MODEL IDENTIFICATION
Vehicle model is identified by the fifth character of Vehicle Identification Number (VIN). The VIN is stamped on metal pad on top of left end of instrument panel, near windshield. See MODEL IDENTIFICATION table.
| Series (1) | Model |
|---|---|
| "C" | 2WD Avalanche, Escalade, Sierra, Silverado, Suburban, Tahoe, Yukon & Yukon XL |
| "G" | RWD Chevy Express & Savana |
| "H" | AWD Chevy Express & Savana |
| "K" | AWD/4WD Avalanche, Escalade, Escalade ESV, Escalade EXT, Sierra, Silverado, Suburban, Tahoe, Yukon & Yukon XL |
| "N" | Hummer H2 |
| "S" | Envoy XL & TrailBlazer EXT (2WD) |
| "T" | Envoy XL & TrailBlazer EXT (AWD/4WD) |
| (1) Vehicle series is fifth character of VIN. | |
| (1) | Vehicle series is fifth character of VIN. |
MODEL IDENTIFICATION
INTRODUCTION
Note. Unless specified otherwise, Sierra and Silverado information also applies to Cab & Chassis Sierra and Cab & Chassis Silverado. Savana information also applies to Savana Special and Savana Camper Special.
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.
Speed Density
Engine is equipped with a Mass Airflow (MAF) sensor, a Manifold Absolute Pressure (MAP) sensor and an Intake Air Temperature (IAT) sensor. (Scheme 1)or (Scheme 7).
The MAF sensor is an airflow meter that measures the amount of air entering the engine. The 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 MAP sensor responds to pressure changes in the intake manifold. The pressure changes occur based on the engine load. The MAP sensor has the following circuits: 5-volt reference, low reference circuit and MAP sensor signal circuit. The PCM supplies 5 volts to the MAP sensor on the 5-volt reference circuit. PCM also provides a ground on the low reference circuit. MAP sensor provides a signal to PCM on 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. PCM should detect a high signal voltage at a high MAP, such as when ignition is on, with engine off, or at a Wide Open Throttle (WOT). The MAP sensor is also used in order to determine the Barometric Pressure (BARO). This occurs when the ignition is on, with 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.
The IAT sensor is a variable resistor. 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 PCM supplies 5 volts to the IAT signal circuit and a ground for the IAT low reference circuit. When IAT sensor is cold, the sensor resistance is high. When air temperature increases, sensor resistance decreases. With high sensor resistance, PCM detects a high voltage on the IAT signal circuit. With lower sensor resistance, PCM detects a lower voltage on the IAT signal circuit. If PCM detects an out-of-range signal voltage, a DTC will set. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.
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 designed to maintain a 14.7:1 air/fuel ratio under most operating conditions. When the ideal air/fuel ratio is maintained, the Three-Way Catalytic (TWC) converter can control Oxides Of Nitrogen (NOx), Hydrocarbon (HC) and Carbon Monoxide (CO) emissions.
The computerized engine control system consists of engine PCM, input devices (sensor and switch input signals) and output signals.
POWERTRAIN CONTROL MODULE
The PCM has a "learning" ability which allows it to make minor corrections for fuel system variations. PCM is located on left front of engine compartment, behind radiator. (Scheme 2) If battery power is interrupted, a vehicle performance change may be noticed. PCM module corrects itself, and normal performance returns if vehicle is allowed to "relearn" optimum control conditions. "Relearning" occurs when vehicle is driven at normal operating temperature under part throttle, moderate acceleration and idle conditions.
Scheme 2
THROTTLE ACTUATOR CONTROL
The Throttle Actuator Control (TAC) system delivers improved throttle response and greater reliability and eliminates the need for mechanical cable. The TAC system performs accelerator pedal position sensing, throttle positioning to meet driver and engine demands, throttle position sensing, internal diagnostics, cruise control functions and manage TAC electrical power consumption.
The TAC system components include the Accelerator Pedal Position (APP) sensor, throttle body assembly, throttle actuator control module and PCM. (Scheme 3)
Scheme 3
INPUT DEVICES
Note. Components are grouped into 2 categories. The first category is INPUT DEVICES, consisting of components which control or produce voltage signals monitored by the control unit. The second category is OUTPUT SIGNALS, consisting of components controlled by the PCM.
