GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2000 🢒 GMC C Pickup - 2WD 🢒 Transmission/Transaxle 🢒 Automatic Transmission - 4L80-E 🢒 DTC P0751 1-2 Shift Solenoid Valve Performance

DTC P0751 1-2 Shift Solenoid Valve Performance Gas


Object Number: 40795 Size: MF
Automatic Transmission Controls Schematics Gas

Circuit Description

The 1-2 shift solenoid (SS) valve controls the fluid flow action on the 1-2 and the 3-4 shift valves. The 1-2 SS valve is a normally-open exhaust valve that is used with the 2-3 SS in order to allow four different shifting combinations.

When the vehicle control module (VCM) detects a 2-2-3-3 gear ratio or a 1-1-4-4 gear ratio, then DTC P0751 sets. DTC P0751 is a type C DTC. For California emissions vehicles DTC P0751 is a type B DTC.

Conditions for Running the DTC
    • No MAF sensor DTCs P0101, P0102 or P0103.
    • No MAP sensor DTCs P0106, P0107, or P0108.
    • No TP sensor DTCs P0121, P0122, or P0123.
    • No OSS DTCs P0502 or P0503.
    • No ISS sensor DTCs P0716 or P0717.
    • No 1-2 SS valve DTC P0753.
    • No 2-3 SS valve DTC P0758.
    • No TFP manual valve position switch DTC P1810.
    • No TCC PWM solenoid valve DTC P1860.
    • No 4WD low DTC P1875.
    • Not in 4WD low.
    • The engine must be running more than 475 RPM for 7 seconds.
    • The vehicle speed is greater than 3 km/h (2.0 mph).
    • The TP sensor is greater than 10%.
    • The engine torque is 80 N·m (60 lb ft) to the following:
       - 542 N·m (400 lb ft) 5.7L
       - 678 N·m (500 lb ft) 7.4L
    • The transmission fluid temperature (TFT) is greater than 20°C (68°F).

Conditions for Setting the DTC

All conditions for running the DTC are met, and one of the following conditions occur:

Stuck Off (after two occurrences)

When the 1-2 SS valve is stuck OFF, first gear is commanded and the ratio is equal to second gear for greater than 2.0 seconds, and 4th gear is commanded with the TCC locked and the ratio equals 3rd gear for greater than 3 seconds.

Stuck On (after five occurrences)

When the 1-2 SS valve is stuck ON, second gear is commanded and the ratio is equal to first gear for greater than 3 seconds.

Action Taken When the DTC Sets
    • The VCM illuminates the malfunction indicator lamp (MIL) for California emissions vehicles.
    • The VCM commands maximum line pressure.
    • The VCM inhibits 3-2 downshifts above 25 mph.
    • The VCM freezes shift adapts.
    • The VCM stores DTC P0751 in VCM history.

Conditions for Clearing the MIL/DTC
    • For California emissions, the VCM turns OFF the MIL during the third consecutive trip in which the diagnostic test runs and passes.
    • A scan tool clears the DTC from VCM history.
    • For California emissions, the VCM clears the DTC from VCM history if the vehicle completes 40 consecutive warm-up cycles without an emission related diagnostic fault occurring.
    • For Federal emissions, the VCM clears the DTC from VCM history if the vehicle completes 40 consecutive warm-up cycles without a non-emission related diagnostic fault occurring.
    • The VCM cancels the DTC default actions when the fault no longer exists and the ignition switch is OFF long enough in order to power down the VCM.

Diagnostic Aids
    • Verify that the transmission meets the specifications in the Shift Speed table.
    • Other internal transmission failures can cause more than one shift to occur.
    • First diagnose and clear any engine DTCs or TP sensor codes that are present. Then inspect for any transmission DTCs that may have reset.
    • Refer to Shift Solenoid Valve State and Gear Ratio table.

Test Description

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

  1. This step tests the function of the TFP manual valve position switch.

    2.

  2. This step tests whether the scan tool commanded all the shifts, or whether all the shift solenoids responded correctly, but all of the correct shifts did not occur.

P0751

Step

Action

Value(s)

Yes

No

1

Did you perform the Powertrain Diagnostic System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check 5.0/5.7L or Powertrain On Board Diagnostic (OBD) System Check 7.4L

2

  1. Install a Scan Tool .
  2. With the engine OFF, turn the ignition switch to the RUN position.

    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 VCM.

  3. Record the Freeze Frame and Failure Records.
  4. Clear the DTC.
  5. Select TFP switch A/B/C on the scan tool.
  6. With the engine operating, apply the brake pedal, and select each transmission range: D1, D2, D3, D4, N, R, and P.

Refer to the Transmission Fluid Pressure Manual Valve Position Switch Logic table.

Does each selected transmission range match the scan tool TFP switch display?

--

Go to Step 3

Go to DTC P1810 TFP Valve Position Switch Circuit

3

  1. Raise and support the drive axle assembly.
  2. Start the engine.
  3. With the transmission in D4 range, use the scan tool in order to command first, second, third, and fourth gears while accelerating the vehicle.

Did you detect a 2-2-3-3 or 1-1-4-4 shift pattern only? (You may need to road test the vehicle).

--

Go to Step 4

Go to Diagnostic Aids

4
  1. Inspect the shift solenoid/hydraulic circuit for a 1-2 SS valve internal malfunction.
  2. Inspect the shift solenoid/hydraulic circuit for a stuck 1-2 shift valve.
  3. Inspect the shift solenoid/hydraulic circuit for damaged seals on one or both of the shift solenoids. The shift solenoids can cause oil to leak into the second gear clutch pack.

    4. Refer to Symptom Diagnosis .

Was the condition found?

