switch energizes the starter relay. The following
actions occur when the starter motor is engaged.
• If the PCM receives the camshaft position sensor
and crankshaft position sensor signals, it energizes
the Auto Shutdown (ASD) relay and fuel pump relay.
If the PCM does not receive both signals within
approximately one second, it will not energize the
ASD relay and fuel pump relay. The ASD and fuel
pump relays supply battery voltage to the fuel pump,
fuel injectors, ignition coil and heated oxygen sen-
sors.
• The PCM energizes the injectors (on the 69°
degree falling edge) for a calculated pulse width until
it determines crankshaft position from the camshaft
position sensor and crankshaft position sensor sig-
nals. The PCM determines crankshaft position within
1 engine revolution.
• After determining crankshaft position, the PCM
begins energizing the injectors in sequence. It adjusts
injector pulse width and controls injector synchroni-
zation by turning the individual ground paths to the
injectors On and Off.
• When the engine idles within 664 RPM of its
target RPM, the PCM compares current MAP sensor
value with the atmospheric pressure value received
during the Ignition Switch On (zero RPM) mode.
Once the ASD and fuel pump relays have been
energized, the PCM determines injector pulse width
based on the following:
• Battery voltage
• Engine coolant temperature
• Engine RPM
• Inlet/Intake air temperature (IAT)
• MAP
• Throttle position
• The number of engine revolutions since cranking
was initiated
During Start-up the PCM maintains ignition tim-
ing at 9° BTDC.
ENGINE WARM-UP MODE
This is an OPEN LOOP mode. The following inputs
are received by the PCM:
• Engine coolant temperature
• Manifold Absolute Pressure (MAP)
• Inlet/Intake air temperature (IAT)
• Crankshaft position (engine speed)
• Camshaft position
• Knock sensor
• Throttle position
• A/C switch
• Battery voltage
• Vehicle speed
• Speed control
• O2 sensors
The PCM adjusts injector pulse width and controls
injector synchronization by turning the individual
ground paths to the injectors On and Off.
The PCM adjusts ignition timing and engine idle
speed. Engine idle speed is adjusted through the idle
air control motor.
CRUISE OR IDLE MODE
When the engine is at operating temperature this
is a CLOSED LOOP mode. During cruising or idle
the following inputs are received by the PCM:
• Inlet/Intake air temperature
• Engine coolant temperature
• Manifold absolute pressure
• Crankshaft position (engine speed)
• Camshaft position
• Knock sensor
• Throttle position
• Exhaust gas oxygen content
• A/C control positions
• Battery voltage
• Vehicle speed
The PCM adjusts injector pulse width and controls
injector synchronization by turning the individual
ground paths to the injectors On and Off.
The PCM adjusts engine idle speed and ignition
timing. The PCM adjusts the air/fuel ratio according
to the oxygen content in the exhaust gas (measured
by the upstream and downstream heated oxygen sen-
sor).
The PCM monitors for engine misfire. During
active misfire and depending on the severity, the
PCM either continuously illuminates or flashes the
malfunction indicator lamp (Check Engine light on
instrument panel). Also, the PCM stores an engine
misfire DTC in memory.
The PCM performs several diagnostic routines.
They include:
• Oxygen sensor monitor
• Downstream heated oxygen sensor diagnostics
during open loop operation (except for shorted)
• Fuel system monitor
• EGR monitor
• Purge system monitor
• All inputs monitored for proper voltage range.
• All monitored components (refer to the Emission
section for On-Board Diagnostics).
The PCM compares the upstream and downstream
heated oxygen sensor inputs to measure catalytic
convertor efficiency. If the catalyst efficiency drops
below the minimum acceptable percentage, the PCM
stores a diagnostic trouble code in memory.
During certain idle conditions, the PCM may enter
a variable idle speed strategy. During variable idle
speed strategy the PCM adjusts engine speed based
on the following inputs.
LH FUEL SYSTEM 14 - 23
DESCRIPTION AND OPERATION (Continued)
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