A/C On/Request Signal
The A/C system can be engaged by either pressing the A/C switch or during automatic operation. The HVAC control module sends a class 2 message to the PCM for A/C compressor engagement. The PCM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil. The A/C compressor diode will prevent a voltage spike, resulting from the collapse of the magnetic field of the coil, from entering the vehicle electrical system when the compressor is disengaged.
Accelerator Pedal Position Sensor
The accelerator pedal assembly contains 2 individual Accelerator Pedal Position (APP) sensors within the assembly. (Scheme 4) The APP sensors 1 and 2 potentiometer-type sensors have a 5-volt reference circuit, low reference circuit and signal circuit. The APP sensors are used to determine pedal angle. The control module provides each APP sensor a 5-volt reference circuit and a low reference circuit. The APP sensors then provide the control module with signal voltage proportional to pedal movement. Both APP sensor signal voltages are low at rest position and increase as pedal is applied.
Scheme 4
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 richen the air/fuel mixture. If voltage swings high, PCM may set a charging system fault code and turn on Malfunction Indicator Light (MIL). If voltage signal swings excessively low (less than 9 volts) or excessively high (16 volts), PCM shuts down for as long as condition exists. If condition is short-term, MIL flickers and vehicle may stumble. Vehicle stalls if condition persists.
Brake Switch Feedback
On models equipped with cruise control systems, PCM may monitor the brake switch circuit to determine when to engage and disengage cruise control.
Camshaft Position Sensor
The Camshaft Position (CMP) sensor is a hall-effect type sensor. The sensor produces one signal for each revolution of the camshaft in order to control the sequential fuel injection. The CMP sensor is designed to detect changes in a magnetic field. The PCM supplies the CMP sensor with a 12-volt reference circuit, a low reference circuit and a signal circuit. The CMP sensor produces a magnetic field whenever ignition is on. The CMP sensor is mounted near a reluctor wheel that is attached to the distributor shaft. (Scheme 5) When distributor shaft rotates, and reluctor wheel tooth passes by the CMP sensor, there is a change in the magnetic field. The CMP sensor converts each change in the magnetic field into a PULSE. If PCM does not detect the CMP signal while the engine is running, a DTC will set. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.
Scheme 5
Cranking Signal
Cranking signal is a 12-volt signal monitored by the PCM. Signal is present when ignition switch is in the START position. The PCM uses this signal to determine the need for starting enrichment. PCM also cancels diagnostics until engine is running and 12-volt signal is no longer present.
Crankshaft Position Sensor
The PCM uses the Crankshaft Position (CKP) sensor to detect crankshaft speed and position. The CKP sensor is located on side of engine block, behind harmonic balancer. (Scheme 6) The CKP sensor connects to the PCM through the 12-volt reference circuit, the low reference circuit and CKP sensor 1 signal circuit. If PCM detects that the CKP sensor signal is incorrect for 3 seconds, a DTC will set. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.
Scheme 6
Engine Coolant Temperature Sensor
The Engine Coolant Temperature (ECT) sensor is a variable resistor, that measures the temperature of engine coolant. ECT sensor is located on side of left cylinder head. (Scheme 1) The PCM supplies 5 volts to the ECT sensor signal circuit and ground for the ECT sensor low reference circuit. When ECT is cold, sensor resistance is high. When ECT increases, sensor resistance decreases. With high sensor resistance, PCM detects a high voltage on ECT sensor signal circuit. With lower sensor resistance, PCM detects a lower voltage on the ECT sensor signal circuit. If PCM detects an out-of-range signal voltage, a DTC will set. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.
Fuel Tank Pressure Sensor
Failure in Fuel Tank Pressure (FTP) sensor circuit will set a related DTC. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.
Intake Air Temperature Sensor
The Intake Air Temperature (IAT) sensor is a variable resistor. IAT sensor is located in fresh air intake tube. (Scheme 7) The IAT sensor has a signal and a low reference circuit. The IAT sensor measures the temperature of the air entering the engine. The PCM supplies 5 volts to the IAT signal circuit and a ground for the IAT low reference circuit. When IAT sensor is cold, the sensor resistance is high. When air temperature increases, the sensor resistance decreases. With high sensor resistance, PCM detects a high voltage on the IAT signal circuit. With lower sensor resistance, PCM detects a lower voltage on the IAT signal circuit. Failure in IAT sensor circuit should set a related DTC. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.
