INTRODUCTION
This article covers the basic description and operation of engine performance related systems and components. Read this article before working on unfamiliar systems. For component location (Scheme 23)
Scheme 23
POWERTRAIN CONTROL MODULE (PCM)
PCM is a digital computer that controls ignition timing, air/fuel ratio, emission control devices, cooling fan, charging system, idle speed, fuel pump and tachometer. The control unit is located in the engine compartment. PCM uses data from various input sources to control output devices in order to achieve optimum engine performance for all operating conditions.
PCM has voltage converters that convert battery voltage to regulated 5-volt and 8-volt outputs. The 8-volt output powers the distributor pick-up and crankshaft position sensor. The 5-volt output powers coolant temperature, charge temperature, fuel temperature, manifold absolute pressure and throttle position sensors.
FUEL INJECTOR DRIVER MODULE
Fuel injector driver module, located near PCM, supplies high current required for injector operation. PCM supplies low current fuel injector signal to fuel injector driver module.
SYSTEM COMPONENTS
The following engine components (as used on CNG vehicle) are identical to components used on gasoline vehicles. For component description, see THEORY/OPERATION - 5.2L GASOLINE article in the ENGINE PERFORMANCE section.
- Throttle Body
- Accelerator Linkage
- Manifold Absolute Pressure (MAP) Sensor
- Emission Systems (Except Evaporative System)
- Engine Coolant Temperature Sensor
- Intake Manifold
- Intake Air Temperature Sensor (5.2L)
- Throttle Position Sensor
- Idle Air Control Motor (Automatic Idle Speed)
- Crankshaft Position Sensor
- Automatic Shutdown Relay
- Oxygen Sensor
- Vehicle Speed Sensor
- Ignition System
- Air Conditioning Components
- Speed Control Components
- Overdrive Components
- Alternator (Generator)
- Park/Neutral Position Switch
- Brake Switch Controls
For service and component removal/installation procedures for shared components, see REMOVE/INSTALL/OVERHAUL - 5.2L GASOLINE article in the ENGINE PERFORMANCE section.
CNG fuel system uses following additional components.
- Unique Powertrain Control Module (PCM)
- CNG Fuel Injector Driver Module
- CNG Fuel Shutoff Solenoid Relay
- High-Pressure Fuel Fill Receptacle With An Internal One-Way Check Valve
- Fuel Filter Mounted In Fuel Pressure Regulator
- Unique CNG Fuel Injectors
- High-Pressure, Stainless Steel Seamless Fuel Tubes
- Stainless Steel, Double-Ferrule, Compression-Type Fuel Tube Fittings (Not At All Fuel Tube Connections Or Components)
- Fuel Rail Mounted Fuel Low-Pressure Sensor
- Unique Fuel Rail To Mount Fuel Injectors
- High-Pressure Fuel Cylinders
- Manual Fuel Control Valves (One On Each Fuel Cylinder)
- Manual (Mechanically Operated) 1/4 Turn Gas Shutoff Valve
- Fuel Rail Mounted Fuel Temperature Sensor
- Fuel Rail Mounted And Electrically Controlled Low-Pressure Fuel Shutoff Solenoid
- Fuel Pressure Regulator Mounted And Electrically Controlled High-Pressure Fuel Shutoff Solenoid
- Fuel Pressure Regulator Warmed By Engine Coolant
- Fuel Gauge High-Pressure Sensor Used To Operate Fuel Gauge
- Separate Frame Rail Mounted Check Valve
INPUT DEVICES
Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input usage on a specific model, see appropriate WIRING DIAGRAM in the WIRING DIAGRAM section at the end of this article. For description of components used in common with gasoline vehicles, see THEORY/OPERATION - 5.2L GASOLINE article in the ENGINE PERFORMANCE section. Available CNG input signals include following
Fuel Low-Pressure Sensor
Fuel low-pressure sensor monitors fuel pressure on the low-pressure side of the fuel system. Sensor is mounted in fuel rail inlet block. PCM uses fuel pressure sensor information to calculate injector timing.
