxle.

Camshaft Position Sensor (3.1L VIN M)

On 3.1L (VIN M), sensor is a Hall Effect sensor, located on the front top of the engine. See component "G" in (Scheme 2). As the camshaft sprocket turns, a magnet in it activates a Hall Effect switch in camshaft sensor. This signal is generated whenever cylinder No. 1 is at TDC of its compression stroke. This signal is used by PCM, in conjunction with the 3X and 24X crankshaft position sensors signals, to trigger the fuel injectors in sequential firing order. If sensor should fail while engine is running, engine will continue to run using the last calculated camshaft sensor signal to maintain sequential fuel injection mode. Upon restart, engine will run with a one in 6 chance of being correct. Vehicle will run, but it may run rough.

Crankshaft Position Sensor 3X (3.1L VIN M)

The 3X crankshaft position signal is generated by a 2-wire, Permanent Magnet (PM) generator crankshaft position sensor which is mounted through the right (rear) side of engine block. See component "E" in (Scheme 2). Ignition module passes this signal on to the PCM. The 3X signal is used by the ignition control module to determine which ignition coil to fire. the 3X signal is used by the PCM to determine RPM, crankshaft position, injector timing and to compute ignition timing at engine speeds greater than 1200 RPM.

Crankshaft Position Sensor 24X (3.1L VIN M)

The 24X signal is generated by a 3-wire Hall Effect switch located in an aluminum mounting bracket and bolted onto the front left side of the engine timing chain cover. See component "J" (Scheme 2). The Hall Effect switch alternately grounds and opens a 12-volt signal circuit to PCM. An air gap separates the Hall Effect switch from a magnet. An interrupter ring containing 24 equally-spaced blades is mounted on the vibration damper and rotates with the crankshaft.

When the Hall Effect switch is shielded from the magnetic field generated by the magnet by one of the interrupter blades, the 12-volt signal circuit is not grounded by the Hall Effect switch. When the Hall Effect switch is exposed to the magnetic field, the 12-volt signal circuit is grounded by the Hall Effect switch. The constant grounding and opening of this circuit results in an ON-OFF signal which is interpreted by the PCM as an RPM (engine speed) signal. PCM uses the 24X signal to compute ignition timing at engine speeds less than 1200 RPM. At speeds greater than 1200 RPM, PCM uses the 3X crankshaft signal to compute ignition timing. Four 24X pulses to be seen between two 3X pulses. If the sequence of these pulses is not correct for 50 engine revolutions, the PCM then will set a DTC PO321, PO341 or PO342. The 24X sensor is used in conjunction with the 3X sensor and the Camshaft Position Sensor sequential firing of the injectors.

Note. For more information about the Camshaft Position Sensor. See the CAMSHAFT POSITION SENSOR (3.1L VIN M) heading above

Crankshaft Position Sensor

The Direct Ignition System (DIS) crankshaft position sensor protrudes through side of engine block to within .05" (1.3 mm) of an internally-mounted crankshaft reluctor ring. The reluctor ring is a special trigger wheel cast into the crankshaft. As crankshaft rotates, notches in reluctor ring change the magnetic field at the tip of the position sensor. This creates an induced AC voltage signal in the sensor windings, resulting in reference signals which are sent to PCM by ignition module. This allows PCM to compute crankshaft position and RPM and fire appropriate ignition coil at the proper time.

Vehicles equipped with HEI-EST distributor systems use RPM reference signal from ignition module in distributor for a crankshaft position signal. TDC intake and TDC exhaust are not differentiated; differentiation is not necessary on non-sequential fuel injected engines. Signal is used to trigger fuel injectors. For additional information, DIRECT IGNITION SYSTEM (DIS) under IGNITION SYSTEM.

Engine Coolant Temperature (ECT) Sensor

The ECT sensor is a thermistor (temperature sensitive resistor) located in an engine coolant passage. The PCM supplies and monitors a 5-volt signal to ECT sensor. This monitored 5-volt signal is then reduced by resistance of the CTS. When coolant temperatures are low, ECT sensor resistance is high, and a high monitored voltage signal is seen by the PCM. When coolant temperatures are high, ECT sensor resistance is low, and a low monitored voltage is seen by the PCM. When fully warmed, ECT sensor should reflect a temperature of at least 185°F (85°C).

Coolant temperature input is used in the control of fuel delivery, ignition timing, idle speed, cooling fan operation, emission control devices and converter clutch application. An ECT sensor which is out of calibration will not set a trouble code but will cause fuel delivery and driveability problems. A coolant sensor circuit problem (open or short to ground) will swing monitored voltage high or low and should set a related trouble code.

Fuel Pump Feedback

On some models, the fuel pump circuit between the relay and fuel pump is monitored by PCM. This enables PCM to determine when the fuel pump relay is energized and voltage is being delivered to fuel pump. Voltage monitored on this circuit is also used in calculations to determine changes in idle speed, air/fuel ratio and ignition dwell. A failure in this monitored circuit will result in the setting of a related trouble code in PCM memory.

Gear Switches

Gear switches are located inside automatic transmission. Switches may be normally open or closed and change status depending upon internal hydraulic pressures. High gear switch information is used by PCM in controlling emission components and engagement of Torque Converter Clutch (TCC).

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.

Knock Sensor

The knock sensor is a piezoelectric device which detects abnormal engine vibrations (spark knock) in the engine. This vibration results in the production of a very low AC signal which is sent from the knock sensor back to knock sensor module, if equipped (mounted on PCM), or to the EEPROM/PROM portion of PCM on models not equipped with a module. The PCM will then retard ignition timing until engine knock ceases. Some models use 2 knock sensors.