--

Go to Step 5

Go to Diagnostic Aids

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 (if traffic and road conditions permit):
    4. • Accelerate the vehicle with the TP angle greater than 12% up to 55 mph.
    • The VCM must see the proper ratio for each commanded gear for greater than one second in D1, D2, D3, and D4 (wait for the TCC Lock-up in 4th gear).
  4. Select Specific DTC.
  5. Enter DTC P0751.

Has the test run and passed?

--

System OK

Go to Step 1

DTC P0751 1-2 Shift Solenoid Valve Performance Diesel


Object Number: 40795 Size: MF
Automatic Transmission Controls Schematics Diesel

Circuit Description

The 1-2 shift solenoid (SS) valve controls the fluid flow action on the 1-2 and the 3-4 shift valves. The 1-2 SS valve is a normally-open exhaust valve that is used with the 2-3 shift solenoid (SS) valve to allow four different shifting combinations.

When the powertrain control module (PCM) detects a 2-2-3-3 gear ratio, then DTC P0751 sets. DTC P0751 is a type B DTC.

Conditions for Running the DTC
    • The system voltage is 8.0-18.0 volts.
    • The engine speed is greater than 475 RPM for greater than 7 seconds.
    • No OSS DTCs P0722 or P0723.
    • No ISS DTCs P0716 or P0717.
    • No TFP manual valve position switch DTCs P1810, P1815, P1816, P1817 or P1818.
    • No shift solenoid electrical DTCs P1842, P1843, P1845 or P1847.
    • TFT is 20-130°C (68-266°F).
    • The APP angle is greater than 10%.
    • The vehicle speed is greater than 3.2 km/h (2 mph).
    • The engine torque is 108-644 N·m (80-475 lb ft).
    • The engine speed is below 3,750 RPM.
    • Not in Four Wheel Drive Low.
    • No Four Wheel Drive DTC P1875.
    • No TCC PWM solenoid valve DTC 1860.
    • No MAP DTCs P0106, P0107 and P0108.

Conditions for Setting the DTC

Stuck Off (for two occurrences)

    • First gear is commanded with a ratio equal to second gear for greater than 1.5 seconds.
    • Fourth gear is commanded, with TCC applied or locked, with a ratio equal to third gear for greater than 3.5 seconds.

Stuck On (for five occurrences)

Second gear is commanded and the gear ratio equals first gear for greater than 2.2 seconds.

Action Taken When the DTC Sets
    • The PCM illuminates the malfunction indicator lamp (MIL).
    • The PCM commands maximum line pressure.
    • The PCM freezes shift adapts.
    • The PCM stores DTC P0751 in the PCM history.

Conditions for Clearing the MIL/DTC
    • For Federal and California emissions, the PCM turns OFF the MIL during the third consecutive trip in which the diagnostic test runs and passes.
    • A scan tool clears the DTC from PCM history.
    • For Federal and California emissions, the PCM clears the DTC from PCM history if the vehicle completes 40 consecutive warm-up cycles without an emission related diagnostic fault occurring.
    • For vehicles equal to or greater than 15,000 lbs GVW, the PCM clears the DTC from PCM history if the vehicle completes 40 consecutive warm-up cycles without a non-emission related diagnostic fault occurring.
    • The PCM cancels the DTC default actions when the fault no longer exists and the ignition switch is OFF long enough in order to power down the PCM.

Diagnostic Aids
    • Verify that the transmission meets the specifications in the Shift Speed table.
    • Other internal transmission failures may cause similar shift patterns.
    • First, diagnose and clear any engine DTCs or APP sensor codes that are present. Then inspect for any transmission DTCs that may have reset.
    • Refer to Shift Solenoid Valve State and Gear Ratio table.

Test Description

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

  1. This step tests the function of the TFP manual valve position switch.

    2.

  2. This step tests whether the scan tool commanded all the shifts, or whether all the shift solenoids responded correctly, but all the shifts did not occur.

DTC P0751 (Diesel)

Step

Action

Value(s)

Yes

No

1

Did you perform the Powertrain Diagnostic System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

  1. Install a Scan Tool .
  2. With the engine OFF, turn the ignition switch to the RUN position.

    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 PCM.

  3. Record the Freeze Frame and Failure Records.
  4. Clear the DTC.
  5. Select TFP switch A/B/C on the scan tool.
  6. With the engine operating, apply the brake pedal, and select each transmission range: D1, D2, D3, D4, N, R, and P.

Refer to the Transmission Fluid Pressure Manual Valve Position Switch Logic table.

Does each selected transmission range match the scan tool TFP switch A/B/C display?

--

Go to Step 3

Go to DTC P1810 TFP Valve Position Switch Invalid Range

3

  1. Raise and support the drive axle assembly.
  2. Start the engine.
  3. With the transmission in D4 range, use the scan tool in order to command first, second, third, and fourth gears while accelerating the vehicle.

Was a 2-2-3-3 shift pattern detected? (You may need to road test the vehicle).

--

Go to Step 4

Go to Diagnostic Aids

4
  1. Inspect the shift solenoid/hydraulic circuits for a 1-2 or 3-4 SS valve internal malfunction.
  2. Inspect the shift solenoid/hydraulic circuit for damaged seals on one or both of the shift solenoids.
  3. Refer to Symptom Diagnosis .

Was the condition corrected?

--

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 (if traffic and road conditions permit):
    4. • Place the transmission in D4.
    • Hold the throttle greater than 10%, and accelerate to 55 mph.
    • The PCM must see the proper gear ratio for each commanded gear, for greater than one second in D1, D2, D3, and D4 with TCC locked.
  4. Select Specific DTC.
  5. Enter DTC P0751.

Has the test run and passed?

--

System OK

Go to Step 1