Fuel Temperature Sensor
Fuel temperature sensor monitors fuel temperature. Sensor is mounted in fuel rail. PCM uses fuel temperature information to adjust air/fuel mixture and idle speed.
OUTPUT SIGNALS
Note. Each vehicle may be equipped with different combinations of computer-controlled components. The following components may NOT be used on all models. For theory and operation on each CNG output component, refer to the indicated system. For description of components used in common with gasoline vehicles, see THEORY/OPERATION - 5.2L GASOLINE article in the ENGINE PERFORMANCE section.
Automatic Idle Speed (AIS) Motor
See IDLE SPEED under FUEL SYSTEM.
Auto Shutdown (ASD) Relay
See MISCELLANEOUS CONTROLS .
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
Fuel Shutoff Solenoid Relay
See FUEL CONTROL under FUEL SYSTEM.
FUEL DELIVERY
Note. A decal indicating the location of the manual shutoff valve is affixed in front of the right rear wheelwell, at the base of the side panel. A label indicating the type of fill nozzle required for fueling and the date the fuel cylinders are required to be tested is located on the fuel filler door.
See AUTO SHUTDOWN (ASD) RELAY under MISCELLANEOUS CONTROLS.
Fuel Cylinders
High-pressure fuel cylinders are used to store natural gas at pressures up to approximately 3600 psi. Cylinders are mounted transversely behind rear axle and/or longitudinally along sides of vehicle. Each cylinder is equipped with its own manually operated fuel control valve and pressure relief device.
Fuel Cylinder Control Valves
Each fuel cylinder is equipped with its own manually operated fuel control valve. The fuel control valves are equipped with a pressure relief device. The pressure relief device is designed to release excess cylinder pressure to atmosphere if temperature increases to more than about 217°F (103°C).
Turn fuel control valve handle fully clockwise to stop gas flow. Turn fuel control valve handle fully counterclockwise to provide gas flow.
Fuel Fill Check Valves
Two different check valves are used with the CNG fuel system. One of the valves is located integrally with fuel fill receptacle. The other valve is located in fuel fill tube near top of left rear shock absorber. Both valves are used to keep fuel from escaping from fuel fill receptacle.
Fuel Injector Rail
A unique fuel injector rail is used with the CNG system. The rail can be disassembled for replacement of inlet blocks, connecting tubes and "O" rings.
Fuel Filter
The fuel filter, located in fuel pressure regulator, requires service only when a fuel contamination problem is suspected.
Fuel Pressure Regulator
Fuel pressure regulator is located under right side of vehicle on frame rail. It is used to lower fuel cylinder pressure from approximately 3600 psi down to approximately 90-140 psi for system operation.
The regulator contains a pressure relief device located on low-pressure side of regulator. It is routed (by a tube) to right side of vehicle. This pressure relief device will release excess pressure to atmosphere when low-pressure side fuel pressure exceeds 225 psi.
Pressure regulator is warmed using engine coolant routed through regulator from cooling system. Two hoses, teed into vehicle heater hoses, are connected to fuel pressure regulator.
Fuel Tubes & Fittings
Due to the high-pressure requirements of CNG system, special stainless steel, SAE straight-thread "O" ring, double-ferrule, compression-type fuel tube fittings are used. These will connect all fuel tubes and components on high-pressure side of system (inlet side of fuel pressure regulator). High-pressure stainless steel, seamless fuel tubes are also used.
Standard NPT and 45-degree fittings are used on low-pressure side of system (outlet side of fuel pressure regulator).
Fittings with "O" rings are used on the high-pressure side of system at following places.