Scheme 7
Knock Sensor
The Knock Sensor (KS) produces an AC voltage at all engine speeds and loads. KS is located on top rear of engine. (Scheme 8) The PCM then adjusts the spark timing based on the amplitude and frequency of the KS signal. PCM uses the KS signal to calculate the average voltage. Then PCM assigns a voltage value. PCM checks the knock sensor and related wiring by comparing the actual knock signal to the assigned voltage range. A normal KS signal should stay within the assigned voltage range. A fault in the KS circuit may set a DTC. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article. When a related DTC is not present and the KS system is the suspected cause of a driveability problem, check of KS system. See SYMPTOMS in SYSTEM & COMPONENT TESTING - 4.8L, 5.3L & 6.0L "C", "G", "H", "K" & "N" SERIES- FLEX FUEL & GASOLINE article.
Scheme 8
Manifold Absolute Pressure Sensor
The Manifold Absolute Pressure (MAP) sensor responds to pressure changes in the intake manifold. MAP sensor is mounted on top of intake manifold. (Scheme 1) The pressure changes occur based on engine load. The MAP sensor has a 5-volt reference circuit, a low reference circuit and a signal circuit. The PCM supplies 5 volts to MAP sensor on the 5-volt reference circuit. PCM also provides a ground on the low reference circuit. The MAP sensor provides a signal to PCM on the MAP sensor signal circuit which is relative to the pressure changes in the manifold. PCM should detect a low signal voltage at a low MAP, such as during an idle or a deceleration. PCM should detect a high signal voltage at a high MAP, such as when ignition is on, with engine off or at a Wide Open Throttle (WOT). The MAP sensor is also used in order to determine the Barometric Pressure (BARO). This occurs when ignition is on, with engine off. The BARO reading may also be updated whenever engine is operated at WOT. The PCM monitors the MAP sensor signal for voltage outside of the normal range. If PCM detects a MAP sensor signal voltage that is out-of-range, a DTC will set. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.
If MAP sensor fails, the PCM substitutes a fixed MAP value, and uses the TP sensor to control fuel delivery. A fault in the MAP circuit should set a related diagnostic trouble code. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.
Heated Oxygen Sensor
| CAUTION | Measure oxygen sensor voltage with a digital volt-ohmmeter (minimum 10-megohm impedance) only. Current drain of a conventional voltmeter could damage sensor. |
Heated Oxygen Sensor (HO2S) is used for fuel control and post catalyst monitoring. Each HO2S compares oxygen content of he surrounding air with the oxygen content in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature. The PCM supplies the HO2S with a reference or bias voltage of about 450 mV. When engine is first started, PCM operates in open loop, ignoring the HO2S voltage signal. Once the HO2S reaches operating temperatures and closed loop is achieved, the HO2S generates a voltage within a range of 0-1000 mV that fluctuates at greater than or less than bias voltage. High HO2S voltage indicates a rich exhaust stream. Low HO2S voltage indicates a lean exhaust stream.
Throttle Actuator Control System
The Throttle Actuator Control (TAC) system delivers improved throttle response and greater reliability and eliminates the need for mechanical cable. The TAC system performs the accelerator pedal position sensing, throttle positioning to meet driver and engine demands, throttle position sensing, internal diagnostics, cruise control functions and manage TAC electrical power consumption.
The TAC system components include the Accelerator Pedal Position (APP) sensors, throttle body assembly, throttle actuator control module and the PCM.
Throttle Position Sensor
The Throttle Position (TP) sensor is used by the PCM to determine the throttle plate angle for various engine management systems. TP sensor is mounted on throttle body assembly. (Scheme 1) The TP sensor is a potentiometer type sensor with a 5-volt reference circuit, a low reference circuit and a sensor signal circuit. PCM provides the TP sensor with 5 volts on the 5-volt reference circuit and a ground on the low reference circuit. Rotation of the TP sensor rotor from the closed throttle position to Wide Open Throttle (WOT) position provides the PCM with a signal voltage from less than one volt to greater than 4 volts through the TP sensor signal circuit. When TP sensor signal voltage is not within the predicted range, a DTC sets. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.