For additional information on knock sensor operation, see KNOCK SENSOR OPERATION under IGNITION TIMING SYSTEMS under IGNITION SYSTEM. A fault in the knock sensor circuit may set a related trouble code. When a related trouble code is not present and the knock sensor system is suspected as the cause of a driveability problem, perform a functional check of the knock sensor system. See appropriate I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section. Refer to the following

1. «SYSTEM/COMPONENT TESTS - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l)
2. «SYSTEM/COMPONENT TESTS - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-22l)

Manifold Absolute Pressure (MAP) Sensor

The MAP sensor measures changes in manifold pressure. Changes in manifold pressure result from engine load and speed changes. The MAP sensor converts these changes in manifold pressure into a voltage output signal to PCM (about 1.5 volts at idle to about 4.5 volts at WOT). The PCM can monitor these signals and adjust air/fuel ratio and ignition timing under various operating conditions.

If MAP sensor fails, the PCM will substitute a fixed MAP value and will use the TP sensor to control fuel delivery. A fault in the MAP circuit should set a related trouble code. If a related trouble code is not present and MAP sensor is suspected of causing a driveability problem, perform functional check of MAP sensor. See appropriate I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section. Refer to the following

1. «SYSTEM/COMPONENT TESTS - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l)
2. «SYSTEM/COMPONENT TESTS - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-22l)

Manifold Air Temperature (MAT) Sensor

The MAT sensor (may also be referred to as an intake air temperature sensor) is a thermistor (temperature sensitive resistor) mounted in the intake manifold. Low intake air temperature produces high internal sensor resistance, while high temperature causes low internal sensor resistance. The PCM supplies and monitors a 5-volt signal to sensor through a resistor in PCM. By monitoring this voltage, PCM determines manifold air temperature. After a vehicle has been parked overnight, MAT and ECT sensor signals (resistance and temperature) should be close to same reading. Failure in MAT sensor circuit (open or short to ground) will cause monitored voltage to swing high or low and should set a related trouble code.

Mass Airflow (MAF) Sensor

The MAF sensor measures flow of air entering the engine in grams per second. This measurement of airflow is a reflection of engine load (throttle opening and air volume), similar to the relationship of engine load to MAP or vacuum sensor signal. MAF signal should remain relatively constant at cruise, gradually changing with throttle angle and rapidly changing on sudden acceleration. The PCM uses this information to control fuel delivery.

This frequency generator type MAF sensor produces a frequency signal that cannot be easily measured in testing (32-150 Hertz). This varying signal is proportional to airflow. A fault in the MAF sensor circuit should set a related trouble code.

Oxygen Sensor (O2S)

The oxygen sensor is mounted in the exhaust system where it monitors oxygen content of exhaust gases. Two oxygen sensors are used on some models. The oxygen content causes the Zirconia/Platinum-tipped oxygen sensor to produce a voltage signal which is proportional to exhaust gas oxygen concentration (0-3%) compared to outside oxygen (20-21%). This voltage signal is low (about .1 volt) when a lean mixture is present and high (about 1.0 volt) when a rich mixture is present. As PCM compensates for a lean or rich condition, this voltage signal constantly fluctuates between high and low, crossing a .45-volt reference voltage supplied by PCM on the oxygen sensor signal line. This is referred to as "cross counts."

The oxygen sensor will not function properly (produce voltage) until its temperature reaches approximately 600°F (316°C). On some models, oxygen sensor is equipped with a sensor heating element. This allows the sensor to reach operating temperature sooner and prevents fuel system from re-entering "open loop" mode due to a cooled sensor (which is a normal occurrence during prolonged idle).

At temperatures less than the normal operating range of the sensor, vehicle will function in "open loop" mode and PCM will not make air/fuel adjustments based upon oxygen sensor signals but will use TP sensor and MAP or MAF values to determine air/fuel ratio from a table built into memory. When PCM reads a voltage signal greater than .45 volt from the oxygen sensor, PCM will begin to alter commands to injector to produce either a leaner or richer mixture.

Once vehicle has entered "closed loop", a cooled-down sensor or a fault in the oxygen sensor circuit (open or shorted circuit) is the only thing which can return it to "open loop". A problem in oxygen sensor circuit should set a related trouble code.

Park/Neutral Position (PNP) Switch

This switch is connected to transmission gear selector. The switch signals PCM when transmission is in Park or Neutral. Information from PNP switch is used by PCM for determining control of ignition timing, converter clutch and idle speed. To check function of PNP switch, perform functional check of switch. See appropriate I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section listed below.

1. «SYSTEM/COMPONENT TESTS - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l)
2. «SYSTEM/COMPONENT TESTS - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-22l)

Power Steering Pressure (PSP) Switch

This switch informs PCM of engine load conditions that exist when steering wheel is turned from center to full lock position. PCM uses information to help control idle speed and, on some models, A/C clutch. To check PSP switch, perform functional check of switch. See appropriate I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section. Refer to the following

1. «SYSTEM/COMPONENT TESTS - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l)
2. «SYSTEM/COMPONENT TESTS - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-22l)

RPM Reference Signal

The RPM is monitored by PCM through tach/pulse signals (circuit No. 430) produced by either the ignition module or crankshaft position sensor (Hall Effect signal on C(3)I, 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 (TP) Sensor

The TP sensor is a variable mechanical resistor connected directly to the throttle shaft linkage. The TP sensor has 3 wires connected to it. One is connected to a 5-volt reference voltage supply from PCM, the second is connected to PCM ground and the third is the signal return which is monitored by PCM. The voltage signal from the TP sensor varies from closed throttle (.5-1.0 volt) to wide open throttle (4.5-5.0 volts). This signal is used by PCM for determining control of fuel, idle speed, spark timing and converter clutch. A problem in the TP sensor circuit may set a related trouble code.

Vehicle Speed Sensor (VSS)

VSS is a Permanent Magnet (PM) generator mounted in transmission. The VSS sends a pulsing signal to PCM, which PCM converts into miles per hour (MPH). This sensor input is used by PCM in controlling converter clutch engagement. Signal may also be shared with instrument cluster and cruise control system.

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

See MISCELLANEOUS CONTROLS .

Air Injection Control Solenoid

See EMISSION SYSTEMS .

Boost Control Solenoid (Supercharger)

See AIR INDUCTION SYSTEM .

