Pontiac Fiero 1988 Service Manual page 686

D R I V E A B I L I T Y A N D E M I S S I O N S - F U E L I N J E C T I O N ( P O R T ) 6 E 3 - C 4 - 1
SECTION C4
I G N I T I O N S Y S T E M / E S T
C O N T E N T S
GENERAL DESCRIPTION . . . . . . C4-1
PU RPOSE
e o o o o o e c e o o o o o o Q o o o o o A o o o o
C4-1
Results of
Incorrect
Operation . . . . . . . C4-1
How Code 42 is Determined • • • • • • • • C4-1
DIAGNOSIS
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C 4 ™ %
GENERAL DESCRIPTION
PURPOSE
The High Energy Ignition (HEI) system controls
fuel combustion by providing a spark to ignite the
compressed air/fuel mixture at the correct time. To
provide improved engine performance, fuel economy,
and control of exhaust emissions, the electronic
control module (ECM) controls distributor spark
advance (timing) with the Electronic Spark Timing
(EST) system.
Only the EST system will be described here.
Additional information on the HEI system is found in
Section "6D M .
To properly control ignition/combustion timing
the ECM needs to know:
• Crankshaft position
• Engine Speed (rpm)
• Manifold pressure
• Engine temperature
The EST system consists of the distributor
module, ECM, and connecting wires. The distributor
has module terminals which are connected directly to
the engine harness connectors. The connector
terminals are lettered as shown in CHART C-4B.
These circuits perform the following functions:
m
Distributor reference (CKT 430).
This provides the ECM with rpm and crankshaft
nosition information.
*> Reference ground (CKT 453).
This wire is grounded in the distributor and
makes sure the ground circuit has no voltage drop,
between the distributor and ECM, which could
affect performance.
•
By-Pass (CKT 424).
At about 400 rpm, and ECM applies 5 volts to this
circuit to switch spark timing control from the
HEI module to the ECM. An open or grounded
bypsss circuit will set a Code 42 and the engine
will run at base timing, plus a small amount of
advance built into the HEI module.
•
EST (CKT 423).
This circuit triggers the HEI module. The ECM
does not know what the actual timing is, but it
does know when it gets the reference signal.
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ON-CAR SERVICE
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C4-2
SETTING Tl f VJ ING
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PARTS INFORMATION C4-2
It then advances or retards the spark from that
point. Therefore, if the base timing is set
incorrectly, the entire spark curve will be
incorrect.
Results of Incorrect Operation
An open or ground in the EST or bypass circuit
will set a Code 42 and cause the engine to run on the
HEI module timing. This will cause poor performance
and poor fuel economy.
How Code 42 is Determined
When the system is running on the HEI module,
that is , no voltage on the by-pass line, the HEI module
grounds the EST signal. The ECM expects to see no
voltage on the EST line during this condition. If it
sees a voltage, it sets Code 42 and will not go into the
EST mode.
When the rpm for EST is reached (about 400 rpm)
the ECM applies 5 volts to the by-pass line and the
EST should no longer be grounded in the HEI module
so the EST voltage should be varying.
If the by-pass line is open or grounded, the HEI
module will not switch to EST and Code 42 will be set.
D I A G N O S I S
The description, operation, and diagnosis of the
HEI system are found in Section "6D" of this manual.
CHART C-4 should be used for diagnosing a no spark
condition.
CODE 12
Code 12 is used during the Diagnostic Circuit
Check procedure to test the code display ability of the
ECM. This code indicates that the ECM is not
receiving the engine rpm (REFERENCE) signal. This
occurs with the ignition key "ON" and the engine not
running.
The "Reference" signal also triggers the fuel
injection system. Without the "Reference" signal the
engine cannot run.