Verifying Vehicle Repair; Pcm Function - Hummer H1 Service Manual

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General Information
The control module system has a computer, Powertrain Con-
trol Module (PCM) to control fuel delivery timing, and some
emission control systems.
The control module system, monitors a number of engine and
vehicle functions and controls the following operations:
• Fuel control
• Fuel injection timing
• Speed Control
• Turbocharger Boost
• Transmission shift and shift quality functions. Specific
transmission control diagnostics are covered in Section
5 of this service manual.
Powertrain Control Module (PCM)
The diesel Powertrain Control Module (PCM) is located in the
passenger compartment and is the control center of the control
module system.
The PCM constantly looks at the information from various sen-
sors, and controls the systems that affect vehicle performance.
The PCM performs the diagnostic function of the system. It
can recognize operational problems, alert the driver through
the MIL (Check Engine), and store one or more DTCs which
identify the problem areas to aid the technician in making re-
pairs.
The PCM is designed to process the various input information
and then send the necessary electrical responses to control fuel
delivery, timing and other emission control systems. The input
information has an interrelation to more than one output, there-
fore, if the one input failed it could effect more than one sys-
tem's operation.

PCM Function

The PCM supplies a buffered 5 or 12 volts to power various
sensors or switches. This is done through resistances in the
PCM which are so high in value that a test light will not light
when connected to the circuit. In some cases, even an ordinary
shop voltmeter will not give an accurate reading because its re-
sistance is too low. Therefore, the use of a 10 megaohm imped-
ance digital voltmeter is necessary to assure accurate voltage
readings.
Reprogramming ("Flashing") The Control
Module
Some vehicles allow reprogramming of the control module
without removal from the vehicle. This provides a flexible and
a cost-effective method of making changes in software and cal-
ibrations.
05745159
PCM/Tech 1 Scan Tool 1

Verifying Vehicle Repair

Verification of vehicle repair will be more comprehensive for
vehicles with OBD II system diagnostics. Following a repair,
the technician should perform the following steps:
1. Review and record the fail records and/or Freeze Frame data
for the DTC which has been diagnosed (Freeze Frame data
will only be stored for an A or B type diagnostic and only
if the MIL has been illuminated).
2. Clear DTC(s).
3. Operate the vehicle within conditions noted in the fail
records and/or Freeze Frame data.
4. Monitor the DTC status information for the specific DTC
which has been diagnosed until the diagnostic test associ-
ated with that DTC runs.
Following these steps is very important in verifying repairs on
OBD II systems. Failure to follow these steps could result in
unnecessary repairs.
Reading DTCs Using A Scan Tool
The recommended method for reading DTC(s) stored in mem-
ory is by using diagnostic Scan Tool plugged into the DLC.
Follow instructions supplied by tool manufacturer.
"On-Board Diagnostic (OBD) System
Check"
After the visual/physical under hood inspection, the
"On-Board Diagnostic (OBD) "System Check" is the starting
point for all diagnostic procedures and for locating the cause of
an emissions test failure.
The correct procedure to diagnose a fault is to follow three ba-
sic steps.
1. Are the on-board diagnostics working? This is determined
by performing the "On-Board Diagnostic (OBD) System
Check." If the on-board diagnostics are not working, refer
to diagnostic charts in this section. If the on-board diag-
nostics are working properly, the next step will be:
2. Is there a DTC stored? If a DTC is stored, go directly to the
numbered DTC chart in this section. This will determine if
the fault is still present. If no DTC is stored, the next step
will be:
3. Scan Tool serial data transmitted by the control module.
This involves displaying the information available on the se-
rial data stream with a Scan Tool or one of the tools avail-
able for that purpose. Information on these tools and the
meaning of the various displays can be found in this section.
Normal readings under a particular operating condition can
be found below the chart "On-Board Diagnostic (OBD) Sys-
tem Check."