Transmission Fluid Pressure Manual Valve Position Switch
The Transmission Fluid Pressure (TFP) manual valve position switch consists of 5 pressure switches (2 normally closed and 3 normally open) on the control valve body that sense whether fluid pressure is present in 5 different valve body passages. The combination of switches that are open and closed is used by the PCM in order to determine the actual manual valve position. The TFP manual valve position switch, however, cannot distinguish between PARK and NEUTRAL because the monitored valve body pressures are identical in both cases. The switches are wired to provide 3 signal lines that are monitored by the PCM. These signals are used to help control line pressure, torque converter clutch apply and shift solenoid valve operation. Voltage at each of the signal lines is either zero or 12 volts. The TFT sensor is part of the TFP manual valve position switch assembly.
Transmission Fluid Temperature Sensor
The Transmission Fluid Temperature (TFT) sensor is a thermistor (temperature sensitive resistor) and is part of the Transmission Fluid Pressure (TFP) manual valve position switch. The PCM supplies and monitors a 5-volt signal to TFT sensor. This monitored 5-volt signal is then modified by resistance of TFT sensor. When transmission fluid temperatures are low, TFT sensor resistance is high and PCM sees a high monitored voltage signal. When transmission fluid temperatures are high, TFT sensor resistance is low and PCM sees a low monitored voltage.
PCM uses TFT sensor input to control Torque Converter Clutch (TCC) application and shift quality. Sensor circuit problem should set a related DTC.
Transmission Range Switch
The Transmission Range (TR) switch is part of the Park/Neutral Position (PNP) and backup light switch assembly, which is externally mounted on the transmission manual shaft. The TR switch contains 4 internal switches that indicate the transmission gear range selector lever position. The PCM supplies ignition voltage to each switch circuit. As the gear range selector lever is moved, the state of each switch may change, causing the circuit to open or close. An open circuit or switch indicates a high voltage signal. A closed circuit or switch indicates a low voltage signal. The PCM detects the selected gear range by deciphering the combination of the voltage signals. PCM compares actual voltage combination of the switch signals to a TR switch combination chart stored in memory.
Vehicle Speed Sensor
The Vehicle Speed Sensor (VSS) is a Permanent Magnet (PM) generator. VSS is mounted in transmission tailshaft. The VSS sends a pulsing signal to the PCM. The PCM then converts this signal into miles per hour by monitoring the time interval between pulses. PCM uses this sensor input in controlling Torque Converter Clutch (TCC) engagement, shift speed, etc.
A/C Clutch Relay
Cruise Control
Electronic Ignition
See IGNITION SYSTEMS .
EVAP Canister
See EVAP CANISTER under EMISSION SYSTEMS & SUB-SYSTEMS.
EVAP Canister Purge Solenoid
See EVAP CANISTER PURGE SOLENOID under EMISSION SYSTEMS & SUB-SYSTEMS.
EVAP Canister Vent Solenoid
See EVAP CANISTER VENT SOLENOID under EMISSION SYSTEMS & SUB-SYSTEMS.
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEMS.
Fuel Pump & Fuel Pump Relay
See FUEL DELIVERY under FUEL SYSTEMS.
See FUEL TANK PRESSURE SENSOR under EMISSION SYSTEMS & SUB-SYSTEMS.
Malfunction Indicator Light
See SELF-DIAGNOSTIC SYSTEM .
Self-Diagnostics
See SELF-DIAGNOSTIC SYSTEM .
Serial Data
See SELF-DIAGNOSTIC SYSTEM .
Shift Solenoids (Electronic Transmission)
See MISCELLANEOUS CONTROLS .
See IDLE SPEED under FUEL SYSTEMS.
Torque Converter Clutch
See MISCELLANEOUS CONTROLS .
Fuel Pressure Regulator
Fuel pressure regulator is a diaphragm-operated relief valve with injector pressure on one side and manifold pressure (vacuum) on the other. Pressure regulator maintains a pressure of 55-62 psi (385-425 kPa) under all operating conditions. Pressure regulator compensates for engine load by increasing fuel pressure when low manifold vacuum is experienced.
Fuel Pump
An in-tank, electric fuel pump delivers fuel to injector(s) through an in-line fuel filter. The pump is designed to supply fuel pressure in excess of vehicle requirements. The pressure relief valve controls maximum fuel pump pressure. Pressure regulator keeps fuel available to injector(s) at a constant pressure. Excess fuel is returned to fuel tank through pressure regulator return line.
