TROUBLE CODE DIAGNOSTICS - 7.4L MPI
SERVICE MANUAL NUMBER 23
90-861326--1 MARCH 1999
Page 5G-61
Code 42 (2 of 3): Ignition Control (IC) Circuit (Scan) Grounded IC Circuit, Open
Bypass Circuit, or Grounded Bypass Circuit
TEST DESCRIPTION:
Step 2. DTC 42 means the ECM has seen an open or short to ground in the bypass cir-
cuit, or a short to ground in the IC circuit. This test confirms a DTC 42 and that the fault
causing the DTC is present.
Step 3. Checks for a normal IC ground path through the Ignition Control (IC) module.
An IC CKT 423 shorted to ground will also read less than 3000 ohms, however, this will
be checked later.
Step 4. As the test light voltage touches CKT 424, the module should switch, causing
the DVOM reading to go from over 3000 ohms to under 1000 ohms. The important thing
is that the module switched.
Step 5. If module did not switch, check for open in bypass CKT 424, bypass CKT 424
shorted to ground, or faulty ignition module.
Step 7. Confirms that DTC 42 is a faulty ECM and not an intermittent in CKT 423 or CKT
424.
STEP
ACTION
VALUE
YES
NO
PROCEED TO
1
Was the “On-Board Diagnostic” (OBD) System
Check Performed?
–
Step 2
Go to
OBD
System
Check
2
1. Clear trouble code 42.
2. Start engine and idle for 2 minutes or un-
til trouble code 42 sets.
Is trouble code 42 present?
–
Step 3
Step13
3
1. Ignition “OFF”.
2. Disconnect ECM harness connectors.
3. Using a DVOM selected for Ohms, probe
ECM harness terminal J2-23 to ground.
Is resistance within the specified value?
3000 to
6000
Ohms
Step 4
Step 8
4
1. Leave DVOM connected from ECM har-
ness terminal J2-23 to ground.
2. Using a test light connected to battery
positive (B+), probe ECM harness terminal
J2-24.
1000
3.As the test light contacts J2-24, the resis-
tance should switch from over 3000 Ohms to
under 1000 Ohms.
Ohms
Does the resistance switch to under the
specified value?
Step 7
Step 5
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