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.
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 6).
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 DTC, go to SELF-DIAGNOSTICS - 4.3L CHEVY EXPRESS & SAVANA 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 mounted on top radiator support. (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
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.
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. When the distributor shaft rotates, and the 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 DTC, see SELF-DIAGNOSTICS - 4.3L CHEVY EXPRESS & SAVANA article.
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 behind the harmonic balancer. (Scheme 3) 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 DTC, see SELF-DIAGNOSTICS - 4.3L CHEVY EXPRESS & SAVANA article.
Scheme 3
Engine Coolant Temperature Sensor
The Engine Coolant Temperature (ECT) sensor is a variable resistor, that measures the temperature of engine coolant. ECT is located on side of left cylinder head. (Scheme 4) The PCM supplies 5 volts to the ECT signal circuit and ground for the ECT 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 signal circuit. With lower sensor resistance, PCM detects a lower voltage on the ECT signal circuit. If PCM detects an out-of-range signal voltage, a DTC will set. To diagnose DTC, see SELF-DIAGNOSTICS - 4.3L CHEVY EXPRESS & SAVANA article.
Scheme 4
Fuel Tank Pressure Sensor
Intake Air Temperature Sensor
The Intake Air Temperature (IAT) sensor is a variable resistor. IAT sensor is located in fresh air intake tube. (Scheme 1) 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 DTC, see SELF-DIAGNOSTICS - 4.3L CHEVY EXPRESS & SAVANA article.
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 5) 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. See A article.
Scheme 5
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 6) 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 DTC, see SELF-DIAGNOSTICS - 4.3L CHEVY EXPRESS & SAVANA 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.
Scheme 6
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.
Park/Neutral Position Switch
The Park/Neutral Position (PNP) switch is connected to transmission gear selector and signals PCM when transmission is in Park or Neutral. PCM uses this information for determining control of ignition timing, Torque Converter Clutch (TCC) and idle speed.
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 5) 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 DTC, see SELF-DIAGNOSTICS - 4.3L CHEVY EXPRESS & SAVANA article.
Transmission Fluid Pressure Switch
The Transmission Fluid Pressure (TFP) switch is actually 5 pressure switches combined into a single unit mounted on transmission valve body. The PCM supplies battery voltage on 3 separate wires to TFP switch. As switches are actuated in various combinations during transmission operation, PCM can detect what gear transmission is in.
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 mounted in transmission. (Scheme 7) 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.
Scheme 7
A/C Clutch Relay
Cruise Control
Electronic Ignition
See IGNITION SYSTEMS .
EVAP Canister
See EVAP CANISTER under EVAPORATIVE EMISSION SYSTEM.
EVAP Purge Solenoid
See EVAP PURGE SOLENOID under EVAPORATIVE EMISSION SYSTEM.
EVAP Vent Solenoid
See EVAP VENT SOLENOID under EVAPORATIVE EMISSION SYSTEM.
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEMS.
Fuel Pump & Fuel Pump Relay
See FUEL DELIVERY under FUEL SYSTEMS.
Idle Air Control Valve
See IDLE SPEED under FUEL 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 .
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 (379-427 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 within 2 seconds after ignition is turned on.
A second control path through the oil pressure switch which will turn the fuel pump on after the switch detects oil pressure. Cranking time will be longer if fuel pump does not receive current until oil pressure switch contacts close.
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.
As a back-up system to fuel pump relay, the oil pressure switch also activates fuel pump. The oil pressure switch is normally open until oil pressure reaches about 4 psi (27.6 kPa). If fuel pump relay fails, the oil pressure switch closes when oil pressure is obtained and operates the fuel pump. Cranking time will be longer if fuel pump does not receive current until oil pressure switch contacts close. Oil pressure switch may be combined into a single unit with an oil pressure gauge sending unit or sensor.
PCM monitors fuel pump circuit between fuel pump relay/oil pressure switch and fuel pump, enabling PCM to determine if fuel pump is being energized by fuel pump relay or oil pressure switch. A failure in this monitored circuit results in the setting of a related diagnostic trouble code in PCM memory.
For additional information on fuel pump activation, see SYSTEM & COMPONENT TESTING - 4.3L CHEVY EXPRESS & SAVANA article.
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 During engine starts, PCM delivers one injector pulse for each distributor reference pulse received (synchronized mode). Injector pulse width is based upon coolant temperature and throttle position. PCM determines air/fuel ratio when throttle position is less than 80 percent open. Engine starting air/fuel ratio ranges from 0.8:1 at -40°F (-40°C) to 16.8:1 at 230°F (110°C). At lower coolant temperatures, injector pulse width is wider (richer air/fuel mixture ratio). When coolant temperature is high, injector pulse width becomes narrower (leaner air/fuel ratio).
