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 9)
CNG Component Locations. Scheme 9
POWERTRAIN CONTROL MODULE (PCM)
The Powertrain Control Module (PCM) is a digital computer that controls ignition timing, air/fuel ratio, emission control devices, radiator cooling fan, charging system, idle speed, fuel pump and tachometer. PCM sequentially energizes fuel injectors and regulates pulse width by switching injector ground path off and on. 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 Camshaft Position (CMP) sensor in distributor and Crankshaft Position (CKP) sensor. The 5-volt output powers Engine Coolant Temperature (ECT), Intake Air Temperature (IAT), fuel temperature, Manifold Absolute Pressure (MAP) and Throttle Position (TP) sensors. PCM is located in engine compartment. See PCM LOCATION table.
| Application | Location |
|---|---|
| Ram Van/Wagon | On Firewall, Near Wiper Motor |
PCM LOCATION
SYSTEM COMPONENTS
The following engine components (as used on CNG vehicle) are identical to components used on gasoline vehicles. For description of components used in common with gasoline powered vehicles, see the THEORY/OPERATION - 5.2L (GASOLINE) article.
- Accelerator Linkage
- Air Conditioning (A/C) Components
- Automatic Shutdown (ASD) Relay
- Brake Switch Controls
- Camshaft Position (CMP) Sensor
- Crankshaft Position (CKP) Sensor
- Emission Systems (Except Evaporative System)
- Engine Coolant Temperature (ECT) Sensor
- Generator
- Idle Air Control (IAC) Motor
- Ignition System
- Intake Air Temperature (IAT) Sensor
- Intake Manifold
- Manifold Absolute Pressure (MAP) Sensor
- Overdrive Components
- Oxygen Sensor (O2S)
- Park/Neutral (P/N) Position Switch (A/T Only)
- Speed Control Components
- Throttle Body
- Throttle Position (TP) Sensor
- Vehicle Speed Sensor (VSS)
For service and component removal/installation procedures for shared components, see the REMOVE/INSTALL/OVERHAUL - 5.2L (CNG) article.
CNG fuel system uses following additional components.
- Fuel Filter Mounted In Fuel Pressure Regulator
- Fuel Gauge High-Pressure Sensor Used To Operate Fuel Gauge
- Fuel Pressure Regulator Mounted And Electrically Controlled High-Pressure Fuel Shutoff Solenoid
- Fuel Pressure Regulator Warmed By Engine Coolant
- Fuel Rail Mounted And Electrically Controlled Low-Pressure Fuel Shutoff Solenoid
- Fuel Rail Mounted Fuel Low-Pressure Sensor
- Fuel Rail Mounted Fuel Temperature Sensor
- High-Pressure Fuel Fill Receptacle With Internal One-Way Check Valve
- High-Pressure Fuel Shutoff Solenoid Relay
- High-Pressure, Stainless Steel Seamless Fuel Tubes
- High-Pressure Fuel Cylinders
- Manual Fuel Control Valves (One On Each Fuel Cylinder)
- Manual (Mechanically Operated) 1/4-Turn Gas Shutoff Valve
- Separate Frame Rail Mounted One-Way Check Valve
- Stainless Steel, Double-Ferrule, Compression-Type Fuel Tube Fittings (Not At All Fuel Tube Connections Or Components)
- Unique CNG Fuel Injectors
- Unique Fuel Rail To Mount Fuel Injectors
- Unique Powertrain Control Module (PCM)
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 WIRING DIAGRAMS - 5.2L article. For description of components used in common with gasoline vehicles, see the THEORY/OPERATION - 5.2L (GASOLINE) article. 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 on fuel rail. PCM uses fuel pressure sensor information to calculate injector timing.
Fuel Temperature Sensor
Fuel temperature sensor monitors fuel temperature. Sensor is mounted at end of 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 the THEORY/OPERATION - 5.2L (GASOLINE) article.
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.
Idle Air Control (IAC) Motor
See IDLE SPEED 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 ASD RELAY & HIGH-PRESSURE FUEL SHUTOFF SOLENOID 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 check valves are used with CNG fuel system. One valve is integrated with fuel fill receptacle. 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 CNG system. Fuel injector rail can be disassembled for replacement of inlet blocks, connecting tubes and "O" rings.
Fuel Filter
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. Regulator is used to lower fuel cylinder pressure from approximately 3600 psi down to approximately 90-140 psi for system operation.
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 by engine coolant routed from cooling system. Two rubber coolant hoses are used to connect regulator to heater hoses in engine compartment.
Fuel Tubes & Fittings
Due to high-pressure requirements of CNG system, special stainless steel, SAE straight-thread "O" ring, double-ferrule, compression-type fuel tube fittings are used. These fittings 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
High-pressure fuel shutoff solenoid is used as an on/off valve to electrically control high-pressure gas flowing through fuel pressure regulator. It is an integral part of fuel pressure regulator and is operated by Powertrain Control Module (PCM) through high-pressure fuel shutoff solenoid relay.
Low-Pressure Fuel Shutoff Solenoid
Low-pressure fuel shutoff solenoid is controlled by the PCM through ASD relay. Solenoid is mounted to inlet side of fuel injector rail. After cylinder pressure has been lowered by fuel pressure regulator, solenoid is used as an on-off valve to electrically control fuel entering fuel rail assembly.