When ignition switch is turned to ON position, PCM turns on electric fuel pump by energizing fuel pump relay. PCM keeps pump on if engine is running or cranking (PCM is receiving reference pulses from ignition module). If there are no reference pulses, PCM turns pump off.
Fuel Pump Relay
When ignition switch is turned to ON position, PCM turns electric fuel pump on by energizing fuel pump relay. PCM keeps relay energized if engine is running or cranking (PCM is receiving reference pulses from ignition module). If there are no reference pulses, PCM turns pump off within 2-20 seconds after key on.
FUEL CONTROL
The PCM, using input signals, determines adjustments to the air/fuel mixture to provide the optimum ratio for proper combustion under all operating conditions. Fuel control systems can operate in the open loop or closed loop mode.
Closed Loop Mode
When HO2S reaches operating temperature, coolant temperature reaches a preset temperature and a specific period of time has passed since engine start-up, PCM operates in closed loop mode. In closed loop mode, PCM controls air/fuel ratio based upon HO2S signals (in addition to other input parameters) to maintain as close to a 14.7:1 air/fuel ratio as possible. If HO2S cools off (due to excessive idling) or a fault occurs in HO2S circuit, vehicle will re-enter open loop mode.
Fuel System Operating Modes
Internal PCM calibration controls fuel delivery during starting, clear flood mode, deceleration and heavy acceleration.
- Starting With ignition switch in the ON position, before engaging starter, the PCM energizes the fuel pump relay for 2seconds allowing fuel pump to build pressure. Speed density is determined by inputs from the engine RPM, the IAT and MAP sensors. The PCM first tests speed density, then switches to the MAF sensor. PCM also uses the ECT, TP and 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 width of the fuel injector pulse.
- Clear Flood If the engine floods, clear the engine by pushing accelerator pedal down to the floor and then crank engine. When TP sensor is at Wide Open Throttle (WOT), the PCM reduces the injector pulse width in order to increase the air-to-fuel ration. The PCM maintains injector rate as long as the throttle stays wide open and engine speed is below a predetermined RPM. If throttle is not held wide open, PCM returns to the starting mode.
- Run Mode The run mode has 2conditions. These conditions are called open loop and closed loop. When engine is first started and engine speed is above a predetermined RPM, the system begins open loop operation. The PCM ignores the signal from the 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 the following conditions are met: Both HO2Ss have varying voltage output, showing that they are hot enough to operate properly. This depends upon engine temperature. The ECT sensor is above a specified temperature. A specific amount of time has elapsed after starting the engine. Specific values for the above conditions exist for each different engine. These values 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, fuel injector ON time, based upon the signal from various sensors, but mainly form the HO2S. This allows the air/fuel ratio to stay very close to 14.7:1.
- Acceleration When the driver pushes on the accelerator pedal, the air flow into the cylinders increases rapidly, while fuel flow tends to lag behind. In order to prevent possible hesitation, the PCM increases the pulse width to the fuel injectors in order to provide extra fuel during acceleration. The PCM determines amount of fuel required based upon the throttle position, coolant temperature, MAP, MAF and engine speed.
- Deceleration When the driver releases the accelerator pedal, the air flow into the engine is reduced. The PCM reads the corresponding changes in the TP, MAP and MAF sensors. The PCM shuts OFF fuel completely if deceleration is very rapid or for long periods, such as during a long, closed-throttle coast-down. The fuel shuts OFF in order to protect the TWC.
- 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 When ignition is turned off, injectors are de-energized to prevent dieseling. Injectors are not energized if RPM reference pulses are not received by the PCM, even with ignition on. This prevents flooding before starting. Fuel cut-off also occurs at high engine RPM or excessive vehicle speed to prevent internal damage to engine. Fuel injector signals are cut off during periods of sudden, closed throttle deceleration (when fuel is not needed).
Open Loop Mode
When engine is cold and engine speed is greater than 400 RPM, PCM operates in open loop mode. In open loop mode, PCM calculates air/fuel ratio based upon coolant temperature and MAP sensor readings. Engine remains in open loop mode until oxygen sensor reaches operating temperature, coolant temperature reaches a preset temperature and a specific period of time has elapsed after engine starts.
