GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2009 🢒 Sierra Denali 🢒 Transmission 🢒 Automatic Transmission - Allison 🢒 DTC P0776 or P0777

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC P0776: Pressure Control (PC) Solenoid 2 - Stuck Off

DTC P0777: Pressure Control (PC) Solenoid 2 - Stuck On

Circuit/System Description

The transmission has 2 pressure control solenoids that controls the flow of transmission fluid to engage clutches. The flow is directed through shift valves that allows it to engage different clutches. The vehicle speed sensor and automatic transmission turbine speed sensor contains a permanent magnet surrounded by a coil that gives off a continuous magnetic field. As the vehicle is driven, a speed sensor rotor located near the magnetic pickup of the speed sensor coil also rotates. This rotation produces a variable AC voltage signal. The frequencies of the signals are proportional to the transmission turbine and output speeds. The transmission control module (TCM) uses the signals to calculate the gear ratio. The TCM compares the computed ratio to the commanded gear ratio.

Conditions for Running the DTC

P0776

    • Output speed is greater than 125 RPM.
    • Turbine speed is greater than 60 RPM.
    • Transmission is at normal operating temperature.
    • DTCs P0716, P0717, P0721, P0722, P0877, and P0878 are not active.

P0777

    • Output speed is greater than 200 RPM.
    • Turbine speed is greater than 200 RPM.
    • Transmission is at normal operating temperature.
    • DTCs P0716, P0717, P0721, P0722, and P0877 are not active.

Conditions for Setting the DTC

P0776

This DTC sets during a shift when the calculated commanded gear ratio differs from the known commanded gear ratio for more than 2 seconds.

P0777

This DTC sets during a shift when the off going gear ratio remains the same even though it is commanded off.

Action Taken When the DTC Sets

P0776

    • The TCM requests the engine control module (ECM) to illuminate the malfunction indicator lamp (MIL).
    • If transmission was in a forward range, transmission will shift back to the previous range. If the transmission was in reverse or neutral, the transmission will shift to neutral.
    • While diagnostic response is active, if shift selector is moved to neutral, the transmission will shift to neutral. If the transmission is shifted to reverse (R), the transmission will shift to reverse or neutral. If the shift selector is moved to a forward range and transmission is compromised by overspeeding or direction change, transmission shifts to neutral.
    • DTC P0776 is stored in TCM history.
    • The TCM inhibits torque converter clutch (TCC) engagement.

P0777

    • The TCM requests the ECM to illuminate the MIL.
    • If transmission was in a forward range, transmission will shift back to the previous range. If the transmission was in reverse or neutral, the transmission will shift to neutral.
    • While diagnostic response is active, if shift selector is moved to neutral, the transmission will shift to neutral. If the transmission is shifted to R (reverse), the transmission will shift to reverse or neutral. If the shift selector is moved to a forward range and transmission is compromised by overspeeding or direction change, transmission shifts to neutral.
    • DTC P0777 is stored in TCM history.
    • The TCM inhibits TCC engagement.

Conditions for Clearing the DTC

DTCs P0776 and P0777 are Type A DTCs.

Diagnostic Aids
    • You will have to drive the vehicle in order to experience a condition.
    • DTC may indicate a component condition with piston seals, clutch plates, or solenoids.
    • DTC may be caused by a faulty speed sensor signal.

Reference Information

Schematic Reference

Automatic Transmission Controls Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

    •  Transmission General Description
    •  Transmission Component and System Description
    •  Electronic Component Description

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification
  1. Inspect the transmission fluid level. Refer to Transmission Fluid Check.
  2. Perform a road test. Refer to Road Test.
  3. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed on the Freeze Frame/Failure Records data.

Circuit/System Testing

    Note: You must perform the Circuit/System Verification first.

  1. Perform a line pressure test. Refer to Line Pressure Check.
    2. ⇒If line pressure is low in all ranges, refer to Low Main Line Pressure in All Ranges.
    ⇒If line pressure is low in a specific range or ranges, refer to Low Main Line Pressure in Specific Ranges, Normal Pressure in Other Ranges.

    Warning: Chock the wheels to prevent the vehicle from moving. Failure to chock the wheels can cause personal injury when the electrical system is repaired.

  2. Perform the clutch test. Refer to Clutch Test.
    3. ⇒If not the specified value, refer to Excessive Slippage and Clutch Chatter.
  3. If the above test is normal, refer to Functional Test.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Speed Sensor Replacement
    •  Control Valve Solenoid Replacement
    •  Control Valve Body Cleaning and Inspection

Repair Verification

If another DTC is present, refer to Diagnostic Trouble Code (DTC) List - Vehicle and perform the appropriate diagnostic procedure.