Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
107
records soft codes for intermittent problems. In a wig-
gle test, tap or wiggle wiring, connectors, and compo-
nents to try to get the problem to occur and reset the
code. Wiggle tests can be done with the key on and the
engine off or with the engine running. Usually, a wiggle
test can be conducted with a scan tool connected. This
allows you to monitor the data stream and watch for pa-
rameters to suddenly change. Be aware, some systems
will not set codes with a scan tool connected and oper-
ating in the diagnostic mode. For these vehicles, either
monitor the data stream, or simply perform the test
without the scan tool. If the problem occurs during the
wiggle test, remember what you did to cause it. After
wiggle testing, check for codes in memory once again
to verify the fault.
Some vehicles have output cycling tests as part of their
diagnostic capabilities. These systems have a bidirec-
tional PCM that accepts commands from a scan tool.
When in the test mode, the PCM generates a signal to
switch an actuator on and off in a controlled condition.
While the actuator is being switched, perform a wiggle
test and take voltmeter readings to pinpoint an intermit-
tent fault.
Check Connectors for Damage
Intermittent problems are often caused by damaged
wiring connectors and terminals. Poor connections cre-
ate high resistance that interrupts the signal. Unplug the
connectors in the problem circuit and inspect them
carefully.
Look for:
• Bent or broken terminals
• Corrosion
• Terminals that have been forced back into the con-
nector shell, causing an intermittent connection
• Loose, frayed, or broken wires inside the connector
shell. Most connectors are repairable, and often
spraying a small amount of electronic contact
cleaner on the terminals will help to remove dirt,
corrosion, and moisture. If damage is severe, re-
place the connector.
PCM/ECM Replacement
Vehicle computers do fail, but not with great frequency
or regularity. Too often the PCM or ECM is replaced sim-
ply because someone thought it might be the problem.
However, from time to time, manufacturer's release re-
vised computers to correct specific vehicle problems.
Fig. 5-28.
Checking for voltage ripple from an AC generator.
Fig. 5-29.
Checking the frequency of a MAP sensor.
Summary of Contents for ASE-A8
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