- Clear Flood If engine is flooded, driver must depress accelerator pedal to Wide Open Throttle (WOT) position. At this position, PCM adjusts injector pulse width equal to an air/fuel ratio of 16.5:1. This air/fuel ratio is maintained as long as throttle remains in wide open position and engine speed is less than 600 RPM. If throttle position becomes less than 65 percent open and/or engine speed exceeds 600 RPM, PCM changes injector pulse width to that used during engine starting (based upon coolant temperature and manifold vacuum).
- Heavy Acceleration PCM provides fuel enrichment during heavy acceleration. Sudden opening of throttle valve causes rapid increase in MAP signal. Pulse width is directly related to MAP, throttle position and coolant temperature. Higher MAP and wider throttle angles give wider injector pulse width (richer mixture). During enrichment, injector pulses are not in proportion to distributor reference signals (non-synchronized). Any reduction in throttle angle cancels fuel enrichment.
- Deceleration During normal deceleration, fuel output is reduced. This reduction in available fuel serves to remove residual fuel from intake manifold. During sudden deceleration, when MAP, throttle position and engine speed are reduced to preset levels, fuel flow is cut off completely. This deceleration fuel cut-off overrides normal deceleration mode. During either deceleration mode, injector pulses are not in proportion to distributor reference signals.
- 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. Some models may also cut off fuel injector signals 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.
The engine idle speed is controlled by the Idle Air Control (IAC) valve. IAC valve is mounted on throttle body. IAC valve pintle moves in and out of an idle air passage bore to control air flow around the throttle plate. The IAC valve consists of a movable pintle, driven by a gear attached to an electric motor called a stepper motor. The stepper motor is capable of highly accurate rotation or of movement, called steps. The stepper motor has 2 separate windings that are called coils. Each coil is supplied current by two circuits from the PCM. When PCM changes polarity of a coil, the stepper motor moves one step. The PCM uses a predetermined number of counts to determine the IAC pintle position. Observe IAC counts with a scan tool. The IAC counts will increment up or down as the PCM attempts to change the IAC valve pintle position. An IAC Reset will occur when the ignition is turned off. First, the PCM will seat the IAC pintle in the idle air passage bore. Second, the PCM will retract the pintle a predetermined number of counts to allow for efficient engine start-up. If engine idle speed is out of range for a calibrated period of time, an idle speed related DTC will set. To diagnose DTC, see SELF-DIAGNOSTICS - 4.3L CHEVY EXPRESS & SAVANA article.
DISTRIBUTOR IGNITION SYSTEM
The Distributor Ignition (DI) 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 single ignition coil and Ignition Control Module (ICM). Spark energy is delivered via a distributor cap, rotor, and secondary spark plug wires. The driver module within the ICM is commanded to operate the coil by the PCM, that has complete control over spark timing.
The DI system consists of Crankshaft Position (CKP) sensor, Camshaft Position (CMP) sensor, ignition coil and ICM, 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 EVAP canister is mounted in front of fuel tank. 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.
The EVAP purge solenoid controls the flow of vapors from the EVAP system to the intake manifold. EVAP purge solenoid is located on top of intake manifold. (Scheme 6) 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 vent solenoid controls fresh air flow into the EVAP canister. EVAP vent solenoid is mounted on frame rail, near fuel tank. (Scheme 8) 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.
Scheme 8
The Fuel Tank Pressure Sensor (FTP) sensor measures the difference between the pressure or vacuum in fuel tank and outside air pressure. The FTP sensor is located on top of fuel tank. The control module provides a 5-volt reference and a ground to FTP sensor. The FTP sensor provides a signal voltage back to the control module that can vary between 0.1-4.9 volts. As FTP increases, FTP sensor voltage decreases, high pressure equals low voltage. As FTP decreases, FTP voltage increases, low pressure or vacuum equals high voltage.
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 TROUBLE SHOOTING - NO CODES - 4.3L CHEVY EXPRESS & SAVANA 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 - 4.3L CHEVY EXPRESS & SAVANA 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 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. To check function of TCC system, perform functional check of system. See MISCELLANEOUS CONTROLS in SYSTEM & COMPONENT TESTING - 4.3L CHEVY EXPRESS & SAVANA article.
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 4 combinations to shift transmission into different gears. The 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. The PCM controls the pressure control solenoid by commanding current of 0.1-1.1 amps. This changes the duty cycle of the solenoid, which can range 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). The PCM commands the line pressure values, using inputs such as engine speed and TP sensor voltage. The pressure control solenoid takes the place of the throttle valve or the vacuum modulator that was used on past model transmissions.
- Torque Converter Clutch Solenoid Valve The 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 TCC 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 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 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.