Engine Control System - 4.2L - Introduction: Overview GMC Envoy II

Engine Controls Schematic Icons

Engine Controls Schematic Icons Icon Icon Definition NOTE: The OBD II symbol is used on the circuit diagrams in order to alert the technician that the circuit is essential for proper OBD II emission control circuit operation. Any circuit which fails and causes the malfunction indicator lamp (MIL) to turn ON, or causes emissions-related component damage, is identified as an OBD II circuit. IMPORTANT: Twisted-pair wires provide an effective shield that helps protect sensitive electronic components from electrical interference. If the wires were covered with shielding, install new shielding. In order to prevent electrical interference from degrading the performance of the connected components, you must maintain the proper specification when making any repairs to the twisted-pair wires shown : The wires must be twisted a minimum of 9 turns per 31 cm (12 in) as measured anywhere along the length of the wires The outside diameter of the twisted wires must not exceed 6.0 mm (0.25 in)

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Malfunction Indicator Lamp (MIL) Operation

The malfunction indicator lamp (MIL) is located in the instrument panel cluster. The MIL will display as either SERVICE ENGINE SOON or one of the following symbols when commanded ON

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The MIL indicates that an emissions related fault has occurred and vehicle service is required.

The following is a list of the modes of operation for the MIL

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

Throttle Actuator Control (TAC) Overview

The throttle actuator control (TAC) system uses vehicle electronics and components to calculate and control the position of the throttle blade. This eliminates the need for a mechanical cable attachment from the accelerator pedal to the throttle body. This system also performs the cruise control functions as well.

The TAC system components include, but are not limited to the following

1. The accelerator pedal position (APP) sensors
2. The throttle body
3. The powertrain control module (PCM)

Each of these components interface together to ensure accurate calculations and control of the throttle position (TP).

CMP Actuator System Operation

The CMP Actuator system is controlled by the Powertrain Control Module (PCM). The PCM sends a pulse width modulated 12 volt signal to a Camshaft Position (CMP) Actuator Solenoid in order to control the amount of engine oil flow to a cam Phaser passage. There are two 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 affect on cam phaser performance. The PCM calculates the optimum cam position through the following inputs

1. Engine Speed
2. Manifold Absolute Pressure (MAP)
3. Throttle Position (TP) Indicated Angle
4. Crankshaft Position (CKP)
5. Camshaft Position (CMP)
6. Engine Load.
7. Barometric (BARO) Pressure

The Cam Phaser default position is 0 degrees. The PCM uses the following inputs before assuming control of the cam phaser

1. Engine Coolant Temperature (ECT)
2. Closed Loop Fuel Control
3. Engine Oil Temperature
4. Engine Oil Pressure
5. Engine Oil Level
6. CMP Actuator Solenoid circuit state
7. Ignition 1 signal voltage
8. Barometric (BARO) Pressure

Fuel System Overview

The fuel system is a returnless on-demand design. The fuel pressure regulator is a part of the fuel sender assembly, eliminating the need for a return pipe from the engine. A returnless fuel system reduces the internal temperature of the fuel tank by not returning hot fuel from the engine to the fuel tank. Reducing the internal temperature of the fuel tank results in lower evaporative emissions.

An electric turbine style fuel pump attaches to the fuel sender assembly inside the fuel tank. The fuel pump supplies high pressure fuel through the fuel filter and the fuel feed pipe to the fuel injection system. The fuel pump provides fuel at a higher rate of flow than is needed by the fuel injection system. The fuel pressure regulator, a part of the fuel sender assembly, maintains the correct fuel pressure to the fuel injection system. The fuel pump and sender assembly contains a reverse flow check valve. The check valve and the fuel pressure regulator maintain fuel pressure in the fuel feed pipe and the fuel rail in order to prevent long cranking times.

Fuel Metering Modes of Operation

The control module monitors voltages from several sensors in order to determine how much fuel to give the engine. The control module controls the amount of fuel delivered to the engine by changing the fuel injector pulse width. The fuel is delivered under one of several modes.

EVAP System Operation

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 canister vent solenoid valve 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 canister purge solenoid valve ON, open, allowing engine vacuum to be applied to the EVAP canister. With the EVAP canister vent solenoid valve OFF, open, fresh air will be drawn through the solenoid valve 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 canister purge solenoid valve into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

Electronic Ignition (EI) System Description

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 powertrain control module (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 following components

Modes of Operation

During normal operation the PCM controls all ignition functions. If either the CKP or CMP sensor signal is lost, the engine will continue to run because the PCM will default to a limp home mode using the remaining sensor input. As mentioned above, each coil is internally protected against damage from excessive voltage. If one or more coils were to fail in this manner, a misfiring condition would result. Diagnostic trouble codes are available to accurately diagnose the ignition system with a scan tool.

Sensor Description

This KS system uses one or two flat response two-wire sensors. The sensor uses piezo-electric crystal technology that produces an AC voltage signal of varying amplitude and frequency based on the engine vibration or noise level. The amplitude and frequency are dependent upon the level of knock that the KS detects. The control module receives the KS signal through a signal circuit. The KS ground is supplied by the control module through a low reference circuit.