Manual Shutoff Valve
A manual 1/4-turn shutoff valve is located inboard of right frame rail, in front of right rear wheelwell. Valve location is identified by a label on vehicle 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 Shutdown (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 power distribution center 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.
Modes Of Operation
As input signals to PCM change, PCM adjusts its response to 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 input from oxygen sensor to fine tune injector pulse width.
The following inputs are used to determine PCM mode
- A/C Switch
- A/C Control Positions
- Battery Voltage
- Camshaft Position (CMP) Sensor
- Crankshaft Position (CKP) Sensor
- Cruise Control Switch
- Engine Coolant Temperature (ECT) Sensor
- Fuel Temperature Sensor
- Fuel Low-Pressure Sensor
- Intake Air Temperature (IAT) Sensor
- Manifold Absolute Pressure (MAP) Sensor
- Oxygen Sensor (O2S)
- Park/Neutral (P/N) Position Switch (A/T Only)
- Throttle Position (TP) Sensor
- Vehicle Speed Sensor (VSS)
From these inputs, PCM determines which mode vehicle is in and the appropriate response. Not all inputs are used in all modes.
Modes of operation are as follows
- Ignition Switch On (Engine Not Running) This is an open loop mode. PCM pre-positions Idle Air Control (IAC) motor based on engine coolant temperature. PCM determines atmospheric pressure from MAP sensor and determines basic fuel strategy. PCM modifies fuel strategy according to intake air temperature, coolant temperature, fuel temperature, fuel pressure and throttle position sensor 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 input from battery voltage, CKP sensor, CMP sensor, ECT sensor, fuel low pressure sensor, fuel temperature sensor, IAT sensor, MAP sensor and TP sensor. Based on these inputs, voltage is applied to fuel injectors with PCM controlling injection sequence, rate, and pulse width. PCM provides ground for injectors to fire in proper order. PCM determines proper ignition timing according to input received from CKP sensor. If PCM does not receive CKP sensor signal within 3 seconds after engine begins cranking, fuel injection system is shut down and a Diagnostic Trouble Code (DTC) is set in PCM memory.
- Engine Warm-Up This is an open loop mode. PCM determines injector pulse width using input information from battery voltage, CKP sensor, ECT sensor, IAT sensor, fuel temperature sensors, MAP sensor and CMP sensor. PCM also monitors A/C request, fuel low-pressure sensor and P/N position switch (A/T only) and VSS signals for fuel calculation. PCM controls engine idle speed through IAC motor. PCM controls ignition timing based on CKP sensor input. PCM also operates A/C compressor clutch (if A/C is requested) through A/C clutch relay. When engine reaches operating temperature, vehicle will go into idle mode and PCM will begin monitoring O2S input and go into closed loop operation.
- Idle This is a closed loop mode. In idle mode, PCM now adds O2S signal to array of inputs used in ENGINE WARM-UP mode. 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 A/C switch, battery voltage, CKP sensor, ECT sensor, IAT sensor, fuel temperature sensors, MAP sensor and CMP sensor. PCM also monitors A/C request, fuel low-pressure sensor and P/N position switch (A/T only), TP sensor and VSS signals for fuel calculation. PCM monitors O2S and adjusts air/fuel ratio as needed. PCM controls engine idle speed through IAC motor. PCM controls spark advance as necessary.
- 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 in response to increased fuel demand.
- Deceleration This is an open loop mode when engine is at operating temperature and under deceleration. 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 IAC motor for smooth transition to idle mode.
- Wide Open Throttle This is a closed loop mode. When PCM senses wide open throttle, it grounds fuel injectors in sequence and it controls pulse width to supply a predetermined amount of additional fuel. Oxygen sensor input is monitored and air/fuel ratio is adjusted. PCM also adjusts spark advance.
- Ignition Switch Off 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 IAC motor into position in anticipation of next start-up.
- Sequential Fuel Injection (SFI) 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. SFI timing is controlled by PCM based on input from crankshaft position sensor and camshaft position sensor. Constant fuel pressure to injectors is maintained by fuel pressure regulator. Air/fuel mixture is regulated by length of time injector stays open (pulse width). PCM controls pulse width using information provided by various sensors.
IAC 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 ECT sensor, VSS, TP sensor and various switch input operations to adjust IAC motor 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 Diagnostic Trouble Code (DTC) to aid technician in diagnosis of system. The malfunction indicator (CHECK ENGINE) light or a scan tool 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.
ASD RELAY & HIGH-PRESSURE FUEL SHUTOFF SOLENOID RELAY
The ASD relay and high-pressure fuel shutoff solenoid relays are cut-off relays for the following components
- Fuel Injectors
- Generator
- Ignition Coil
- Low-Pressure Fuel Shutoff Solenoid
- O2S Heating Element
When ignition switch is turned to RUN position, PCM energizes ASD relay and high-pressure fuel shutoff relay solenoid through a common ground. If PCM does not receive a crankshaft signal shortly after ignition switch is turned to RUN position, PCM will de-energize ASD and high-pressure fuel shutoff solenoid relays, and power to these components is cut off.