Pontiac Fiero 1988 Service Manual page 548

DRIVEABILITY AND EMISSIONS - FUEL INJECTION (TBI) 6E2-C4-1
SECTION C4
DIRECT IGNITION SYSTEM (DIS) / EST - 2.5L
CONTENTS
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G E N E R A L D E S C R I P T I O N
S Y S T E M O P E R A T I O N . *
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S Y S T E M C O M P O N E N T S .
C r a n k s h a f t S e n s o r * «
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Ignition Coils
" D I S " M o d u l e . . . . . .
Electronic Spark T i m i n g (EST)
DIAGNOSIS • . . .
S Y M P T O M D I A G N O S I S
C H E C K I N G EST P E R F O R M A N C E
GENERAL DESCRIPTION
C4-1
C4-1
C4-1
C 4 1
C 4 - 2
C 4 - 2
C 4 - 2
C 4 - 3
C4~3
C4~3
SYSTEM OPERATION
The Direct Ignition System (DIS) does not use the
conventional distributor and coil. This ignition
system consists of two separate ignition coils, a "DIS"
ignition module and Crankshaft Sensor as well as the
related connecting wires and the EST (Electronic
Spark Timing) portion of the ECM.
A distributorless ignition system, such as this one,
uses a "waste spark" method of spark distribution.
Each cylinder is paired with the cylinder that is
opposite it (1-4 or 2-3). The spark occurs
simultaneously in the cylinder coming up on the
compression stroke and in the cylinder coming up on
the exhaust stroke.
The cylinder on the exhaust stroke requires very
little of the available energy to fire the spark plug.
The remaining energy will be used as required by the
cylinder on the compression stroke. The same process
is repeated when the cylinders reverse roles.
It is possible in a no load condition for one plug to
fire even though the spark plug lead from the same
coil is disconnected from the other spark plug. The
disconnected spark plug lead acts as one plate of a
capacitor, with the engine being the other plate.
These two "capacitor plates" are charged as a current
surge (spark) jumps across the gap of the connected
spark plug. The "plates" are then discharged as the
secondary energy is dissipated in an oscillating
current across the gap of the spark plug still
connected. Because of the direction of current flow in
the primary winding and thus in the secondary
winding, one plug will fire from the center electrode to
the side electrode while the other will fire from side
electrode to center electrode.
These systems utilize the EST signal from the
ECM, as do distributor type ignition systems equipped
with EST, to control spark timing. Under 400 rpm,
the "DIS" module controls spark timing (module
C o d e 12 • • .
C o d e 4 2 • • • • » • » *
S e t t i n g T i m i n g
ON-CAR S E R V I C E . . . . .
" D I S A S S E M B L Y . . . .
C R A N K S H A F T S E N S O R
IGNITION COIL(S)
IGNITION M O D U L E
P A R T S I N F O R M A T I O N
C 4 - 3
C4~3
C4-3
C 4 - 3
C 4 - 3
C 4 - 3
C 4 - 4
C 4 - 4
C 4 - 4
timing mode) and over 400 rpm, the ECM controls
spark timing (EST mode). To properly control ignition
timing, the ECM relies on the following information:
• Engine load (manifold pressure or vacuum).
• Atmospheric (barometric) pressure.
• Engine temperature.
• Manifold air temperature.
• Crankshaft position.
9 Engine speed (rpm).
SYSTEM COMPONENTS
Crankshaft Sensor
This system uses a magnetic crankshaft sensor,
mounted to the bottom of the "DIS" module which
protrudes into the block, within approximately .050"
of the crankshaft reluctor. Figure C4-1 illustrates a
typical sensor in relationship to the crankshaft
reluctor. The reluctor is a special wheel cast into the
crankshaft with seven slots machined into it, six of
which are equally spaced (60° apart). A seventh slot is
spaced 10° from one of the other slots and serves to
generate a "sync-pulse". As the reluctor rotates as
part of the crankshaft, the slots change the magnetic
field of the sensor, creating an induced voltage pulse.
Based on the crank sensor pulses, the "DIS"
module sends reference signals to the ECM which are
used to indicate crankshaft position and engine speed.
The "DIS" module will continue to send these
reference pulses to the ECM at a rate of one per each
180° of crankshaft rotation. The ECM will activate
the fuel injector based on the recognition of every
other reference pulse beginning at a crankshaft
position of 120° after top dead center. By comparing
the time between pulses, the "DIS" module can
recognize the pulse representing the seventh slot (sync
pulse) which starts the calculation of ignition coil
sequencing.