The control module learns a minimum noise level, or background noise, at idle from the KS and uses calibrated values for the rest of the RPM range. The control module uses the minimum noise level to calculate a noise channel. A normal KS signal will ride within the noise channel. As engine speed and load change, the noise channel upper and lower parameters will change to accommodate the normal KS signal, keeping the signal within the channel. In order to determine which cylinders are knocking, the control module only uses KS signal information when each cylinder is near top dead center (TDC) of the firing stroke. If knock is present, the signal will range outside of the noise channel.

If the control module has determined that knock is present, it will retard the ignition timing to attempt to eliminate the knock. The control module will always try to work back to a zero compensation level, or no spark retard. An abnormal KS signal will stay outside of the noise channel or will not be present. KS diagnostics are calibrated to detect faults with the KS circuitry inside the control module, the KS wiring, or the KS voltage output. Some diagnostics are also calibrated to detect constant noise from an outside influence such as a loose/damaged component or excessive engine mechanical noise.

Secondary Air Injection (AIR) System Description

The Secondary Air Injection (AIR) System reduces exhaust emissions following initial engine start up. This occurs when the engine Start-up coolant temperature is between 3-50°C (37-122°F) and the intake air temperature (IAT) is more than 1°C (32°F). The AIR pump will operate until Closed Loop operation is achieved.

The powertrain control module (PCM) activates the AIR system by suppling a ground to the AIR pump relay and AIR solenoid relay simultaneously. This action closes the internal contacts of the AIR pump relay, energizing the AIR pump and also closes the internal contacts of the AIR solenoid relay, energizing the AIR solenoid, opening the shut-off valve. The AIR pump forces pressurized fresh air into the pipes/hoses and past the check valve into the exhaust manifold, accelerating catalyst operation. When the AIR system is inactive, the AIR shut-off valve prevents airflow in either direction.

The AIR system includes the following components

1. The AIR pump The AIR pump supplies pressurized, filtered air to the exhaust stream. The AIR pump is a turbine type pump which is permanently lubricated ad requires no periodic maintenance. The AIR pump for the 4.2 liter engine will draw a steady 35-40 amps under normal operation. The AIR pump has an internal circuit breaker to protect the pump from overheating. The circuit breaker is an integral part of the AIR pump.
2. The AIR shut-off valve The AIR shut-off valve has an electronic solenoid mounted on the valve. The resistance of the solenoid is 4-7 ohms and the current draw is 2-3.5 amps. The solenoid opens the shut-off valve when battery voltage is applied to the valve. Once opened, pressurized air from the AIR pump flows past the check valve and is directed into the bank 1 exhaust manifold through an outlet pipe. The shut-off valve prevents fresh air from being drawn into the exhaust manifold by providing a positive seal when the AIR system is inactive. The solenoid, valve, and outlet pipe are serviced as an assembly. IMPORTANT: An audible exhaust noise may be heard at the inlet of the shut-off valve, when the shut-off valve is opened and the AIR pump outlet hose is removed from the shut-off valve.
3. The AIR pump relay The AIR pump relay supplies high current and battery voltage to the AIR pump. The resistance of the AIR pump relay coil is 55- 68 ohms.
4. The AIR solenoid relay The AIR solenoid relay supplies high current and battery voltage to the AIR solenoid that is an integral part of the shut-off valve. The PCM commands the AIR solenoid relay ON by suppling a ground on the control circuit of the relay. The resistance of the AIR pump relay coil is 80-90 ohms.
5. The pipes and hoses The pipes/hoses carry the air from the AIR pump past the AIR shut-off valve, and into the exhaust manifold. A pipe connects the shut-off valve to the exhaust manifold. The AIR System also utilizes a hose to carry the filtered air to the inlet of the AIR pump.
6. The inlet filter The filter utilizes the air filter for the engine. This system draws filtered air directly from the air cleaner assembly.

Results of Incorrect Operation

The PCM can detect an AIR System airflow fault by monitoring the heated oxygen sensor (HO2S) bank 1 sensor 1 during normal engine operation. This is an active test. The PCM will command the AIR system ON during Closed Loop operation to perform this test. The active test will pass or fail based on the response from the HO2S 1. The active test consists of three tests run at 3-second intervals. A decreasing HO2S voltage parameter response indicates that the secondary AIR system is functioning properly. If the PCM does not detect a decreasing response from the HO2S 1, DTC P0410 will set.

Air Intake System Description

The air induction system provides contaminant filtration and is equipped with two resonators to filter unwanted induction noise. As the throttle plate is opened, air is drawn into the air filter housing. The air is pulled through the air filter element which provides maximum air filtration without restricting air flow. From the filter element, the air is drawn into the air cleaner outlet duct and past the first air resonator. This resonator is actually the upper half of the air filter housing. Air flows through the duct, past the intake air temperature (IAT), and into the second air resonator that is mounted directly on top of the engine. Air exiting the second resonator flows directly to the throttle body and into the engine.

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