- Main Fuel Tube-To-Rear Cylinder Fuel Control Valve (90-Degree Fitting)
- Main Fuel Tube-To-Side Cylinder Fuel Control Valve (Straight Fitting
- Connecting Fuel Tube Between Rear Cylinder Control Valves (Straight Fittings)
High-Pressure Fuel Shutoff Solenoid
The high-pressure fuel shutoff solenoid is used as an on/off valve to electrically control high-pressure gas flowing through the fuel pressure regulator. It is an integral part of fuel pressure regulator and is operated by Powertrain Control Module (PCM) through CNG fuel shutoff solenoid relay.
Low-Pressure Fuel Shutoff Solenoid
Low-pressure fuel shutoff solenoid is controlled by the PCM through ASD relay. The solenoid is mounted to inlet side of fuel injector rail. After cylinder pressure has been lowered by fuel pressure regulator, this solenoid is used as an on-off valve to electrically control fuel entering fuel rail assembly.
Manual Shutoff Valve
A manual one-quarter turn shutoff valve is located inboard of the right frame rail, in front of the right rear wheelwell. Valve location is identified by a label on the body. This valve isolates the fuel cylinders from the rest of the fuel system, the fuel fill tube and the tube to the engine.
Fuel injectors are similar in appearance and function to conventional gasoline fuel injectors; however, they cannot be interchanged. Power to injectors is controlled by Automatic Shut-Down (ASD) relay. Injectors are grounded through PCM. The PCM determines injector timing and pulse width.
When ground is supplied to injector by PCM, armature and pintle inside injector move a short distance against spring and open a small orifice. Since fuel is under high pressure, a fine spray is developed.
High Pressure Fuel Shutoff Solenoid Relay
This relay is located in engine compartment and is comparable to fuel pump relay used on gasoline powered vehicles. Relay is controlled by PCM and is used to control the high-pressure fuel shutoff solenoid in the fuel pressure regulator. Relay also causes ASD relay to deactivate due to a common ground circuit.
Multiport Fuel Injection (MFI)
Individual, electrically pulsed injectors (one per cylinder) are located in intake manifold fuel rails. These injectors are located next to intake valves in cylinder head.
MFI system features sequential injection. Injection timing is controlled by PCM based on input from crankshaft position sensor and sync pick-up. Constant fuel pressure to the injectors is maintained by the fuel pressure regulator. Air/fuel mixture is regulated by the length of time injector stays open (pulse width). The PCM controls pulse width using information provided by various sensors.
Modes Of Operation
As input signals to PCM change, PCM adjusts its response to the output devices. Modes of operation come in 2 types: open loop and closed loop. In open loop mode, PCM is not using input from oxygen sensor and is responding to preset programming to determine injector pulse width and ignition timing. In closed loop mode, PCM is adjusting ignition timing and using the input from oxygen sensor to fine tune injector pulse width.
The following inputs are used to determine PCM mode.
- Charge Temperature Sensor
- Coolant Temperature Sensor (CTS)
- Fuel Temperature Sensor
- Fuel Low-Pressure Sensor
- Idle Contact Switch
- MAP Sensor
- Engine Speed
- Throttle Position (TP) Sensor
- Gear Position
- A/C Switch
- Battery Voltage
- Oxygen Sensor (O2S)
- A/C Control Positions
From these inputs, PCM determines which mode vehicle is in and the appropriate response. Not all inputs are used in all modes. The 8 modes of operation are as follows
- Ignition Switch In ON Position
This is an open loop mode. PCM determines atmospheric pressure from MAP sensor and determines basic fuel strategy. PCM modifies fuel strategy according to ambient air temperature, coolant temperature, fuel temperature and fuel pressure inputs. PCM activates auto shutdown relay, which in turn activates low-pressure fuel shutoff solenoid and ignition circuit for approximately 2 seconds unless engine is cranked. PCM also activates CNG fuel shutoff solenoid relay, which in turn activates high-pressure fuel shutoff solenoid and O2S heater element for approximately 2 seconds unless engine is cranked.