Sequential Fuel Injection System
Fuel injectors are pulsed sequentially in spark plug firing order. 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 depending upon engine operating conditions. PCM senses engine operating conditions and determines best idle speed. Idle speed is controlled through Throttle Actuator Control (TAC) system. If engine idle speed is out of range for a calibrated period of time, an idle speed related DTC will set. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.
ELECTRONIC IGNITION SYSTEM
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 consists of a separate ignition coil connected to each spark plug by a short secondary wire. The driver modules within each coil assembly are commanded ON/OFF by the PCM. The PCM primarily uses engine speed and position information from the CKP and CMP sensors to control the sequence, dwell, and timing of the spark.
The EI system consists of Crankshaft Position (CKP) sensor, Camshaft Position (CMP) sensor, ignition coils, secondary ignition components and portion of the PCM.
IGNITION TIMING CONTROL
Ignition spark timing and ignition dwell time are controlled entirely by the PCM. The PCM monitors information from various engine sensors, computes the desired spark timing and dwell, and firing of the ignition coil via ignition control circuit to the coil driver.
CATALYTIC CONVERTER
A Three-Way Catalytic (TWC) converter is used to reduce exhaust emissions. This type of converter can reduce Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides Of Nitrogen (NOx).
EVAPORATIVE EMISSION SYSTEM
The Evaporative Emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid to atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid ON (open), allowing engine vacuum to be applied to EVAP canister. With EVAP vent solenoid OFF (open), fresh air will be drawn through the solenoid and vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.
The canister is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapor is stored in the canister until the control module determines that the vapor can be consumed in the normal combustion process. EVAP canister is located near fuel tank. (Scheme 9)
Scheme 9
The EVAP canister purge solenoid controls the flow of vapors from the EVAP system to the intake manifold. EVAP canister purge solenoid is located on top of intake manifold. (Scheme 1) This normally closed solenoid is Pulse Width Modulated (PWM) by the control module to precisely control the flow of fuel vapor to engine. The solenoid will also be opened during some portions of EVAP testing, allowing engine vacuum to enter the EVAP system.
The EVAP canister vent solenoid controls fresh air flow into the EVAP canister. EVAP canister vent solenoid is mounted on frame rail, near fuel tank. (Scheme 9) The solenoid is normally open. The control module will command the solenoid closed during some EVAP tests, allowing the system to be tested for leaks.
The Fuel Tank Pressure (FTP) sensor measures the difference between the pressure or vacuum in the fuel tank and outside air pressure. The control module 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. A high FTP sensor voltage indicates a low fuel tank pressure or vacuum. A low FTP sensor voltage indicates a high fuel tank pressure.
POSITIVE CRANKCASE VENTILATION
The Positive Crankcase Ventilation (PCV) system provides effective evacuation 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 the PCV valve and 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 (according to manifold vacuum) of the blow-by vapors. When manifold vacuum is high (at idle), the PCV valve restricts the flow to maintain a smooth idle.
Under conditions in which abnormal amounts of blow-by gases are produced (such as worn cylinders or rings), system is designed to allow excess gases to flow back through crankcase vent hose into air inlet.
Spring pressure holds PCV valve closed when engine is not running. This prevents hydrocarbon fumes from collecting in the intake manifold, a condition which could result in hard starting.
During engine operation, manifold vacuum pulls the valve closed against spring pressure. As vacuum decreases with increased engine load, spring pressure begins to overpower vacuum strength. This allows PCV valve to open proportional to engine load and evacuation requirements. Should the engine backfire, the PCV valve closes to prevent ignition of fumes in the crankcase.
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 MIL located on instrument cluster. When a malfunction is detected and MIL is turned on, a corresponding DTC will be stored in PCM memory. Malfunctions are designated as either "emission related" or as "non-emission related", and are divided into 4 code types to identify type of fault. The 4 code types are defined as follows
- Type "A" Emission related faults that illuminate MIL at first occurrence of a fail condition. PCM records the operating conditions at the time diagnostic failed into Failure Records and Freeze Frame.
- Type "B" Emission related faults that illuminate MIL if a fault occurs in 2 consecutive ignition cycles. PCM records the operating conditions at the time diagnostic failed the first time into Failure Records and Freeze Frame. When diagnostic fails a second time, PCM records the operating conditions at the time diagnostic failed into Freeze frame and updates Failure Records.
