Pontiac Fiero 1988 Service Manual page 633

6E3-A-50 DRfWABlllTY AND
EMISSIONS -
FUEL
INJECTION (PORT)
"C"
IGN.
BLK.
CONN.
GRY.
CONN.
PICK-UP
COSL
TACH.
IGNITION COIL
CONNECTOR
DISTRIBUTOR
GRY.
CONN.
5-19-87
DISTRIBUTOR CONNECTOR
I CM
PNK ~ J
WHT
423 WHT
430 PPI/WHT
424 TAN/BLK
453 BLK/RED
B4
B5
D5
B3
EST
— REFERENCE
— BY-PASS
TACH. LEAD
5-8-87
*5S1923-6E
C O D E 42
ELECTRONIC SPARK TIMING (EST) CIRCUIT
2.8L " P " SERIES (PORT)
Circuit Description:
When the system is running on the ignition module, that is no voltage on the bypass line, the ignition
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) and bypass voltage applied, the EST should on longer be
grounded in the ignition module so the EST voltage should be varying.
If the bypass line is open or grounded, the ignition module will not switch to EST mode so the EST voltage
will be low and Code 42 will be set.
If the EST line is grounded, the ignition module will switch to EST, but because the line is grounded there
will be no EST signal. A Code 42 will be set.
Test Description:
Numbers below refer to circled
numbers on the diagnostic chart.
1. Code 42 means the ECM has seen an open or short
to ground in the EST or by-pass circuits. This test
confirms Code 42 and that the fault causing the
code is present.
2. Checks for a normal EST ground path through the
ignition module. An EST CKT 423 shorted to
ground will also read less than 500 ohms,
however, this will be checked later.
3. As the test light voltage touches CKT 424, the
module should switch, causing the ohmmeter to
"overrange" if the meter is in the 1000-2000 ohms
position. Selecting the 10-20,000 ohms position
will indicate above 5000 ohms. The important
thing is that the module "switched".
4. The module did not switch and this step checks for:
• EST CKT 423 shorted to ground
• Bypass C KT 424 open
• Faulty ignition module connection or module
5. Confirms that Code 42 is a faulty ECM and not an
intermittent in CKTS 423 or 424.
Diagnostic Aids:
The "Scan" tool does not have any ability to help
diagnose a Code 42 problem.
A PROM not fully seated in the ECM can result in
a Code 42.
Refer to "Intermittents" in Section "B".