Canister Purge Solenoid

See EMISSION SYSTEMS .

Computer Controlled Coil Ignition (C(3)I)

See IGNITION SYSTEM .

Cooling Fan Relay

See MISCELLANEOUS CONTROLS .

Digital EGR Valve

See EMISSION SYSTEMS .

Direct Ignition System (DIS)

See IGNITION SYSTEM .

EGR Control Solenoid

See EMISSION SYSTEMS .

Electronic Variable Orifice (EVO) Actuator

See MISCELLANEOUS CONTROLS .

Fuel Injectors

See FUEL CONTROL .

Fuel Pump & Fuel Pump Relay

See FUEL DELIVERY .

HEI-EST Ignition

See IGNITION SYSTEM .

HOT Light Or Coolant Temperature (TEMP) Light

See MISCELLANEOUS CONTROLS .

Idle Air Control (IAC) Valve

See IDLE SPEED .

Integrated Direct Ignition (IDI) System

See IGNITION SYSTEM .

Knock Control System

See IGNITION SYSTEM .

Malfunction Indicator Light (MIL)

See SELF-DIAGNOSTIC SYSTEM .

Self-Diagnostics

See SELF-DIAGNOSTIC SYSTEM .

Serial Data

See SELF-DIAGNOSTIC SYSTEM .

Shift Light

See MISCELLANEOUS CONTROLS .

Shift Solenoids (Electronically-Controlled Auto Transmission)

See MISCELLANEOUS CONTROLS .

Torque Converter Clutch

See MISCELLANEOUS CONTROLS .

Fuel Pump

An in-tank electric fuel pump delivers fuel to injectors through an in-line fuel filter. The pump is designed to supply fuel pressure in excess of vehicle requirements. The pressure relief valve in the fuel pump controls maximum fuel pump pressure.

A pressure regulator, mounted in fuel rail (port injection systems) or on throttle body unit (throttle body injection systems), keeps fuel available to injectors at a constant pressure. Excess fuel is returned to fuel tank through pressure regulator return line. For fuel pressure specifications, see SPECIFICATIONS article in the ENGINE PERFORMANCE section.

When the ignition switch is turned to ON position, PCM will turn on the electric fuel pump by energizing the fuel pump relay. The PCM will continue to energize relay if the engine is running or cranking (PCM is receiving reference pulses from the ignition module). If no reference pulses exist, PCM de-energizes fuel pump relay within 2 seconds after ignition is turned on. For additional information, see FUEL PUMP RELAY below.

Fuel Pump Relay

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

As a back-up system to fuel pump relay, fuel pump is also activated by the oil pressure switch. The oil pressure switch is normally open until oil pressure reaches approximately 4 psi (.28 kg/cm 2 ). If fuel pump relay fails, the oil pressure switch closes when oil pressure is obtained, operating the fuel pump. An inoperative fuel pump relay may result in extended cranking times due to the time required to build up oil pressure. Oil pressure switch may be combined into a single unit with an oil pressure gauge sender or sensor.

For additional information on fuel pump activation, refer to BASIC TESTING and appropriate I - SYS/COMP TESTS articles in the ENGINE PERFORMANCE section. Refer to the following

1. «SYSTEM/COMPONENT TESTS - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l)
2. «SYSTEM/COMPONENT TESTS - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-22l)

Fuel Pressure Regulator (MFI Systems)

Fuel pressure regulator on MFI systems is a diaphragm-operated relief valve with injector pressure on one side and manifold pressure (vacuum) on the other. Pressure regulator compensates for engine load by increasing fuel pressure when low manifold vacuum is experienced.

During periods of high manifold vacuum, regulator-to-fuel tank return orifice is fully open, keeping fuel pressure on the low side of its regulated range. As throttle valve opens, vacuum to regulator diaphragm decreases, allowing spring tension to gradually close off return passage. At wide open throttle, when vacuum is at its lowest, return orifice is restricted, providing maximum fuel volume and maintaining constant fuel pressure to injectors.

FUEL CONTROL

The PCM, using input signals, determines adjustments to the air/fuel mixture in order to provide the optimum ratio for proper combustion under all operating conditions. One of 2 types of fuel control systems are used: throttle body injection or port fuel injection. These systems can operate in the "open loop" or "closed loop" mode. Description of these modes is as follows

Open Loop

When engine is cold and engine speed is greater than 400 RPM, PCM operates in "open loop" mode. In "open loop", PCM calculates air/fuel ratio based upon coolant temperature and Manifold Absolute Pressure (MAP) or Mass Airflow (MAF) sensor readings. Engine will remain in "open loop" operation until oxygen sensor reaches operating temperature, coolant temperature reaches preset temperature and a specific period of time has elapsed after engine start-up.

Closed Loop

When oxygen sensor has reached operating temperature, coolant temperature has reached a preset temperature and a specific period of time has passed since engine start-up, PCM operates in "closed loop". In "closed loop", PCM controls air/fuel ratio based upon oxygen sensor signals (in addition to other input parameters) to maintain as close to a 14.7:1 air/fuel mixture as possible. If oxygen sensor cools off (due to excessive idling) or a fault occurs in the oxygen sensor circuit, vehicle will once again enter "open loop" mode.

Battery Voltage Correction

PCM compensates for low battery voltage by increasing injector pulse width and increasing idle RPM. PCM is able to perform these commands because of a built-in memory/learning function.

Fuel Cut-Off

Injectors are de-energized when ignition is turned off to prevent dieseling. Injectors will not be energized if RPM reference pulses are not received by the PCM, even with ignition on. This prevents flooding before starting. Fuel cut-off will also occur at high engine RPM to prevent internal damage to engine. On some models, fuel injector signals may also be cut off during periods of high speed, closed throttle deceleration (when fuel is not needed).

Multiport Fuel Injection (MFI)

Individual, electrically pulsed injectors (one per cylinder) are located in intake manifold fuel rails. These injectors are next to intake valves in cylinder head.