- Type "C" Non-emission related faults that do not illuminate MIL but the DTC will be recorded in memory. PCM records the operating conditions at the time diagnostic failed into Failure Records, no Freeze Frame information will be saved. Driver information center (if equipped) may display a message.
A current DTC clears when the diagnostic runs and passes. A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other diagnostic. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES - 4.8L, 5.3L & 6.0L "C", "G", "H", "K" & "N" SERIES- FLEX FUEL & GASOLINE article.
As a bulb and system check, the MIL will illuminate when ignition switch is turned to ON 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. MIL may be used on some models to display a stored DTC. To access DTCs, see SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.
PCM has a serial data line. Serial data is a stream of electrical impulses which can be exchanged between control modules. Serial data can be interpreted using a special scan tool. Access serial data by connecting a scan tool to DLC. Update intervals and information contained within data stream vary with model application.
MISCELLANEOUS CONTROLS
Note. Although not considered true engine performance-related systems, some devices may affect driveability if they malfunction.
A/C CLUTCH
PCM regulates operation of the A/C clutch through a relay. The PCM disengages 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 to less than or rises to greater than normal operating levels.
Refrigerant pressure is sensed through the monitoring of high and/or low pressure switch(es) or a pressure sensor which registers either high or low pressure levels. Hot restart is monitored through the Engine Coolant Temperature (ECT) sensor. For component application and related wiring, see appropriate A/C COMPRESSOR CLUTCH CONTROLS article in AIR CONDITIONING & HEATING.
A/C Pressure Switches
A/C high and low pressure switches may be used in the A/C compressor clutch or compressor clutch relay circuit. Switches are normally closed, completing the circuit which energizes the compressor clutch. When system refrigerant pressure increases beyond a certain point, high side switch opens, causing compressor clutch to disengage.
If system refrigerant level decreases (causing refrigerant pressure to drop), low side pressure switch opens, preventing compressor damage by causing compressor clutch to disengage.
On models equipped with cruise control, the system is operated by the PCM. PCM receives inputs from Vehicle Speed Sensor (VSS), servo diaphragm position sensor, cruise control switch and brake release switch. Based on these inputs, PCM controls position of cruise control stepper motor. PCM prevents system engagement at speeds of less than 25 MPH. PCM is not serviceable; if defective, it must be replaced. A system fault is stored as a DTC in PCM memory.
The transmission Torque Converter Clutch (TCC) eliminates power loss of torque converter stage when vehicle is in a cruise condition, allowing driver the convenience of an automatic transmission while providing the fuel economy of a manual transmission.
The 2nd, 3rd and 4th gear hydraulic apply switches (located within transmission) may also be in series with solenoid power or ground circuit. Switch status may only be monitored by PCM, without sharing power or ground with TCC solenoid. For wiring reference, see appropriate DIAGNOSIS article in AUTOMATIC TRANSMISSIONS.
The TCC engages when vehicle is moving faster than a pre-calibrated speed, engine is at normal operating temperature, TP sensor output is not changing (indicating a steady road speed) and transmission 3rd gear or high gear switch (if equipped) and brake switch are closed.
When vehicle speed is great enough (about 20-45 MPH as indicated by vehicle speed sensor), PCM energizes TCC solenoid mounted in transmission, allowing torque converter to directly connect engine to the transmission. When operating conditions indicate transmission should operate as normal, TCC solenoid is de-energized, allowing transmission to return to normal automatic operation. Since power for the TCC solenoid is delivered through the brake switch, transmission also returns to normal automatic operation when brake pedal is depressed.
Electronic Transmission
PCM controls transmission and other vehicle functions. PCM monitors a number of engine/vehicle functions and uses data to control shift solenoid valves and TCC solenoid. PCM also regulates TCC engagement, upshift pattern, downshift pattern and line pressure (shift quality).
- 1-2 & 2-3 Shift Solenoid Valves The 1-2 and 2-3 shift solenoid valves (also called "A" and "B" solenoids) are identical devices that control the movement of the 1-2 and 2-3 shift valves (the 3-4 shift valve is not directly controlled by a shift solenoid). The solenoids are normally-open exhaust valves that work in four combinations to shift the transmission into different gears. PCM energizes each solenoid by grounding the solenoid through an internal quad driver. This sends current through the coil winding in the solenoid and moves the internal plunger out of the exhaust position. When on, the solenoid redirects fluid to move a shift valve. The PCM controlled shift solenoids eliminate the need for TV and governor pressures to control shift valve operation.
