Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
108
Most newer control modules can be "flash" pro-
grammed to electronically update the internal program-
ming, rather than physically replacing the PCM or ECM.
Check these items before replacing the PCM or ECM:
1. Batter voltage supply to the PCM or ECM and main
system ground - Be sure the battery is fully
charged and provides at least 9.6 to 10 volts dur-
ing cranking. Be sure the charging system is main-
taining correct battery charge. Most control sys-
tems receive battery voltage through a fuse, fusible
link, or both. Be sure that battery voltage is avail-
able at the specified terminal of the main harness
connector. Most systems are grounded remotely
through a wire in the harness. Trace and check the
ground connection to ensure good continuity.
2. Operation of a system power relay - The power to
the PCM or ECM may be supplied through a sys-
tem power relay. If so, check the relay.
3. Sensor reference voltage and ground circuits -
Many sensors share a common reference voltage
supply from the PCM or ECM and a common
ground. Incorrect or erratic reference voltage or
bad common ground can affect operation of sev-
eral sensors simultaneously. The system may ap-
pear as if the PCM or ECM has a major internal
problem.
4. Resistance and current flow through all controlled
solenoids and relays - Every output device con-
trolled by the PCM or ECM has a minimum resis-
tance specification, Actuator resistance limits the
current through the PCM or ECM output control
circuit. If the actuator is shorted, current can ex-
ceed the maximum safe resistance and internally
damage the PCM or ECM. Before replacing a PCM
or ECM, check all output circuits for shorts or low
resistance that could cause internal damage to the
computer.
As a general rule, the PCM or ECM should be at the bot-
tom of the list of things to replace. It can fail, but a
sensor, actuator, faulty wiring, poor connection, or me-
chanical failure is more likely the cause of the problem.
Interrelated Systems
There are many systems on a late-model vehicle that di-
rectly or indirectly interface with the PCM. When diag-
nosing any driveability concern check for any of the fol-
lowing systems and verify they are operating properly
before condemning the PCM or its inputs and outputs:
• Cruise control system
• Security alarm or other anti-theft device
• Traction and/or torque control systems
• Climate control systems
• Aftermarket components of any type
Begin your diagnosis by disabling or monitoring the in-
terfacing system that may be causing the concern
Cruise control systems directly or indirectly control the
throttle opening and may be the root cause of a throttle
control problem.
Keep in mind that most anti-theft devices shutoff the
fuel to the engine and may also disable the starter cir-
cuit. A system that is not operating correctly may cause
the engine to fail to start or start and then stall. If the
starter circuit is affected generally a failure in the theft
deterrent system will result in a no-crank concern. Trac-
tion and torque control systems send a message to the
PCM requesting a reduction in engine torque to help re-
duce wheel spin or reduce transmission wear during up-
shifts.
Fig. 5-30.
Checking the duty cycle of a canister purge solenoid.
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