Hummer H1 Service Manual page 1069

4 PCM/Tech 1 Scan Tool
POWERTRAIN OBD SYSTEM CHECK
Circuit Description
The On Board Diagnostic System Check is an organized ap-
proach to identifying a problem created by an electronic engine
system fault. The OBD system check is the starting point for
any driveability diagnosis. The OBD system check directs the
service technician to the next step in diagnosing the complaint.
Do not perform this check if no driveability complaint exists.
Understanding the table correctly reduces the diagnostic time.
Understanding the table correctly prevents the replacement of
good parts.
Diagnostic Aids
Important: Do not clear the DTCs unless directed by a diagnos-
tic procedure. Clearing the DTCs will also clear valuable
freeze frame and failure records data.
Inspect all related wiring and connections including the con-
nections at the PCM. These may cause and intermittent mal-
function.
Check any circuitry that is suspected of causing an intermittent
problem for the following conditions:
• Backed out terminals
• Improper mating
• broken locks
• Improperly formed or damaged terminals
• Poor terminal to wiring connections
• Physical damage to the wiring harness
• Corrosion
• Poor pin tension
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Test Description
The numbers below refer to the step number in the diagnostic
table.
1. When the ignition is turned on, the MIL will momentarily
2. The diagnostic tables in this section are designed for use
with a properly functioning scan tool. Before beginning any
other diagnostic procedures, correct the serial data commu-
nications. The Class II serial data will not transmit if the sys-
tem voltage measures below 9v or above 16v.
3. Whenever multiple DTCs are stored, refer to the DTC tables
in the following order:
5. By storing the Freeze Frame Data and the Failure records se-
lection in the scan tool, an electronic copy of the freeze
frame data and the failure records taken when the fault oc-
curred is stored, which can be referred to later.
6. Checking the sensors for proper operation during warm up
can be a crucial step in correctly diagnosing any driveability
concern. Careful observation of these sensors during the en-
gine warm up may reveal a slow responding sensor or a sen-
sor that malfunctions only within a small portion of its
range.
After the engine is at the normal operating temperature, a com-
parison of the actual control system data with the typical values
is a quick way to determine if any parameter is not within lim-
its. Keep in mind that a base engine problem may substantially
alter sensor values.
Check the following sensors for proper operation:
flash ON and OFF and then remain on until the engine is
running if no Diagnostic Trouble Codes are stored.
a. PCM Error DTCs
b. System voltage DTCs
c. Component level DTCs (DTCs that indicate a faulty
part
d. System level DTCs (DTCs that indicate a system
fault).
• The Engine Coolant Temperature Sensor (ECT) for ini-
tial coolant temperature reading at ambient. Then ob-
serve the rise in the temperature while the engine is
warming up.
• The Intake Air Temperature Sensor (IAT) for initial air
temperature reading at ambient. Then observe the rise in
the temperature while the engine is warming up.
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