- 3-2 Shift Solenoid Valve The 3-2 shift solenoid valve assembly is a normally closed, 3-port, ON/OFF device that is used in order to improve the 3-2 downshift. The solenoid regulates the release of the 3-4 clutch and the 2-4 band apply.
- Transmission Pressure Control Solenoid The transmission pressure control solenoid is an electronic pressure regulator that controls pressure based on the current flow through its coil winding. The magnetic field produced by the coil moves the solenoid's internal valve which varies pressure to the pressure regulator valve. PCM controls the pressure control solenoid by commanding current between 0.1-1.1 amps. This changes the duty cycle of the solenoid, which can range between 5-95 percent (typically less than 60 percent). High amperage (1.1 amps) corresponds to minimum line pressure, and low amperage (0.1 amp) corresponds to maximum line pressure (if the solenoid loses power, the transmission defaults to maximum line pressure). PCM commands the line pressure values, using inputs such as engine speed and throttle position sensor voltage. The pressure control solenoid takes the place of the throttle valve or the vacuum modulator.
- Torque Converter Clutch Solenoid Valve The Torque Converter Clutch (TCC) solenoid valve is a normally-open exhaust valve that is used to control torque converter clutch apply and release. When grounded (energized) by the PCM, the TCC solenoid valve stops converter signal oil from exhausting. This causes converter signal oil pressure to increase and move the TCC solenoid valve into the apply position.
- TCC Pulse-Width Modulation Solenoid Valve The Torque Converter Clutch Pulse Width Modulation (TCC PWM) solenoid valve controls the fluid acting on the converter clutch valve. The converter clutch valve controls the TCC apply and release. This solenoid is attached to the control valve body assembly within the transmission. The TCC PWM solenoid valve provides a smooth engagement of the torque converter clutch by operating during a duty cycle percent of ON time.
- Transmission Fluid Pressure Manual Valve Position Switch The Transmission Fluid Pressure (TFP) manual valve position switch consists of 5 pressure switches (2 normally closed and 3 normally open) on the control valve body that sense whether fluid pressure is present in 5 different valve body passages. The combination of switches that are open and closed is used by the PCM in order to determine the actual manual valve position. The TFP manual valve position switch, however, cannot distinguish between PARK and NEUTRAL because the monitored valve body pressures are identical in both cases. The switches are wired to provide 3 signal lines that are monitored by the PCM. These signals are used to help control line pressure, torque converter clutch apply and shift solenoid valve operation. Voltage at each of the signal lines is either zero or 12 volts. In order to monitor the TFP manual valve position switch operation, the PCM compares the actual voltage combination of the switches to a TFP combination table stored in its memory. The Transmission Fluid Temperature (TFT) sensor is part of the TFP manual valve position switch assembly.
- Transmission Fluid Temperature Sensor The automatic Transmission Fluid Temperature (TFT) sensor is part of the automatic Transmission Fluid Pressure (TFP) manual valve position switch. The TFT sensor is a resistor or thermistor, which changes value based on temperature. The sensor has a negative-temperature coefficient. This means that as the temperature increases, resistance decreases and as temperature decreases, resistance increases. PCM supplies a 5-volt reference signal to the TFT sensor and measures voltage drop in the circuit. When transmission fluid is cold, sensor resistance is high and PCM detects high signal voltage. As fluid temperature warms to a normal operating temperature, resistance becomes less and signal voltage decreases. PCM uses the TFT sensor information to control shift quality and TCC application.
- Transmission Range Switch The Transmission Range (TR) switch is part of the Park/Neutral Position (PNP) and backup light switch assembly, which is externally mounted on the transmission manual shaft. The TR switch contains four internal switches that indicate the transmission gear range selector lever position. PCM supplies ignition voltage to each switch circuit. As the gear range selector lever is moved, the state of each switch may change, causing the circuit to open or close. An open circuit or switch indicates a high voltage signal. A closed circuit or switch indicates a low voltage signal. PCM detects the selected gear range by deciphering the combination of the voltage signals. PCM compares the actual voltage combination of the switch signals to a TR switch combination chart stored in memory.