Hummer H1 (2006+). Manual — part 113
OBD-296
On-Board Diagnostics
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Conditions for Setting the DTC
DTC P0748 sets when the transmission control module (TCM) detects one of the following conditions:
• A short to power
• A short to ground
• High or low resistance in PC solenoid A
• High or low resistance in PC solenoid A wiring
Action Taken When the DTC Sets
• The TCM illuminates the malfunction indicator lamp (MIL).
• When in a forward range, the transmission fails to 1st, 3rd, or 5th range while diagnostic response is active.
• If the shift selector is moved to NEUTRAL, the transmission will shift to NEUTRAL.
• If the shift selector is moved to REVERSE, the transmission will shift to REVERSE, unless the transmission is compro-
mised by a direction change, then the transmission will shift to or lock in NEUTRAL.
• If the shift selector is returned to a forward range and the transmission is compromised by over speeding or direction
change, the transmission will lock in NEUTRAL.
• DTC P0748 will be stored in TCM history.
• The TCM freezes shift adapts.
• The TCM inhibits TCC engagement.
Conditions for Clearing the DTC
A scan tool can clear the code from TCM history. The TCM automatically clears the DTC from TCM history if the vehicle com-
pletes 40 warm-up cycles without failure.
Diagnostic Aids
• Inspect the wiring for poor electrical connections at the TCM. Inspect for the following conditions:
• A bent terminal
• A backed-out terminal
• A damaged terminal
• Poor terminal tension
• A chafed wire
• A broken wire inside the insulation
• When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a
change.
• You may have to drive the vehicle in order to experience a condition.
• Solenoids are sensitive to battery voltage spikes and poor ground connections. This condition may not be indicated with
your scan tool, or with a DVOM.
• The installation of aftermarket devices such as remote starters may interfere with proper TCM voltage supply.
• A faulty ignition switch may cause improper voltage supply to the TCM.
• Power or ground corrosion may cause voltage variation.
• Voltage may vary by 0.5 volts based on the state of charge in the battery.
• PC solenoid A operates on 1,000 Hz and can be measured using a DVOM.
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On-Board Diagnostics OBD-297
5745804
Step
Action
Value(s)
Yes
No
1
NOTE: Before clearing DTC(s) use the scan tool “Capture info”
to record Freeze Frame and Failure Records for reference, as data
will be lost when “Clear Info” function is used
Was the “On-Board Diagnostic (OBD) System Check” per-
formed?
Go to step 2.
Go to OBD Sys-
tem Check.
2
1.
Install the scan tool.
2.
Start the engine.
NOTE:
• Before clearing the DTC, use the scan tool in order to
record the Freeze Frame and Failure Records for refer-
ence. The Clear Info function will erase the data.
• This diagnostic table should only be used if the DTC is
currently failing. If the DTC is not active, refer to Test-
ing for Intermittent Conditions and Poor Connections
in Wiring Systems until the DTC becomes active.
3.
Record the DTC Freeze Frame and Failure Records.
4.
Clear the DTC.
5.
Using the scan tool, measure ignition voltage.
Is voltage within the specified range?
9-18 Volts
Go to Step 4
Go to DTC
P0562 for low
voltage or DTC
P0563 for high
voltage
3
1.
Turn the ignition OFF.
2.
Disconnect C78 at the TCM.
3.
Install the J 39700 breakout box and the J 43799 breakout
box adapter.
4.
Turn ON the ignition, with the engine OFF.
5.
Using a DVOM, measure voltage at C78 terminals 2, 4, and
3 to a known good ground.
Is the voltage within the specified value?
B+
Go to Step 4
Go to Diagnos-
tic Aids
4
1.
Turn the ignition OFF.
2.
Disconnect the C79 at the TCM.
3.
Install the J 39700 breakout box and the J 43799 breakout
box adapter.
4.
Using a DVOM, measure voltage between connector C79
terminal 22 and terminal 23.
5.
Turn ON the ignition, with the engine OFF.
6.
Using the scan tool, command PC solenoid A ON.
Is the voltage within the range specified when PC solenoid A is
commanded ON?
7-7.5 Volts
Go to Step 5
Go to Step 11
5
1.
Turn the ignition OFF.
2.
Using a DVOM measure the resistance between connector
C79 terminal 22 and terminal 23.
Is the resistance within the specified value?
5.5 ohms
[commat]
20°C
(68°F)
Go to Diagnos-
tic Aids
Go to Step 6
OBD-298
On-Board Diagnostics
_____________________________________________
6
1.
Connect the C79 to the TCM.
2.
Disconnect the transmission 20-way connector C34.
3.
Using a DVOM, measure the resistance between terminals L
and M of the transmission side of the 20-way connector.
Is resistance within the specified value?
5.5 ohms
[commat]
20°C
(68°F)
Go to Step 9
Go to Step 7
7
1.
Remove the oil pan. Refer to Transmission Oil Pan Replace-
ment.
2.
Remove the internal wiring harness at the PC Trim solenoid
A connector.
3.
Using the DVOM, measure the resistance of the PC solenoid
A.
Is the solenoid resistance within the specified value?
5.5 ohms
[commat]
20°C
(68°F)
Go to Step 8
Go to Step 10
8
Replace the internal wiring harness.
Did you complete the replacement?
Go to Step 12
9
Inspect the cowl top harness for an open or short to ground on cir-
cuits 773 (YL) and 774 (WH) between C79 and C83.
Did you find and correct a condition?
Go to Step 12
10
Replace PC solenoid A.
Did you complete the replacement?
Go to Step 12
11
NOTE: In most cases, the TCM is not at fault. Investigate thor-
oughly before replacing the TCM.
Replace the faulty TCM.
Did you complete the replacement?
Go to Step 12
12
Perform the following procedure in order to verify the repair:
1.
Clear the DTC.
2.
Drive the vehicle under normal operating conditions.
3.
Select Specific DTC.
4.
Enter DTC P0748.
Has the test run and passed?
System OK
Go to Step 1
Step
Action
Value(s)
Yes
No
_____________________________________________
On-Board Diagnostics OBD-299
5745804
DTC P0746 Shift Solenoid C Electrical (Figure OBD-78)
Figure OBD-78: Shift Solenoid C Circuit
Circuit Description
Pressure control (PC) solenoid A is used to control oncoming, off-going, and holding pressure to any one of the five clutches. This
solenoid is referred to as a pressure proportional to current (PPC) solenoid since the output hydraulic pressure supplied by this so-
lenoid is proportional to the controlled current command.
The solenoid operates using a very high frequency pulse width modulation signal of 1,000 Hz. Unlike the torque converter clutch
pulse width modulated (TCC PWM) solenoid, where the ball follows the pulse width square wave, the PPC ball remains stationary.
The ball clearance is balanced between a coil current force on one side of the ball and the clutch trim pressure force on the other
side of the ball. The ball-to-seat clearance adjusts the bleed-off rate in the signal pocket. At high bleed levels, the resulting flow
across the feed orifice results in a high pressure drop. This produces the desired signal pressure level used to trim the clutch.
DTC P0748 is a type A DTC.
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