90-823225--1 10965E-94 - ELECTRONIC FUEL INJECTION (MULTI-PORT AND THROTTLE BODY)
Code 42: Ignition Control (IC) Circuit (Non-Scan)
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TO DISTRIBUTOR
IGNITION CONTROL (IC)
DIST. REFERENCE “HIGH”
BYPASS
DIST. REFERENCE
“LOW”
902 RED
TO IN-LINE HARNESS (TACH)
BA
121 WHT
121 GRY
3 PNK
IGNITION CONTROL (IC)
MODULE
CIRCUIT DESCRIPTION:
When the system is running on the ignition module,
that is, no voltage on the bypass line, the ignition
module grounds the IC signal. The ECM expects to
detect no voltage on the IC line during this condition.
If it detects a voltage, it sets Code 42 and will not go
into the IC mode.
When the RPM for IC is reached (about 300 RPM),
and bypass voltage applied, the IC should no longer
be grounded in the ignition module, so the IC voltage
should be varying.
If the bypass line is open or grounded, the ignition
module will not switch to IC mode so the IC voltage
will be low and Code 42 will be set.
If the IC line is grounded, the ignition module will
switch to IC but, because the line is grounded, there
will be no IC signal. A Code 42 will be set.
TEST DESCRIPTION:
Number(s) below refer to circled number(s) on the
diagnostic chart.
1. Code 42 means the ECM has seen an open or
short to ground in the IC or bypass circuits. This
test confirms Code 42 and that the fault causing
the code is present.
2. Check for a normal IC ground path through the
ignition module. An IC CKT 423 shorted to
ground will also read more than 3000 ohms; how-
ever, this will be checked later.
3. As the test light voltage touches CKT 424, the
module should switch, causing the DVM reading
to go from over 3000 ohms to under 1000 ohms.
The important thing is that the module “switched.”
4. The module did not switch and this step checks
for:
• IC CKT 423 shorted to ground.
• Bypass CKT 424 open.
• Faulty ignition module connection or module.
5. Confirms that Coded 42 is a faulty ECM and not
an intermittent in CKT 423 or CKT 424.
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