Standard MFI systems feature simultaneous double-fire injection. Fuel injectors are pulsed once for each engine revolution, each spray providing 1/2 the fuel required for the combustion process. Thus, 2 injections of fuel (2 rotations of crankshaft) are mixed with incoming air to produce the fuel charge for each combustion cycle.

Some models use Sequential Fuel Injection (SFI). Injectors on these models are pulsed sequentially in spark plug firing order. The main differences between sequential and simultaneous systems are injectors, wiring and the PCM.

In all systems, constant fuel pressure is maintained to the injectors. Air/fuel mixture is regulated by amount of time injector stays open (pulse width). Various sensors provide information to the PCM to control pulse width.

IDLE SPEED

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

Idle Air Control Valve

The Idle Air Control (IAC) valve controls engine idle speed during engine load changes to prevent stalling. The IAC valve is mounted on throttle body and controls the amount of air by-passed around the throttle plate. To control engine idle speed, the IAC valve moves its pintle in and out in steps referred to as "counts" (zero counts, fully seated; 255 counts, fully retracted). Counts can be measured using a scan tester plugged into the Data Link Connector (DLC).

Normal counts on an idling engine should be 4-60. When engine is idling, PCM determines proper positioning of IAC valve based on battery voltage, coolant temperature, engine load and engine RPM. If engine RPM is too low, pintle is retracted and more air is by-passed around the throttle plate to increase engine RPM. If engine RPM is too high, pintle is extended and less air is by-passed around the throttle plate to decrease engine RPM.

If IAC valve is disconnected or connected with engine running, IAC loses its reference point and has to be reset. Resetting of IAC is accomplished on some models by turning ignition on and off. On other models, driving vehicle at normal operating temperature and speed greater than 35 MPH with circuit properly connected may be necessary. Problems in IAC circuit should set a related code.

The IAC valve affects only the idle system. If valve is stuck fully open, excessive airflow into the manifold creates a high idle speed. Valve stuck closed allows insufficient airflow, resulting in low idle speed. For calibration purposes, several different design IAC valves are used. Ensure proper design valve is used during replacement.

IGNITION SYSTEM

All vehicles are equipped with a DIS ignition system capable of producing in excess of 50,000 volts.

DIS is a distributorless system used on 2.2L and 3.1L models. The operation of both DIS and IDI is quite similar to operation of the C(3)I system. Systems consist of 2 (4-cylinder), 3 (V6) or 4 (V8) ignition coils, spark plug wires, ignition module (located under coil pack), a crankshaft position sensor, necessary wiring and knock control system portion of the PCM. On 2.3L, coils, module and spark plug connectors are all combined into one unit which plugs directly onto spark plugs.

Spark is timed by a signal sent from a crankshaft position sensor mounted through side of engine block instead of from a crankshaft position sensor mounted at crankshaft pulley (such as C(3)I). This signal is received by PCM (through ignition module) and is used to trigger each coil at the proper time. See CRANKSHAFT POSITION SENSOR under INPUT DEVICES . As with the C(3)I system, each cylinder is fired consecutively with the cylinder opposite it in the firing order. On V6, cylinder No. 1 is paired with No. 4, No. 2 with No. 5, and No. 3 with No. 6. On 4-cylinder, cylinder No. 1 is paired with No. 4 and cylinder No. 2 is paired with No. 3. Each pair of cylinders is fired by its own ignition coil.

On all models, crankshaft position sensor is mounted on bottom of DIS ignition module or near the ignition module.

The reluctor is a piece of metal, cast with the crankshaft. On all engines, reluctor has 7 slots machined into it, 6 of which are equally spaced (60 degrees apart). The seventh slot is spaced about 10 degrees from one of the other slots and generates a synchronization pulse signal. On all engines, as crankshaft rotates, notches in reluctor ring change the magnetic field at the tip of position sensor. This creates an induced AC voltage signal in the sensor windings, resulting in RPM reference signals which are sent to PCM by the ignition module. This allows PCM to compute crankshaft position and RPM.

Ignition Timing Advance

At engine speeds less than 400 RPM, the ignition module controls spark advance by triggering coils at a predetermined interval based only on engine speed. At engine speeds greater than 400 RPM (EST mode), the PCM takes over control of the ignition timing.

PCM controls ignition timing based upon input signals from the engine RPM reference line (ignition module), coolant temperature sensor, manifold air temperature sensor, throttle position sensor, knock sensor, vehicle speed sensor, gear position switch and the MAF or MAP sensor.

The PROM/MEM-CAL portion of the PCM has a programmed spark advance curve based on engine speed. Spark timing is calculated by PCM whenever an ignition pulse is present. Spark advance is controlled only when engine is running (not during cranking). Input signal values are used by PCM to modify PROM/MEM-CAL information, increasing or decreasing spark advance to achieve maximum performance with minimum emissions. To check ignition system operation, see BASIC TESTING or I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section. Refer to the following

1. «SYSTEM/COMPONENT TESTS - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l)
2. «SYSTEM/COMPONENT TESTS - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-22l)

Although several types of ignition systems are used, all ignition systems use the same 4 basic ignition circuits. For description of fuel control and sync signals, see IGNITION SYSTEM.

The ignition module is connected to PCM by 4 EST circuits. Circuits perform the following functions

1. By-Pass

When an engine speed signal of approximately 400 RPM is received by the PCM, PCM considers engine to be running and applies 5 volts to the ignition module on the by-pass wire. This causes ignition module to switch timing control over to the variable timing control circuit in the PCM. On some models, this by-pass wire contains a connector located between the 4-wire connector and PCM. This is disconnected when adjusting base timing. On all models, an open or grounded by-pass circuit will set a related trouble code in PCM memory. The engine will run at base timing plus a small amount of advance built into the HEI module.

1. EST

When 5 volts is present on the by-pass circuit and ignition module has turned control of engine timing over to PCM, the PCM advances or retards spark on this circuit based on calculations involving the reference signal and other sensor input signals. If base timing is incorrectly set, entire advance curve will be incorrect.

