ENGINE CONTROLS 1F – 31
DAEWOO V–121 BL4
PCM/ECM OUTPUT DIAGNOSIS
Circuit Description
The powertrain control module (PCM)/engine control
module (ECM) controls most components with electronic
switches which complete a ground circuit when turned on.
These switches are arranged in groups of 4 and 7, and are
called either a surface mounted quad driver module, which
can independently control up to 4 output terminals or an
Output Driver Module (ODM), which can independently
control up to 7 outputs. Not all of the outputs are always
used.
Drivers are fault protected. If a relay or solenoid is shorted,
having very low or zero resistance, or if the control side of
the circuit is shorted to voltage, it would allow too much
current flow into the PCM/ECM. The driver senses this
and the output is turned OFF or its internal resistance in-
creases to limit current flow and protect the PCM/ECM
and driver. The result is high output terminal voltage when
it should be low. If the circuit from B+ to the component or
the component is open, or the control side of the circuit is
shorted to ground, terminal voltage will be low. Either of
these conditions is considered to be a driver fault.
Drivers also have a fault line to indicate the presence of a
current fault to the PCM/ECM’s central processor. A scan
tool displays the status of the driver fault lines as 0=OK
and 1=Fault.
Diagnostic Aids
The scan tool has the ability to command certain compo-
nents and functions ON and OFF. If a component or func-
tion does not have this capability, operate the vehicle dur-
ing its normal function criteria to check for an open or
shorted circuit.
An open or short to ground will appear in the open posi-
tions on the scan tool only when it is not commanded by
the PCM/ECM or the scan tool, while a short to voltage will
appear in the short positions on the scan tool only while the
component is being commanded by the PCM/ECM or
scan tool.
Test Description
Number(s) below refer to the step number(s) on the Diag-
nostic Table.
1. The Powertrain On–Board Diagnostic (OBD II) Sys-
tem Check prompts the technician to complete
some basic checks and store the freeze frame and
failure records data on the scan tool if applicable.
This creates an electronic copy of the data taken
when the malfunction occurred. The information is
then stored on the scan tool for later reference.
2. A 1 in any of the positions indicates that an open or
short is present in the corresponding circuit for that
position.
4. An open or short to ground will appear in the open
positions on the scan tool only when it is not com-
manded by the PCM/ECM or scan tool, while a
short to voltage will appear in the short positions on
the scan tool only while the component is being
commanded by the PCM/ECM or scan tool.
5. Proper operation of any component of function with
no 1 in any operation of the positions indicates that
system operation is normal at this time.
6. A component or function that failed to operate at
this point indicates that the fault is not on the PCM/
ECM side of the circuitry.
8. The 1 going away after disconnecting the compo-
nent electrical connector indicates that the compo-
nent or component side wiring is at fault. If the scan
tool indicates a fault after disconnecting the compo-
nent electrical connector and verifying that no open
or short is present in the circuit, then the PCM/ECM
is faulty.
9. The replacement PCM/ECM must be repro-
grammed. Refer to the latest Techline procedure for
PCM/ECM reprogramming.
10. If no faults have been found at this point, refer to
”Diagnostic Aids” in this section for additional
checks and information.
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