GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2007 🢒 Lucerne 🢒 DTC 🢒 DTC P0751

Object Number: 1550350 Size: LF
Master Electrical Component List
Automatic Transmission Controls Schematics
OBD II Symbol Description Notice
Control Module References

Circuit Description

The 1-2 shift solenoid (SS) valve is a normally-open exhaust valve that, in conjunction with the 2-3 shift solenoid (SS) valve, allows 4 different shifting combinations. The 1-2 SS valve is attached to the lower control valve body, within the transmission.

The transmission control module (TCM) monitors the actual gear ratio, and compares the actual gear ratio with the commanded gear ratio. DTC P0751 sets under two conditions:

    • A stuck OFF 1-2 SS valve
    • A stuck OFF 1-2 shift valve

If the TCM detects a 2-2-3-3 shift pattern, then DTC P0751 sets. DTC P0751 is a type B DTC.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0751 1-2 Shift Solenoid Valve Performance - No First or Fourth Gear

Conditions for Running the DTC
    • No MAF DTCs P0101, P0102 or P0103.
    • No MAP DTCs P0106, P0107 or P0108.
    • No OSS DTCs P0722 or P0723.
    • No AT ISS DTCs P0716 or P0717.
    • No shift solenoid electrical DTCs P0973, P0974, P0976 or P0977.
    • No TCC stuck ON DTC P0742.
    • The engine run time is greater than 5 seconds.
    • The time since the gear select lever change is 1 second.
    • The transmission fluid temperature is 20-130°C (68-266°F).
    • The TP angle is 8 percent or greater.
    • L37: The transmission output shaft speed is 239 RPM or greater.
    • The engine torque is 50-1492 N·m (37-1100 lb ft).

Conditions for Setting the DTC

The following conditions occur twice during the same trip:

    • The TCM commands 1st gear and the gear ratio is 1.54:1-1.71:1 for 2 seconds.
    • The TCM commands 4th gear and the gear ratio is 0.95:1-1.05:1 for 4 seconds.

Action Taken When the DTC Sets
    • The TCM requests the engine control module (ECM) to illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the Conditions for Setting the DTC are met.
    • The TCM commands maximum line pressure.
    • The TCM freezes transmission adaptive functions.
    • The ECM records the operating conditions when the Conditions for Setting the DTC are met. The ECM stores this information as Freeze Frame and Failure Records and the TCM records the operating conditions when the Conditions for Setting the DTC are met. The TCM stores this information as Failure Records.
    • The TCM stores DTC P0751 in TCM history during the second consecutive trip in which the Conditions for Setting the DTC are met.

Conditions for Clearing the MIL/DTC
    • The ECM turns OFF the MIL after the third consecutive drive trip in which the TCM does not send a MIL illumination request.
    • The TCM clears the DIC message when the condition no longer exists.
    • A scan tool can clear the MIL/DTC.
    • The TCM clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without an emission-related diagnostic fault occurring.
    • The TCM cancels the DTC default actions when the ignition switch is OFF long enough in order to power down the TCM.

Diagnostic Aids
    • If you clear DTC P0751, and then cannot reset DTC P0751, the following conditions may exist:
       - Fluid contamination
       - Plugged fluid circuits
       - Restricted fluid circuits
       - Sticking 1-2 SS
       - Sticking 1-2 shift valve
    • Refer to Shift Solenoid Valve State and Gear Ratio .

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. This step verifies a 2-2-3-3 shift pattern.

    2.

  2. This step tests for a mechanical or hydraulic condition causing the 1-2 SS valve to be stuck OFF, or the circuit to be released.

DTC P0751

Step

Action

Values

Yes

No

1

Did you perform the Diagnostic System Check - Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

Did you perform the Transmission Fluid Checking Procedure?

--

Go to Step 3

Go to Transmission Fluid Check

3

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.

    3. Important: Before clearing the DTC, use the scan tool in order to record the Freeze Frame and Failure Records. Using the Clear Info function erases the Freeze Frame and Failure Records from the TCM.

  3. Record the DTC Freeze Frame and Failure Records.
  4. Clear the DTC.
  5. Use the scan tool snapshot mode in order to record commanded gear and gear ratio.
  6. Drive the vehicle in D4, in order to obtain 1-2, 2-3 and 3-4 shifts with a TP angle of 8% or greater.

Are the commanded gear and gear ratio within the specified parameters?

1st/2nd

1.54:1-1.71:1

3rd/4th

0.95:1-1.05:1

Go to Step 4

Go to Diagnostic Aids

4

  1. Inspect the 1-2 shift circuit for the following conditions:
    2. • 1-2 SS valve mechanically stuck OFF.
    • Refer to Shift Solenoid Leak Test .
    • 1-2 SS valve O-ring worn or damaged.
    • 1-2 shift valve stuck in the released position.
  2. Repair the circuit as necessary.

Refer to Control Valve Lower Body Disassemble .

Did you complete the repair?

--

Go to Step 5

--

5

Perform the following procedure in order to verify the repair:

  1. Select DTC.
  2. Select Clear Info.
  3. Operate the vehicle under the following conditions:
    4. • Drive the vehicle in D4 with a TP angle of 8% or greater.
    • Allow the transmission to shift through all the gears.
    • Monitor the commanded gear and gear ratios. The actual gear ratios must match the commanded gears for 0.9 seconds, for all gears.
  4. Select Specific DTC.
  5. Enter DTC P0751.

Has the test run and passed?

1st

2.75:1-3.17:1

2nd

1.51:1-1.74:1

3rd

0.93:1-1.07:1

4th

0.63:1-0.73:1

Go to Step 6

Go to Step 2

6

With the scan tool, observe the stored information, capture info. and DTC info.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK