Fuel Injection System

POWERTRAIN CONTROL MODULE (PCM) DESCRIPTION

Fig. 1 PCM Location:

The Powertrain Control Module (PCM) is located in the engine compartment.

The PCM operates the fuel system. The PCM was formerly referred to as the SBEC or engine controller. The PCM is a pre-programmed, dual microprocessor digital computer. It regulates ignition timing, air- fuel ratio, emission control devices, charging system, speed control, air conditioning compressor clutch engagement and idle speed. The PCM can adapt its programming to meet changing operating conditions.

The PCM receives input signals from various switches and sensors. Based on these inputs, the PCM regulates various engine and vehicle operations through different system components. These components are referred to as Powertrain Control Module (PCM) Outputs. The sensors and switches that provide inputs to the PCM are considered Powertrain Control Module (PCM) Inputs.

The PCM adjusts ignition timing based upon inputs it receives from sensors that react to: engine rpm, manifold absolute pressure, engine coolant temperature, throttle position, transmission gear selection (automatic transmission), vehicle speed and the brake switch.

The PCM adjusts idle speed based on inputs it receives from sensors that react to: throttle position, vehicle speed, transmission gear selection, engine coolant temperature and from inputs it receives from the air conditioning clutch switch and brake switch.

Based on inputs that it receives, the PCM adjusts ignition coil dwell. The PCM also adjusts the generator charge rate through control of the generator field and provides speed control operation.

NOTE: Powertrain Control Module (PCM) Inputs:
- A/C request (if equipped with factory A/C)
- A/C select (if equipped with factory A/C)
- Auto shutdown (ASD) sense
- Battery temperature
- Battery voltage
- Brake switch
- CCD bus (+) circuits
- CCD bus (-) circuits
- Camshaft position sensor signal
- CNG fuel pressure (low-pressure at fuel rail)
- CNG fuel temperature
- Crankshaft position sensor
- Data link connection for DRB scan tool
- Engine coolant temperature sensor
- Fuel level (monitors CNG high-pressure)
- Generator (battery voltage) output
- Ignition circuit sense (ignition switch in ON/OFF/ crank/run position)
- Intake manifold air temperature sensor
- Manifold absolute pressure (MAP) sensor
- Oil pressure
- Output shaft speed sensor
- Overdrive/override switch
- Oxygen sensors
- Park/neutral switch (auto. trans. only)
- Power ground
- Sensor return
- Signal ground
- Speed control multiplexed single wire input
- Throttle position sensor
- Transmission governor pressure sensor
- Transmission temperature sensor
- Vehicle speed inputs from ABS or RWAL system

NOTE: Powertrain Control Module (PCM) Outputs:
- A/C clutch relay
- Auto shutdown (ASD) relay (controls 12V+ output to CNG low-pressure fuel shut-off solenoid)
- CCD bus (+A) circuits for: speedometer, voltmeter, fuel gauge, oil pressure gauge/lamp, engine temp. gauge and speed control warn. lamp
- CNG high-pressure fuel shut-off solenoid relay
- CNG low-pressure fuel shut-off solenoid (controlled through ASD relay)
- Data link connection for DRB scan tool
- EGR valve control solenoid (if equipped)
- Five volt sensor supply (primary)
- Five volt sensor supply (secondary)
- Fuel injectors
- Generator field driver (-)
- Generator field driver (+)
- Generator lamp (if equipped)
- Idle air control (IAC) motor
- Ignition coil
- Malfunction indicator lamp (Check Engine Lamp). Driven through CCD circuits
- Speed control vacuum solenoid
- Speed control vent solenoid
- Tachometer (if equipped)
- Transmission torque convertor clutch
- Transmission overdrive indicator lamp
- Transmission 3-4 shift solenoid
- Transmission variable force solenoid
- Transmission variable force solenoid relay

MODES OF OPERATION
As input signals to the Powertrain Control Module (PCM) change, the PCM adjusts its response to output devices. For example, PCM must calculate different injector pulse width and ignition timing for idle, than it does for wide open throttle (WOT).

The PCM will operate in two different modes: Open Loop and Closed Loop.

During Open Loop modes, PCM receives input signals and responds only according to preset PCM programming. Input from oxygen (02S) sensor is not monitored during Open Loop modes.

During Closed Loop modes, PCM will monitor oxygen (O2S) sensor input. This input indicates to PCM whether or not calculated injector pulse width results in ideal air-fuel ratio. By monitoring exhaust oxygen content through 02S sensor, PCM can fine tune injector pulse width. This is done to achieve optimum fuel economy combined with low emission engine performance.

The fuel injection system recognizes the following operations:
- Ignition switch ON (engine not running)
- Engine start-up (crank)
- Engine warm-up
- Idle
- Cruise
- Acceleration
- Deceleration
- Wide open throttle (WOT)
- Ignition switch OFF

The ignition switch ON (engine not running), engine start-up (crank), engine warm-up, acceleration and deceleration are Open Loop modes. The idle mode, cruise modes and wide open throttle modes, with engine at operating temperature are Closed Loop modes.

Ignition Switch ON Mode-ZERO RPM
This is an Open Loop mode. When the system is activated by the ignition switch, the following actions occur:
- The PCM pre-positions the idle air control motor based on engine coolant temperature
- The PCM determines atmospheric air pressure from MAP sensor input to determine basic fuel strategy
- The PCM monitors engine coolant temperature sensor input.
- The PCM monitors CNG fuel temperature sensor input
- The PCM monitors CNG fuel low-pressure sensor input
- Intake manifold air temperature sensor input is monitored
- Throttle position sensor (TPS) is monitored
- The high-pressure fuel shut-off solenoid is energized by the PCM through the CNG high-pressure fuel shut-off solenoid relay. This solenoid will operate for approximately 2 seconds unless the engine is operating or starter motor is engaged.
- The low-pressure fuel shut-off solenoid is energized by the PCM through the ASD relay. This solenoid will operate for approximately 2 seconds unless the engine is operating or starter motor is engaged.
- The O2S sensor heater element is energized through the automatic shut-down (ASD) relay. Although, O2S input is not used by the PCM to calibrate air-fuel ratio during Open Loop mode of operation

Engine Start-up Mode
This is an Open Loop mode. The following actions occur when starter motor is engaged.

The PCM receives inputs from:
- Battery voltage
- Engine coolant temperature sensor
- Crankshaft position sensor
- CNG fuel low-pressure sensor
- CNG fuel temperature sensor
- Intake manifold air temperature sensor
- Manifold absolute pressure (MAP) sensor
- Throttle position sensor (TPS)
- Camshaft position sensor, in distributor

The PCM monitors the crankshaft position sensor. If the PCM does not receive a sensor signal within 3 seconds of cranking engine, it will shut-down the fuel injection system. A Diagnostic Trouble Code (DTC) will be set.

The high-pressure fuel shut-off solenoid is energized by the PCM through the high-pressure fuel shut-off solenoid relay. This solenoid will operate for approximately 2 seconds unless the engine is operating or starter motor is engaged.

The low-pressure fuel shut-off solenoid is energized by the PCM through the ASD relay. This solenoid will operate for approximately 2 seconds unless the engine is operating or starter motor is engaged.

Voltage is applied to fuel injectors with the PCM controlling injection sequence, rate and injector pulse width. This is done by turning the ground circuit to each individual injector ON-and-OFF at the PCM.

The PCM determines the proper ignition timing according to the input received from the crankshaft position sensor. This is done by switching the coil ON- and-OFF through control of its ground path.

Engine Warm-up Mode
This is an Open Loop mode. During engine warm-up, PCM receives inputs from:
- Battery voltage
- Crankshaft position sensor
- Engine coolant temperature sensor
- Intake manifold air temperature sensor
- CNG fuel low-pressure sensor
- CNG fuel temperature sensor
- Manifold absolute pressure (MAP) sensor
- Throttle position sensor (TPS)
- Camshaft position sensor, in distributor
- Park/Neutral Switch (gear indicator signal- auto. trans. only)
- Air conditioning select signal, if equipped
- Air conditioning request signal, if equipped
- Vehicle speed sensor

Based on these inputs, the following occurs:
- Voltage is applied to the fuel injectors with the PCM controlling injection sequence, rate and injector pulse width. This is done by turning the ground circuit to each individual injector ON-and-OFF at the PCM.
- The PCM begins firing ignition based on crankshaft position sensor input.
- The PCM adjusts the engine idle speed through the idle air control motor
- The PCM operates the air conditioning (A/C) compressor clutch through the A/C clutch relay. This is done if A/C has been selected by vehicle operator and requested by A/C thermostat
- When the engine has reached operating temperature, the PCM will begin monitoring 02S sensor input. The system will leave warm-up mode and go into Closed Loop operation

Idle Mode
When engine is at operating temperature, this is a Closed Loop mode. At idle speed PCM receives inputs from:
- Air conditioning select signal, if equipped
- Air conditioning request signal, if equipped
- Battery voltage
- Crankshaft position sensor
- Engine coolant temperature sensor
- Intake manifold air temperature sensor
- CNG fuel low-pressure sensor
- CNG fuel temperature sensor
- Manifold absolute pressure (MAP) sensor
- Throttle position sensor (TPS)
- Camshaft position sensor, in distributor
- Park/Neutral Switch (gear indicator signal- auto. trans. only)
- Oxygen sensor
- Vehicle speed sensor

Based on these inputs, the following occurs:
- Voltage is applied to the fuel injectors with the PCM controlling injection sequence and injector pulse width. This is done by turning the ground circuit to each individual injector ON-and-OFF at the PCM.
- The PCM monitors O2S sensor input and adjusts air-fuel ratio by varying the injector pulse width. It will then adjust engine idle speed through the idle air control motor
- The PCM adjusts ignition spark advance as necessary for rpm and temperature.
- The PCM operates the A/C compressor clutch through the clutch relay. This is done if A/C has been selected by vehicle operator and requested by A/C thermostat

Cruise Mode
When engine is at operating temperature, this is a Closed Loop mode. At cruising speed the PCM receives inputs from:
- Air conditioning select signal, if equipped
- Air conditioning request signal, if equipped
- Battery voltage
- Engine coolant temperature sensor
- Crankshaft position sensor
- Intake manifold air temperature sensor
- CNG fuel low-pressure sensor
- CNG fuel temperature sensor
- Manifold absolute pressure (MAP) sensor
- Throttle position sensor (TPS)
- Camshaft position sensor, in distributor
- Cruise control switch inputs
- Park/Neutral switch (gear indicator signal- auto. trans. only)
- Oxygen sensor
- Vehicle speed sensor

Based on these inputs, the following occurs:
- Voltage is applied to the fuel injectors with the PCM adjusting injector pulse width. This is done by turning the ground circuit to each individual injector ON-and-OFF at the PCM.
- The PCM monitors O2S sensor input. It then adjusts air-fuel ratio as needed
- The PCM adjusts spark advance as necessary for rpm, temperature and load. This is done for maximum power and efficiency
- The PCM operates the air conditioning (A/C) compressor clutch through the A/C clutch relay. This is done if A/C has been selected by vehicle operator and requested by A/C thermostat
- The idle air control (IAC) motor is positioned in anticipation of the next deceleration

Acceleration Mode
This is an Open Loop mode. The PCM recognizes an abrupt increase in throttle position or MAP pressure as a demand for increased engine output and vehicle acceleration. The PCM increases the injector pulse width in response to increased throttle opening.

Deceleration Mode
When engine is at operating temperature, and under deceleration, this is an Open Loop mode. During hard deceleration the PCM receives the following inputs:
- Air conditioning select signal, if equipped
- Air conditioning request signal, if equipped
- Battery voltage
- Engine coolant temperature sensor
- Crankshaft position sensor
- Intake manifold air temperature sensor
- CNG fuel low-pressure sensor
- CNG fuel temperature sensor
- Manifold absolute pressure (MAP) sensor
- Throttle position sensor (TPS)
- Camshaft position sensor, in distributor
- Park/Neutral switch (gear indicator signal- auto. trans. only)
- Vehicle speed sensor

If the vehicle is under hard deceleration with proper rpm and closed throttle conditions, the PCM will ignore oxygen sensor input signal. The PCM will enter a fuel cut-off strategy; until idle is approached. in which it will not supply a ground signal to injectors. If a hard deceleration does not exist, the PCM will determine proper injector pulse width. Fuel injection will continue.

Based on the preceding inputs, the PCM will adjust engine idle speed through the idle air control motor. This is done to provide a smooth transition to expected idle mode.

Wide Open Throttle Mode
This is normally an Open Loop mode on gasoline powered vehicles. On CNG powered vehicles, this is an Closed Loop mode. During wide open throttle operation, the PCM receives the following inputs:
- Battery voltage
- Crankshaft position sensor
- Engine coolant temperature sensor
- Intake manifold air temperature sensor
- CNG fuel low-pressure sensor
- CNG fuel temperature sensor
- Manifold absolute pressure (MAP) sensor
- Throttle position sensor (TPS)
- Oxygen sensor (02S)
- Camshaft position sensor (in distributor)
- Vehicle speed sensor

During wide open throttle conditions the following occurs:
- Voltage is applied to the fuel injectors with the PCM controlling injection sequence and injector pulse width. This is done by turning the ground circuit to each individual injector ON-and-OFF at the PCM. The PCM also monitors 02S sensor input. It then adjusts air-fuel ratio as needed.
- The PCM adjusts spark advance as necessary for rpm, temperature and load. This is done for maximum power and efficiency

Ignition Switch OFF Mode
When ignition switch is turned to the OFF position, the PCM stops operating the fuel injectors, ignition coil, automatic shut-down (ASD) relay and CNG high-pressure fuel shut-off solenoid relay.

Fuel flow is electrically stopped (shut-down) at both the low-pressure and high-pressure fuel shut-off solenoids.

AUTOMATIC SHUT-DOWN (ASD) RELAY

Fig. 2 Power Distribution Center (PDC):

The Automatic Shut-Down (ASD) relay is located in the Power Distribution Center (PDC). The PDC is located in the engine compartment.

The ASD relay is used to supply the 12V + power source to the CNG low-pressure fuel shut-off solenoid, 025 heating elements, all fuel injectors and ignition coil.

The Powertrain Control Module (PCM) operates and controls both the ASD relay and the CNG high- pressure fuel shut-off solenoid relay. The PCM operates the relays by switching the ground path ON and OFF (both relays turn ON and OFF at the same time).

The PCM turns the ground path OFF when the ignition switch is in the OFF position. Both relays (ASD and CNG high-pressure fuel shut-off) are off. When the ignition switch is in the ON or CRANK position, the PCM monitors the crankshaft position sensor and camshaft position sensor signals to determine engine speed and ignition timing (coil dwell). If the PCM does not receive the necessary crankshaft position sensor and camshaft position sensor signals when the ignition switch is in the RUN position (indicating the engine has not started and is not running), it will deenergize both relays. When the relays are de-energized, battery voltage is not supplied to the fuel injectors, ignition coils, CNG high or low-pressure fuel shut-off solenoids and oxygen sensor (O2S) heating elements.