Speed Density
Engine is equipped with a Manifold Absolute Pressure (MAP) sensor and an Intake Air Temperature (IAT) sensor. (Scheme 1)
The Manifold Absolute Pressure (MAP) sensor responds to pressure changes in the intake manifold. The pressure changes occur based on the engine load. The MAP sensor has a 5-volt reference circuit, low reference circuit and MAP sensor signal circuit.
The Powertrain Control Module (PCM) supplies 5 volts to the 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. The PCM should detect a low signal voltage at a low MAP, such as during an idle or a deceleration. The PCM should detect a high signal voltage at a high MAP, such as 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 switch is turned 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. If PCM detects an out-of-range signal voltage, a DTC will set. To diagnose, go to SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.
The Intake Air Temperature (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, go to SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT 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 right front of engine. (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. 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.
PCM monitors the A/C refrigerant pressure sensor signal circuit. The voltage signal on this circuit is proportional to the refrigerant pressure inside the A/C high side pressure line. As the pressure inside the line increases, so does the voltage signal. If the pressure is greater than 350 psi (2413 kPa), the A/C compressor output is disabled. When pressure drops to 229 psi (1578 kPa), the PCM enables the compressor to operate.
Accelerator Pedal Position Sensor
The Accelerator Pedal Position (APP) sensors 1 and 2 are located within the accelerator pedal assembly. (Scheme 3) Each sensor has a 5-volt reference circuit, low reference circuit and a signal circuit. This provides the PCM with a signal voltage proportional to accelerator pedal movement. The APP sensor 1 signal voltage at rest position is about zero volts and increases as the pedal is actuated. The APP sensor 2 signal voltage at rest position is about 5 volts and decreases as pedal is actuated. When APP sensor 1 signal voltage is not within the specified range, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.
Scheme 3
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, most models), 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 triggered by a notched reluctor wheel built into the exhaust camshaft sprocket. The CMP sensor is located on top front of engine. (Scheme 4) The CMP sensor provides 6 signal pulses every camshaft revolution. Each notch or feature of the reluctor wheel is of a different size for individual cylinder identification. This means the CMP and CKP signals are pulse-width encoded to enable the PCM to constantly monitor their relationship. This relationship is used to determine camshaft actuator position and control its phasing at the correct value. The PCM also uses this signal to identify the compression stroke of each cylinder and for sequential fuel injection. The CMP sensor is connected to the PCM by a 12-volt reference, a low reference, and a signal circuit.
Scheme 4
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 Crankshaft Position (CKP) sensor utilizes a pick-up coil-type sensor mounted on lower rear side of engine block. (Scheme 5) The CKP sensor monitors crankshaft position and sends signals to ignition control module. These signals provide PCM with a Top Dead Center (TDC) position reference for each piston, as well as supplying an engine speed (RPM) signal. This allows PCM to fire appropriate ignition coil at the proper time, determine triggering of the fuel injectors, and to compute crankshaft position and RPM. CKP sensor signal may also used to detect a cylinder misfire by monitoring changes in crankshaft speed. For additional information, see IGNITION SYSTEMS.
Scheme 5
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 top front of cylinder head, near PCM. (Scheme 2) 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, see SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.
Fuel Pump Feedback
PCM monitors fuel pump circuit between fuel pump relay/oil pressure switch and fuel pump. This enables the PCM to determine if fuel pump is being energized by fuel pump relay or back-up oil pressure switch. A failure in this monitored circuit results in the setting of a related DTC in PCM memory.
Intake Air Temperature Sensor
The Intake Air Temperature (IAT) sensor is a thermistor (temperature sensitive resistor) mounted in the fresh air intake tube. (Scheme 1) 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 pull-down resistor in PCM.
IAT sensor allows PCM to determine intake air temperature. If intake air temperature becomes excessively high, PCM compensates by slightly retarding ignition timing. After a vehicle has cooled overnight, IAT and ECT sensor signals (resistance and temperature) should be close to the same reading. Failure in IAT sensor circuit should set a related DTC. To diagnose, see SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.
Knock Sensor
The Knock Sensor (KS) is a piezoelectric device which detects abnormal engine vibrations (spark knock) in the engine. Engine is equipped with 2 knock sensors and are located on right side of engine block. (Scheme 5) This vibration results in the production of a very low AC signal, which is sent from the KS to the KS module (integral to PCM). The PCM then retards ignition timing until the engine knock ceases.
A fault in the KS circuit may set a DTC. See SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.
Manifold Absolute Pressure Sensor
The Manifold Absolute Pressure (MAP) sensor measures changes in manifold pressure. MAP sensor is located on top of intake manifold. (Scheme 1) 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 (1.5 volts at idle to about 4.5 volts at wide open throttle). The PCM can monitor these signals and adjust air/fuel ratio and ignition timing under various operating conditions.
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.