1. Ground

This is the reference ground circuit. It is grounded at distributor and PCM, ensuring no voltage drop occurs in the EST circuit which could affect ignition operation.

1. Reference (RPM)

Alternating current signals from the PM generator (DIS) are converted by the ignition module converter to digital signals for use by PCM. This supplies RPM data and crankshaft position reference to PCM. Because signal on this circuit is used as an injector trigger reference, engine will not run if circuit is open or grounded.

ESC Detonation Retard Operation

In conjunction with the HEI-EST system, an Electronic Spark Control (ESC) retard system is used. System consists of a detonation (knock) sensor, a high energy ignition system, an ESC controller and PCM. On some models, the function of the ESC controller is built into the Memory Calibration (MEM-CAL) unit of the PCM.

When detonation (engine knock) occurs, detonation sensor produces a low voltage AC signal. This signal goes to the ESC controller or directly to the MEM-CAL unit inside the PCM, depending upon application.

On vehicles using PCMs containing MEM-CAL units, the PCM supplies a 5-volt DC reference signal on the knock sensor signal line. Internal circuitry of the knock sensor will pull this voltage down to about 2.5 volts. When knock occurs, the knock sensor produces an AC voltage signal which rides on the 2.5-volt DC signal back to the PCM. The voltage and frequency of this signal depend upon knock signals received by the sensor. The PCM will retard spark timing until signals from detonation sensor cease.

A malfunction in the ESC circuit should set a related trouble code. If a code is not present and ESC system is suspected as the cause of driveability problems, perform functional check of ESC system. See the appropriate I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section. Refer to the following

1. «SYSTEM/COMPONENT TESTS - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l)
2. «SYSTEM/COMPONENT TESTS - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-22l)

EMISSION SYSTEMS

Note. To determine emission systems usage, refer to appropriate EMISSION APPLICATIONS article in the ENGINE PERFORMANCE.

AIR INJECTION SYSTEM

This system helps reduce hydrocarbon (HC) and carbon monoxide (CO) exhaust emissions by injecting air into the exhaust system. The induction of additional air promotes further oxidation (combustion) of unburned and partially burned exhaust gases. During cold engine operation, air is injected into exhaust manifold. This quickly warms up catalytic converter and oxygen sensor. When vehicle warms up, air is diverted to atmosphere or, on models with a TWC/OC, to the catalytic converter. See CATALYTIC CONVERTER .

Note. Always cover centrifugal filter fan before cleaning engine to prevent liquid from entering air pump. DO NOT oil air pump.

Air Pump (Belt-Driven)

The air pump is a belt-driven, positive displacement vane-type pump. Air drawn into pump is purged of dirt and contaminants by a centrifugal filter mounted behind the pulley. Air pump is permanently lubricated and requires no periodic service.

Air Pump (Electric)

Air pump is a sealed, non-serviceable, electric-motor type. Pump is energized by an PCM-controlled relay, which is activated when fuel system is functioning in "open loop" mode and/or less than a predetermined amount of time has passed since relay was energized. See ELECTRIC AIR PUMP RELAY heading below.

Note. Air control (divert) valve and air switching valve may be separate or combined into a single assembly.

Air Injection Reaction Management System

When PCM energizes the air control (divert) and air switching valves on a cold vehicle, air is allowed to flow through the control valve to the air switching valve. The air switching valve then directs this air to the exhaust port.

During warm engine operation ("closed loop"), PCM de-energizes the air switching valve. This causes air switching valve to direct air to the catalytic converter.

If air control (divert) valve detects a rapid increase in manifold vacuum (deceleration condition) or if high RPM operation causes pump output pressure to exceed normal operating range, air is mechanically diverted to the air cleaner by the air control (divert) valve. If PCM detects any failure in the computerized engine control system, air control (divert) valve will be de-energized, also causing air to be diverted to the air cleaner or atmosphere. To check function of AIR system, perform functional check of system. See appropriate I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section. Refer to the following

1. «SYSTEM/COMPONENT TESTS - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l)
2. «SYSTEM/COMPONENT TESTS - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-22l)

Check Valve

The check valve prevents the backflow of exhaust gases into the air injection system. The check valve closes when exhaust gas pressure in exhaust manifold exceeds pressure delivered by pump. This occurs when air pump by-passes at high speeds, air delivery is switched to catalytic converter, air is diverted to atmosphere or air cleaner, or air pump malfunctions.

Electric Air Divert/Electric Air Switching Valves

Electric divert and electric switching valves are used on some models. System may combine both divert function and air switching function into one integral component.

The valves are electrically controlled by the PCM and operated by air pump pressure. The operation of the valves is not dependent on intake manifold vacuum.

For cold engine ("open loop") operation, the divert solenoid is energized and air flows to exhaust ports. In warm engine ("closed loop") operation, the divert solenoid is de-energized and switching solenoid is energized. This forces airflow to the converter. In the divert mode, both solenoids are de-energized and airflow is allowed to vent to atmosphere.

Divert will occur during rich operating condition, when the PCM recognizes a problem and turns on the Malfunction Indicator Light (MIL), during deceleration (high vacuum) and during heavy acceleration when air pressure exceeds the setting of the relief valve in the air divert valve.

Electric Air Divert Valve (EADV)

The Electric Air Divert Valve performs normal diverter valve operation and may provide air divert to the air cleaner for catalytic converter protection during wide open throttle and high temperature conditions.

The PCM de-energizes EADV solenoid (located in EADV), preventing manifold vacuum from entering the chamber during the previously described conditions. Spring tension against the lower diaphragm pushes the diaphragm up, diverting air to air cleaner. Air from the air pump is always shut off from the engine unless PCM grounds EADV circuit (solenoid energized).