- Engine Start-Up
This is an open loop mode. When starter is engaged, PCM receives crankshaft position and distributor signal and energizes Auto Shutdown (ASD) relay. Once ASD relay is energized, PCM determines proper injector pulse width and ignition timing from input signals and sends a signal to fuel injector driver module to power up fuel injectors. PCM provides ground for injectors to fire in proper order. If PCM does not receive crankshaft position sensor signal within 3 seconds after engine begins cranking, fuel injection system is shut down and a fault code is set in PCM memory.
- Engine Warm-Up
This is an open loop mode. PCM determines injector pulse width using information from battery voltage, crankshaft position sensor, coolant, charge and fuel temperature sensors, MAP sensor and distributor pick-up. PCM also monitors A/C request, fuel low-pressure sensor and Park/Neutral switch (if equipped) signals for fuel calculation. PCM controls engine idle speed and ignition timing.
- Idle
This is an closed loop mode. In idle mode, PCM now adds O2S signal to the array of inputs. PCM maintains correct air/fuel ratio by adjusting injector pulse width and ignition timing. PCM also controls A/C clutch operation if A/C is requested.
- Cruise
When engine is at operating temperature, this is a closed loop mode. Using information from various inputs, PCM determines injector pulse width and ignition timing under different engine load conditions. PCM determines proper air/fuel ratio using input from O2S.
- Acceleration
This is an open loop mode. When PCM recognizes an abrupt increase in throttle position or manifold pressure as a demand for increased engine output, it increases injector pulse width through fuel injector driver module in response to increased fuel demand.
- Deceleration
This is an open loop mode. When PCM receives inputs signaling a closed throttle and an abrupt decrease in manifold pressure, it reduces injector pulse width to lean air/fuel mixture. Under certain RPM and closed throttle position conditions, O2S signals are ignored and PCM cuts off fuel injection until idle speed is reached. PCM also drives AIS motor for smooth transition to idle mode.
- Wide Open Throttle
This is an closed loop mode. When PCM senses wide open throttle, it grounds EGR and evaporative purge solenoids to prevent EGR and canister purge functions. Oxygen sensor input is ignored and PCM adjusts injector pulse width to supply a predetermined amount of additional fuel.
- Ignition Switch In OFF Position
This is an open loop mode. All fuel injection is stopped and fuel flow is electrically shut down at both low-pressure and high-pressure fuel shutoff solenoids. PCM de-energizes auto shutdown relay and drives AIS motor into position in anticipation of next start-up.
AIS motor adjusts idle speed to compensate for engine load and ambient temperature by adjusting amount of air flowing through by-pass in throttle body. PCM uses coolant temperature, distance (vehicle speed) sensor, throttle position and various switch input operations to adjust AIS to obtain optimum idle conditions. Deceleration stall is prevented by increasing airflow when throttle is closed suddenly.
SELF-DIAGNOSTIC SYSTEM
The PCM monitors several different circuits of engine control system. If a problem is sensed with a monitored circuit, PCM will store a trouble code to aid technician in diagnosis of system. The CHECK ENGINE light or DRB can be used to read trouble codes.
MISCELLANEOUS CONTROLS
Note. Although not strictly considered part of engine performance system, some controlled devices can adversely affect driveability if they malfunction.
The ASD relay is a cut-off relay for the following components.
- Fuel Injector Driver Module
- Ignition Coil
- Low-Pressure Fuel Shutoff Solenoid
When ignition switch is turned to RUN position, PCM energizes ASD relay, which powers these components. If PCM does not receive a crankshaft signal shortly thereafter, PCM will de-energize ASD relay, and power to these components is cut off.
CNG FUEL SHUTOFF SOLENOID RELAY
The CNG fuel shutoff solenoid relay is a cut-off relay for the following components.
- High-Pressure Fuel Shutoff Solenoid
- O2S Heating Element
When ignition switch is turned to RUN position, PCM energizes CNG fuel shutoff solenoid relay, which powers these components. If PCM does not receive a crankshaft signal shortly thereafter, PCM will de-energize CNG fuel shutoff solenoid relay, and power to these components is cut off.
