GM Service Manual Online
For 1990-2009 cars only

Object Number: 1201772  Size: SF
Master Electrical Component List
Automatic Transmission Controls Schematics
OBD II Symbol Description Notice

Circuit Description

The 1-2 shift solenoid (1-2 SS) valve is supplied with fused ignition voltage. The ground path is controlled by the transmission control module (TCM). When the TCM commands the solenoid OFF, a ground path is not provided and the filtered line pressure that is fed to the solenoid is exhausted. When the TCM commands the solenoid ON, a ground path is provided and the exhaust port is blocked, stopping the exhaust of line pressure.

The TCM identifies a 1-2 SS valve condition by monitoring the commanded gear versus gear ratio. Refer to Shift Solenoid Valve State and Gear Ratio .

If the TCM detects a second gear ratio when first gear is commanded and a third gear ratio when fourth gear is commanded, 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 TP sensor DTC P0120 or P0220.
    • No ISS DTC P0716 or P0717.
    • No OSS DTC P0722 or P0723.
    • No TCC system stuck ON DTC P0742.
    • No 1-2 SS valve electrical DTC P0973 or P0974.
    • No 2-3 SS valve electrical DTC P0976 or P0977.
    • The engine speed is greater than 500 RPM for 5 seconds.
    • The calc. throttle position is 8 percent or greater.
    • The transmission fluid temperature is between 20°C (68°F) and 130°C (266°F).
    • The engine torque is greater than 50 N·m (37 lb ft).
    • The transmission ISS is 150 RPM or greater.
    • The transmission OSS is 300 RPM or greater.
    • The time since the last gear change is 1.5 seconds.

Conditions for Setting the DTC

The following conditions both occur twice in 1 trip:

Condition 1

The TCM commands first gear and the gear ratio indicates second gear, 1.48:1-1.62:1, for 2 seconds.

Condition 2

The TCM commands fourth gear and the gear ratio indicates third gear, 0.95:1-1.05:1, for 3 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 commands an alternate coast pattern where downshifts occur at higher vehicle speeds while coasting.
    • The TCM freezes transmission adaptive functions.
    • The TCM disables the touch activated power (TAP) shift operation.
    • The TCM records the operating conditions when the Conditions for Setting the DTC are met. The TCM stores this information as Freeze Frame and 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 trip in which the TCM does not send an MIL illumination request.
    • 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 DTC P0751 is cleared, then will not set again when the vehicle is driven, some of the following conditions may exist:

    • Fluid contamination
    • Plugged fluid circuits
    • Restricted fluid circuits

Test Description

The number below refers to the step number on the diagnostic table.

  1. This step confirms the TCM commanded all the shifts, but all the shifts did not occur.

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 DTCs, use the scan tool in order to record the Freeze Frame and Failure Records for reference. Using the Clear Info function will erase the stored Freeze Frame and Failure Records from the TCM.

  4. Record the Freeze Frame and Failure Records.
  5. Clear the DTCs.
  6. Accelerate the vehicle in D4 range, ensuring the TCM commands first, second, third, and fourth gears.
  7. With the scan tool, observe Commanded Gear and Gear Ratio for each gear.

Did the gear ratios indicate a 2-2-3-3 shift pattern?

First: 2.87:1-2.97:1

Second: 1.52:1-1.62:1

Third: 0.95:1-1.05:1

Fourth: 0.65:1-0.75:1

Go to Step 4

Go to Diagnostic Aids

4

Inspect the 1-2 shift solenoid valve/hydraulic circuit for the following conditions:

    • The 1-2 shift solenoid mechanically stuck OFF
    • A stuck or binding 1-2 shift valve
    • Damaged seals on the shift solenoid valve

Refer to:

    •  Control Valve Body Disassemble in Transmission Unit Repair Manual

Did you find and correct the condition?

--

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.
    • Calc. throttle position must be 8% or greater.
    • Ensure the gear ratios are as specified for first and fourth gear.
  5. Select Specific DTC.
  6. Enter DTC P0751.

Has the test run and passed?

First: 2.87:1-2.97:1

Second: 1.52:1-1.62:1

Third: 0.95:1-1.05:1

Fourth: 0.65:1-0.75:1

Go to Step 6

Go to Step 2

6

With a 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