INTRODUCTION
This article covers the basic description and operation of engine performance related systems and components. Read this article before working on unfamiliar systems. CNG fueled vehicles are limited to RWD medium and heavy-duty vans with a 5.2L engine. The CNG-fueled vehicle is similar to gasoline-fueled vehicles. For theory and operation that applies to both CNG and gasoline-fueled vehicles, see THEORY & OPERATION - TRUCKS & RWD VANS - GASOLINE article. For CNG-specific components (Scheme 4)
Scheme 4
POWERTRAIN CONTROL MODULE (PCM)
The PCM is a digital computer that controls ignition timing, air/fuel ratio, fuel injector pulse width, ignition coil(s), spark advance, emission control devices, cooling fan, charging system, idle speed, cruise control (if equipped), fuel pump and tachometer. PCM is located on firewall, near wiper motor. 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 output. The 5-volt output powers battery temperature sensor, distributor, Crankshaft Position (CKP) sensor, Engine Coolant Temperature (ECT) sensor, Intake Air Temperature (IAT) sensor, logic circuits, Manifold Absolute Pressure (MAP) sensor, Throttle Position (TP) sensor and Vehicle Speed Sensor (VSS).
Components are grouped into 2 categories. The first category, INPUT DEVICES , includes components that control or produce voltage signals monitored by the PCM. The second category, OUTPUT SIGNALS , includes components controlled by the PCM (this is accomplished by the PCM grounding individual circuits).
SYSTEM COMPONENTS
Following components are shared by gasoline-fueled and CNG-fueled vehicles. For component description, see THEORY & OPERATION - TRUCKS & RWD VANS - GASOLINE article. For removal and installation procedures, see REMOVAL & INSTALLATION - TRUCKS & RWD VANS - GASOLINE article.
- Throttle Body
- Accelerator Linkage
- Manifold Absolute Pressure (MAP) Sensor
- Engine Coolant Temperature (ECT) Sensor
- Intake Manifold
- Intake Air Temperature (IAT) Sensor
- Throttle Position (TP) Sensor
- Idle Air Control (IAC) Motor
- Camshaft Position (CMP) Sensor
- Crankshaft Position (CKP) Sensor
- Auto Shut Down (ASD) Relay
- Oxygen (O2) Sensors
- Vehicle Speed Sensor (VSS) Signal
- Distributor
- Air Conditioning
- Cruise Control
- Overdrive (OD)
- Generator
- Park/Neutral Switch (A/T Models)
- Brake Switch
CNG-specific components include the following
- CNG-Specific PCM
- CNG High-Pressure Fuel Shutoff Solenoid Relay
- CNG High-Pressure Fill Receptacle
- One-Way Check Valve
- CNG Fuel Filter
- CNG Injectors
- High-Pressure Stainless Steel Seamless Fuel Lines
- Special High-Pressure Line Fittings
- Fuel Low-Pressure Sensor
- Fuel Rail
- Some Wiring Harnesses
- Fuel Cylinders
- Manual Fuel Control Valves On Each Cylinder
- Manual Shutoff Valve
- Fuel Temperature Sensor
- Low-Pressure Fuel Shutoff Solenoid
- Chassis Frame Members
- Automatic Transmission Dipstick Tube
- Fuel Pressure Regulator (Warmed By Engine Coolant)
- Temperature Compensated Fuel Gauge High-Pressure Sensor (Fuel Gauge Sensor)
INPUT DEVICES
Note. CNG-fueled vehicles are similar to gasoline-fueled vehicles. For input devices that apply to both CNG and gasoline-fueled vehicles, see THEORY & OPERATION - TRUCKS & RWD VANS - GASOLINE article. For CNG-specfic components, see the following.
Fuel Low-Pressure Sensor
Located on fuel rail, provides fuel pressure information to PCM for fuel adjustment.
Fuel Temperature Sensor
Located on fuel rail, provides fuel temperature information to PCM for fuel adjustment.
Temperature Compensated Fuel Gauge High-Pressure Sensor
Located at fuel control valve on inside left side cylinder. Sensor monitors fuel pressure in high-pressure side of fuel system. PCM monitors sensor resistance (8-100 ohms).
OUTPUT SIGNALS
Note. CNG-fueled vehicles are similar to gasoline-fueled vehicles. For input devices that apply to both CNG and gasoline-fueled vehicles, see THEORY & OPERATION - TRUCKS & RWD VANS - GASOLINE article. For theory and operation on each output component, refer to indicated system.
CNG High-Pressure Fuel SHUTOFF Solenoid Relay
See CNG HIGH-PRESSURE FUEL SHUTOFF RELAY under MISCELLANEOUS CONTROLS.