Electric Air Pump Relay

When vehicle is cold ("open loop" mode), PCM provides a ground for the electric air pump relay. When relay is energized, power is supplied to the electric air pump. When fuel system goes into "closed loop" or electric air pump has been on for more than a precalibrated period, the PCM opens the ground circuit. When relay is de-energized on, air is diverted to the atmosphere until air pump stops spinning, or an internal stop valve closes when relay is de-energized.

CATALYTIC CONVERTER

A 3-way catalytic (TWC) converter is used on all vehicles to reduce exhaust emissions. This type of converter reduces hydrocarbon (HC), carbon monoxide (CO) and oxides of nitrogen (NOx) levels.

TWC

Converter contains a reducing agent (Rhodium and Platinum) to reduce NOx and an oxidizing agent (Palladium and Platinum) to oxidize HC and CO. This causes HC and CO to oxidize (break down with the addition of oxygen and heat) into the harmless base elements: water (H2O) and carbon dioxide (CO2). Oxygen is removed from NOx, causing it to reduce to the harmless base elements nitrogen (N) and oxygen (O2).

EXHAUST GAS RECIRCULATION (EGR)

The Exhaust Gas Recirculation (EGR) system is designed to reduce oxides of nitrogen (NOx) emissions by lowering combustion temperatures. This is accomplished when a metered amount of exhaust gas is recirculated into the intake manifold and mixed with the air/fuel mixture.

The 3 types of EGR systems used are pulse width modulated backpressure (positive and negative) EGR using an EGR solenoid and either ported or manifold vacuum, pulse width modulated without backpressure EGR, and digital or linear EGR.

On computer-controlled EGR systems using a solenoid, PCM controls ported or manifold vacuum to EGR valve through solenoid valve. Solenoid may be normally open or normally closed, depending upon application.

PCM uses coolant temperature, throttle position and manifold pressure signals to determine vacuum solenoid operation. During cold engine operation and idle, EGR is not desired; PCM causes solenoid to block vacuum to EGR valve. During warm engine operation and at speeds greater than idle, vacuum is allowed through solenoid, opening EGR valve. To check EGR system, perform functional check of system. See appropriate I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section. Refer to the following

1. «SYSTEM/COMPONENT TESTS - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l)
2. «SYSTEM/COMPONENT TESTS - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-22l)

Digital EGR System

The digital EGR valve is designed to accurately supply EGR to engine, independent of intake manifold vacuum. The valve controls EGR flow from exhaust to intake manifold through 3 internally-mounted solenoids. When each solenoid is energized, a pintle is lifted to allow exhaust gas to flow through valve. Solenoids are energized individually, in pairs or together to provide 7 different EGR flow ratios. This enables ECM to tailor EGR flow to specific engine requirements.

Exhaust Backpressure EGR System

EGR uses positive and negative backpressure EGR valves. These valves may be identified by the letter in the last position of part number; "P" designates a positive backpressure valve and "N" a negative backpressure valve. Backpressure EGR may also use an PCM-controlled solenoid to regulate vacuum signal to EGR valve.

1. Negative Backpressure EGR Valve

Vacuum is applied to upper EGR diaphragm via a hose connected to intake manifold vacuum. Manifold vacuum is also applied to lower EGR diaphragm (through intake port at base of EGR valve).

When manifold vacuum in lower chamber is insufficient to overcome spring tension on lower diaphragm, bleed valve will be closed, allowing vacuum in upper chamber to open EGR valve. With engine at idle or under light load, high manifold vacuum applied to lower chamber opens air bleed valve in lower diaphragm. This bleeds off vacuum in upper chamber, keeping the EGR valve closed.

1. Positive Backpressure EGR Valve

A control valve, located in EGR valve, acts as a vacuum regulator valve. Control valve regulates amount of vacuum to EGR diaphragm chamber by bleeding vacuum to atmosphere during certain operating conditions.

When control valve receives backpressure signal through hollow shaft of EGR valve, pressure on bottom of control valve closes control valve. When control valve closes, maximum vacuum signal is applied directly to EGR valve allowing exhaust gas recirculation.

Pulse Width Modulated (PWM) EGR System

This system is controlled entirely by the PCM. PCM regulates EGR vacuum signal by controlling an electrical signal to a solenoid vacuum valve. The PCM-controlled vacuum solenoid valve is located in series between vacuum source and EGR valve. The solenoid is pulsed at a rate of up to 32 times per second. The PCM uses a ported vacuum signal to determine the flow rate signal to the solenoid. PWM systems also use a backpressure EGR valve to prevent EGR function until engine loads are present. See EXHAUST BACKPRESSURE EGR SYSTEM.

EVAPORATIVE EMISSION CONTROL

Carbon canister storage is used for evaporative fuel control on all vehicles. The function of evaporative emission control system is to store gasoline fumes from fuel tank in a carbon canister until fumes can be drawn into engine for burning during combustion process.

Evaporative emission system uses 3 basic components

1. Activated carbon canister (may be sealed or open at top or bottom for fresh air intake).
2. Tank pressure control valve (mounted internally or externally to fuel tank).
3. PCM-controlled solenoid (mounted remotely or on canister).

For specific component application, see appropriate EMISSION APPLICATIONS article in the ENGINE PERFORMANCE. For vacuum hose routing, see VACUUM DIAGRAMS article in the ENGINE PERFORMANCE.

Carbon Canister

Evaporative fumes from the fuel tank are vented through hoses into a canister containing activated carbon. The activated carbon absorbs and holds fuel vapors when the engine is not operating. When the engine is started and engine speed is greater than idle (purge at idle would cause too rich a mixture), engine vacuum draws fuel vapors from the canister into the engine. Regulation of vapors through this purge line may be controlled by a vacuum canister purge valve, an PCM-controlled solenoid or both.

Carbon canisters are either open or closed design. When the engine is started on open canister models, engine vacuum draws outside air into canister either through the top or through a filter in bottom of canister. This helps to purge vapors from the activated carbon.

Note. Models without fuel tank pressure control valves may use a special pressure/vacuum relief fuel tank filler cap or other external relief device.