Heated Oxygen Sensor
| CAUTION | Measure Oxygen Sensor (O2S) 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) sensors 1 and 2 are located within the throttle body assembly. Each sensor has a 5-volt reference circuit, low reference circuit and a signal circuit. This provides the PCM with a signal voltage proportional to throttle plate movement. TP sensor 1 signal voltage at closed throttle is about 5-volt reference and decreases as the throttle plate is opened. TP sensor 2 signal voltage at closed throttle is about low reference and increases as the throttle plate is opened. When TP sensor 1 signal voltage is not within the predicted range, a DTC sets. To diagnose, ee SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.
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 on the transmission. (Scheme 6) 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 6
A/C Clutch Relay
See MISCELLANEOUS PCM CONTROLS .
Camshaft Actuator System
See EMISSION SYSTEMS .
Cruise Control
See MISCELLANEOUS PCM CONTROLS .
Electronic Ignition
See IGNITION SYSTEMS .
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 .
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 48-54 psi (334-375 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 fuel pump provides fuel at a higher rate of flow than is needed by the fuel injection system. 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 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.
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.
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 on Sequential Fuel Injection (SFI) system 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.
Throttle Actuator Control Motor
The Throttle Actuator Control (TAC) motor is controlled by the PCM. The DC motor located in the throttle body, drives the throttle plate. (Scheme 1) In order to decrease idle speed, the PCM commands the throttle closed reducing air flow into the engine and the idle speed decreases. In order to increase idle speed, the PCM commands the throttle plate open, allowing more air to pass the throttle plate. This system also performs the cruise control functions as well. If actual idle RPM does not match the desired idle RPM within a calibrated time, a DTC will set.
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 directly to each spark plug, known as coil on plug. These coil assemblies are located in the center of the camshaft cover. 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 crankshaft and camshaft position sensors to control the sequence, dwell, and timing of the spark.
The EI system consists of the Camshaft Position (CMP) sensor, Crankshaft Position (CKP) sensor, ignition coils and 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.
The Camshaft Position (CMP) actuator system is used for a variety of engine performance enhancements. These enhancements include lower emission output through exhaust gas recirculation control, a wider engine torque range, improved gas millage and improved engine idle stability. The CMP actuator system accomplishes this by controlling the amount of intake and exhaust valve overlap.
The CMP actuator system is controlled by the PCM. The PCM sends a pulse-width modulated 12 volt signal to a CMP actuator solenoid in order to control the amount of engine oil flow to a cam phaser passage. There are 2 different passages for oil to flow through a passage for cam advance and a passage for cam retard. The cam phaser is attached to a camshaft and is hydraulically operated in order to change the angle of the camshaft relative to crankshaft position. Engine oil pressure, viscosity, temperature and engine oil level can have an adverse effect on cam phaser performance. The PCM calculates the optimum cam position through the engine speed, MAP sensor, TP sensor indicated angle, CKP sensor, CMP sensor, engine load and BARO pressure.
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.
Scheme 7
EVAP Canister
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 Purge Solenoid
The EVAP purge solenoid controls the flow of vapors from the EVAP system to the intake manifold. 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.
EVAP Vent Solenoid
The EVAP vent solenoid controls fresh air flow into the EVAP canister. EVAP vent solenoid is mounted on fuel tank. (Scheme 7) 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.
Fuel Tank Pressure Sensor
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 rear of fuel tank. (Scheme 7) 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.
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 - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT 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.
COOLING FAN CONTROL
The cooling system uses an Electro-Viscous (EV) fan clutch is to maintain engine cooling requirements. The PCM monitors the following sensors to regulate the fan speed; ECT sensor, A/C refrigerant pressure sensor, VSS, IAT sensor, transmission fluid temperature sensor and ambient air temperature sensor. The PCM controls EV fan clutch engagement and regulates a 12-volt Pulse-Width Modulated (PWM) signal to the cooling fan relay. The PWM signal determines the ON time of the relay. As the commanded state of the fan clutch increases, so does the ON time of the relay. This ON time directly affects the amount of time the solenoid, which is internal to the fan clutch, is energized. When solenoid in fan clutch is energized, it opens the spring loaded valve and allows fluid to flow from the storage chamber to the fluid coupling of the cooling fan clutch, increasing fan speed. When solenoid is de-energized, the spring loaded valve closes and blocks the path of the fluid to the fluid coupling of the fan clutch, reducing fan speed.
The fan has the ability to create a feedback signal, so the PCM has an actual fan speed input. This is done with a hall effect sensor internal to the fan clutch. PCM supplies a 5-volt reference and a low reference to the hall effect sensor. The hall effect sensor returns a signal pulse through the cooling fan speed signal circuit in response to the reluctor track passing by the magnetic field of the hall effect sensor.
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.
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 (TFP) Manual Valve Position Switch The 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.