CNG High-Pressure Fuel SHUTOFF Solenoid
See CNG HIGH-PRESSURE FUEL SHUTOFF SOLENOID under MISCELLANEOUS CONTROLS.
CNG Fuel Injectors
Fuel injectors are controlled by PCM. Injector pulse width is adjusted based on various inputs. Because no liquid (gasoline) is in injector, operating noise may be louder.
FUEL SYSTEM
CNG is stored under high pressure in 4 cylinders located under floor. When control valves on each cylinder and main SHUTOFF valve are open, CNG is controlled by high-pressure fuel shutoff solenoid. When ignition is on, solenoid is opened. Fuel flows through regulator and low-pressure fuel shutoff solenoid, located on fuel rail. PCM operates fuel rail mounted injectors to control air/fuel mixture into engine. PCM uses inputs including fuel temperature sensor and low-pressure sensor located on fuel rail to modify air/fuel ratio.
IGNITION SYSTEM
CNG-fueled PCM operation is similar to gasoline-fueled PCM. The PCM completely controls ignition system. During crank/start mode, PCM will set a fixed amount of spark advance for an efficient engine start. Amount of spark advance or retard is determined by inputs that PCM receives from ECT sensor, engine vacuum and engine RPM. During engine operation, PCM can supply an infinite number of advance curves to ensure proper engine operation.
HALL EFFECT IGNITION SYSTEM
This system is equipped with a Hall Effect distributor. Shutter(s) attached to distributor shaft rotate through distributor Hall Effect switch, also referred to as a CMP sensor, which contains a distributor pick-up (a Hall Effect device and magnet). As shutter blade(s) pass through pick-up, magnetic field is interrupted and voltage is toggled between high and low. PCM uses this cylinder position data from CMP sensor, along with engine speed (RPM) and CKP sensor data, to control ignition timing and injector pulse width to maintain optimum driveability.
EMISSION SYSTEMS
Note. CNG-fueled vehicle emission systems are similar to gasoline-fueled vehicle systems. Evaporative system is not used due to different fuel system. To determine component usage, see EMISSION APPLICATIONS - TRUCKS article.
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 Light (MIL), or a scan tool can be used to read DTCs. For additional information, see SELF-DIAGNOSTICS - JEEPS, TRUCKS & RWD VANS article.
MALFUNCTION INDICATOR LIGHT
Malfunction Indicator Light (MIL) comes on and remains on for 3 seconds as a bulb test each time ignition switch is turned to ON position. If PCM receives an incorrect signal or receives no signal from battery voltage input, charging system, ECT sensor, MAP sensor or TP sensor, MIL will come on. MIL will also come on if certain emission-related faults exist. This warns driver that PCM is in limp-in mode and immediate repairs are necessary. MIL can also be used to display Diagnostic Trouble Codes (DTCs). For additional information, see SELF-DIAGNOSTICS - JEEPS, TRUCKS & RWD VANS article.
SERIAL COMMUNICATIONS INTERFACE (SCI)
SCI circuit is used by PCM to send data to and receive data and sensor activation signals from scan tool. Scan tool uses signals sent on SCI to display fault messages or Diagnostic Trouble Codes (DTCs), sensor voltages and device states (On/Off). Scan tool uses SCI to send solenoid and switch activation commands to PCM so that devices and circuits can be tested. SCI is also used to write SRI mileage to PCM.
MISCELLANEOUS CONTROLS
Note. Although not strictly considered part of engine performance system, some controlled devices can adversely affect driveability if they malfunction.
Relay is used in place of gasoline-fueled fuel pump relay. Relay controls operation of CNG high-pressure fuel shutoff solenoid. PCM controls relay through ground circuit. When ignition is off, relay is off. When ignition switch is in ON or CRANK position, PCM monitors crankshaft position sensor and camshaft position sensor to determine engine speed. If PCM does not detect engine speed, relay is not energized.
Solenoid is used as an on/off valve to control high pressure gas flow through pressure regulator. Solenoid is controlled by PCM, using CNG high-pressure fuel shutoff solenoid relay.
AUTO SHUTDOWN (ASD) RELAY
ASD relay is energized when ignition is on. This relay is controlled through PCM by switching a common ground circuit on and off. Following components are controlled by ASD relay
- Fuel Injectors
- Generator Field Winding
- Ignition Coil
- O2 Sensor Heater
- CNG High-Pressure Fuel Shutoff Solenoid Relay
- HO2S Heating Element
When ignition switch is turned to RUN position, PCM energizes ASD relay which powers these components. If PCM does not receive a CMP and CKP sensor signal within one second of engine cranking (start-up), PCM will turn ground circuit off and de-energize ASD relay.
GENERATOR
Powertrain Control Module (PCM) regulates charging system voltage.