Fuel Tank Pressure Control Valve

Fuel tank pressure control valve is a vacuum regulated/pressure control valve located in fuel tank or in vapor delivery hose between fuel tank and carbon canister. When engine is not running and tank pressure is less than .9 psi (.06 kg/cm 2 ), internal spring pressure holds valve in the closed position.

This causes fuel tank low-pressure vapors to be vented through a restriction in valve. This restriction will retain most fuel tank vapors in fuel tank. When tank pressure rises and overrides spring tension, fumes are vented to the carbon canister. When engine is running, vacuum is applied to upper port of valve, opening passage between fuel tank and carbon canister, which is purged by engine vacuum.

Purge Solenoid Valve

Purge solenoid valve is controlled by the PCM. Current is supplied to solenoid when the ignition is on. Solenoid is energized when PCM provides a ground circuit for solenoid. Solenoid may be normally closed or normally open. When solenoid valve is open, charcoal canister is purged using manifold or ported vacuum. When solenoid valve is closed, purge vacuum to canister is blocked.

The PCM will allow vacuum to pass through solenoid when engine has been running for more than one minute, coolant temperature is greater than 176°F (80°C), vehicle speed is greater than 5 MPH and throttle is off idle. This solenoid (if used) is located in the purge line between charcoal canister and vacuum purge port or on top of canister.

POSITIVE CRANKCASE VENTILATION (PCV)

The PCV system is used to provide more effective elimination of crankcase vapors. Fresh air from the air filter housing is supplied to the crankcase where it is mixed with blow-by gases and passed through a PCV valve into the intake manifold. This mixture is then passed into the combustion chamber and burned.

The PCV valve provides primary control in this system by metering the flow of the blow-by vapors, according to manifold vacuum. When manifold vacuum is high (at idle), the PCV restricts the flow to maintain a smooth idle condition.

Under conditions in which abnormal amounts of blow-by gases are produced (such as worn cylinders or rings), the system is designed to allow the excess gases to flow back through crankcase vent hose into the air inlet to be consumed during normal combustion.

SELF-DIAGNOSTIC SYSTEM

The PCM is equipped with a self-diagnostic system which detects system failures or abnormalities. When a malfunction occurs, PCM will illuminate the Malfunction Indicator Light (MIL) on instrument panel. When malfunction is detected and MIL is turned on, a corresponding trouble code will be stored in PCM memory. Malfunctions are designated as either "hard failures" or as "intermittent failures". To retrieve stored codes, see appropriate G - TESTS W/ CODES article in the ENGINE PERFORMANCE section below.

1. «TESTS W/CODES - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-tests-wcodes-31l)
2. «TESTS W/CODES - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-tests-wcodes-22l)

"HARD FAILURES"

Hard failures cause MIL to glow and remain on until the malfunction is repaired. On models using digital display on dash to indicate codes, codes may be accompanied by a "current" or "history" indication for intermittent and hard codes. If light comes on and remains on during vehicle operation, cause of malfunction must be determined using diagnostic charts located in appropriate G - TESTS W/ CODES article in the ENGINE PERFORMANCE section below. If a sensor fails, PCM will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional but loss of good driveability is likely.

1. «TESTS W/CODES - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-tests-wcodes-31l)
2. «TESTS W/CODES - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-tests-wcodes-22l)

"INTERMITTENT FAILURES"

Intermittent failures cause MIL to flicker or glow and go out about 10 seconds after the intermittent fault goes away. The corresponding trouble code, however, will be retained in PCM memory. On models using digital display on dash to indicate codes, codes may be accompanied by a "current" or "history" indication for intermittent and hard codes. If related fault does not reoccur within 50 engine restarts, related trouble code will be erased from PCM memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See TESTS W/O CODES article in the ENGINE PERFORMANCE section.

As a bulb and system check, MIL will glow when ignition switch is turned to ON position and engine is not running. When engine is started, light should go out. If light does not go out, a malfunction has been detected in the computerized engine control system or MIL circuit is faulty. Light may be used on some models to display stored trouble codes. To access codes using "scan" or "non-scan" methods, see appropriate G - TESTS W/ CODES article in the ENGINE PERFORMANCE section below.

1. «TESTS W/CODES - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-tests-wcodes-31l)
2. «TESTS W/CODES - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-tests-wcodes-22l)

PCM is equipped with a serial data line. Serial data is a stream of electrical impulses which can be interpreted by special testers of other control modules. On some models, serial data and codes must be accessed using special "scan" testers connected to the Data Link Connector (DLC). Update intervals and information contained within the data stream vary with model application.

On some models, serial data may be accessed using the Driver Information Center (DIC) and Climate Control Panel (CCP). On these models, serial data may be shared with A/C controller, supplemental restraint controller, anti-lock brake controller and even cruise control unit.

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 the A/C clutch through an PCM-controlled 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 below or rises above 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 A/C wiring schematics under MISCELLANEOUS CONTROLS in appropriate I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section. Refer to the following

1. «SYSTEM/COMPONENT TESTS - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l)
2. «SYSTEM/COMPONENT TESTS - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-22l)

Some models are equipped with an air conditioner pressure sensor which is used to inform PCM of A/C system pressure levels. Low pressure signal will cause A/C compressor to disengage to prevent system damage. High pressure levels cause PCM to engage high speed fans while A/C compressor clutch is engaged. Extremely high pressure levels will cause PCM to disengage A/C compressor clutch to prevent system damage.

A/C high and low pressure switches may be used in the PCM-monitored A/C request circuit. Switches are normally closed, completing the circuit between ignition and PCM. PCM will engage or disengage A/C clutch relay based upon status of this circuit. When system refrigerant pressure increases beyond a certain point, high side switch will open, causing A/C request line voltage to drop.

If system refrigerant level decreases, causing refrigerant pressure to drop below normal, low side pressure switch will open, causing A/C request line voltage to drop. Switches may be used as normal clutch cycling devices or as safety devices which prevent compressor damage in the event of excessively high or low refrigerant pressure.

COOLING FAN

On many models, PCM regulates operation of electric cooling fan through a PCM-controlled relay which controls the ground circuit or power circuit for the cooling fan. This allows PCM to operate cooling fan based upon engine temperature.

Most systems will engage electric cooling fan whenever A/C clutch is engaged, regardless of engine temperature. As a back-up system, many models use a coolant override switch that will also engage the cooling fan if PCM fails to energize the cooling fan relay or the cooling fan relay malfunctions. A malfunction of the cooling fan will cause engine overheating and possible detonation.

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 refrigerant temperature sensors or high pressure switches) during periods of engine overheating or high A/C refrigerant pressures.

For component application and related wiring, see wiring schematics under MISCELLANEOUS CONTROLS in appropriate I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section. Refer to the following

1. «SYSTEM/COMPONENT TESTS - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l)
2. «SYSTEM/COMPONENT TESTS - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-22l)

HOT LIGHT OR COOLANT TEMPERATURE LIGHT

When engine coolant temperature sensor input indicates temperature exceeds specified range, PCM will turn on the TEMP or HOT light by providing a ground for the light circuit. As a bulb check, PCM also supplies a ground to turn on light when ignition is first turned on.

Torque Converter Clutch (Non-Electronic Transmission)

The purpose of the transmission/transaxle converter clutch feature is to eliminate power loss of torque converter stage when vehicle is in a cruise condition. This allows convenience of automatic transmission/transaxle and fuel economy of a manual transmission.

Fused battery ignition is supplied to converter solenoid through a brake switch. On some models, 2nd, 3rd and 4th gear hydraulic apply switches (located within the transmission) may also be in series with solenoid power or ground circuit. On other models, switch status may only be monitored by the PCM, without sharing power or ground with the converter solenoid. For wiring reference, see wiring schematics under MISCELLANEOUS CONTROLS in appropriate I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section. Refer to the following

1. «SYSTEM/COMPONENT TESTS - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l)
2. «SYSTEM/COMPONENT TESTS - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-22l)

Converter clutch will engage when vehicle is moving faster than a precalibrated speed, engine is at normal operating temperature, throttle position sensor output is not changing (indicating a steady vehicle speed), transmission 3rd gear or high gear switch is closed (if equipped) and brake switch is closed.

When vehicle speed is great enough (about 20-45 MPH as indicated by the vehicle speed sensor), PCM energizes converter clutch solenoid mounted in transmission. This allows torque converter to directly connect engine to the transmission. When operating conditions indicate transmission should operate as normal, converter clutch solenoid is de-energized.

This allows transmission to return to normal automatic operation. Since power for the converter solenoid is delivered through the brake switch, transmission will also return to normal automatic operation when brake pedal is depressed. To check function of converter clutch system, perform functional check of system. See MISCELLANEOUS CONTROLS in appropriate I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section. Refer to the following

1. «SYSTEM/COMPONENT TESTS - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l)
2. «SYSTEM/COMPONENT TESTS - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-22l)

Torque Converter Clutch (Electronic Transmission)

The torque converter clutch functions similarly to the non-electronic type except instead of a single internal TCC solenoid, 2 solenoids are used. A standard TCC solenoid is used in conjunction with a Pulse Width Modulated (PWM) solenoid that regulates hydraulic pressure to make locking and unlocking of the TCC smoother.

Electronic Transmission

On vehicles equipped with electronic transmission, transmission is controlled by the Powertrain Control Module (PCM). PCM controls other vehicle functions as well as the transmission. The PCM monitors a number of engine/vehicle functions and uses the data to control shift solenoid "A", shift solenoid "B", TCC and the force motor to regulate TCC engagement, upshift pattern, downshift pattern and line pressure (shift quality).

1. Shift Solenoid "A"

Shift solenoid "A" is attached to the valve body and is a normally open exhaust valve. PCM activates solenoid by grounding it through an internal quad-driver. Solenoid "A" is on in 1st and 4th gears but off in 2nd and 3rd gears. When on, solenoid redirects fluid to act on the shift valves.

1. Shift Solenoid "B"

Shift solenoid "B" is attached to the valve body and is a normally open exhaust valve. PCM activates solenoid by grounding it through an internal quad-driver. Solenoid "B" is on in 3rd and 4th gears but off in 1st and 2nd gears. When on, solenoid redirects fluid to act on the shift valves.

1. Force Motor

Force motor is attached to the valve body and controls line pressure by moving a pressure regulator valve against spring pressure. Force motor takes the place of the throttle valve or vacuum modulator used on past model transmissions. PCM varies line pressure based upon engine load. Engine load is calculated from various inputs, especially the TP sensor.

1. Line pressure is actually varied by changing the amperage applied to the force motor from zero (high pressure) to 1.1 amps (low pressure). The force motor is periodically pulsed to prevent the pressure regulator valve from sticking due to fluid contamination.

The shift light is used on M/T vehicles. Light indicates the best transmission shift point for maximum fuel economy. Power for light is supplied through the GAUGES fuse. Light glows when PCM supplies a ground circuit for bulb. For wiring reference, see MISCELLANEOUS CONTROLS in appropriate I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section. Refer to the following

1. «SYSTEM/COMPONENT TESTS - 3.1L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l)
2. «SYSTEM/COMPONENT TESTS - 2.2L»(/buick/century/v-1982-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-22l)

See also:
• TESTS W/CODES - 3.1L
• TESTS W/CODES - 2.2L
• SYSTEM/COMPONENT TESTS - 3.1L
• SYSTEM/COMPONENT TESTS - 2.2L
• SPECIFICATIONS
• BASIC TESTING
• TESTS W/O CODES
• DIRECT IGNITION SYSTEM (DIS)
• MISCELLANEOUS CONTROLS
• EMISSION SYSTEMS
• IGNITION SYSTEM
• FUEL CONTROL
• IDLE SPEED
• SELF-DIAGNOSTIC SYSTEM
• INPUT DEVICES
• CATALYTIC CONVERTER