4L80-E / 4L85-E Automatic Transmission

Diagnostic Chart (Part 1 Of 3):

Diagnostic Chart (Part 2 Of 3):

Diagnostic Chart (Part 3 Of 3):

Schematic:

CIRCUIT DESCRIPTION
The Pressure Control (PC) valve is a Vehicle Control Module (VCM) controlled device that regulates the transmission line pressure. The VCM compares Throttle Position (TP) sensor voltage, engine RPM, and other inputs in order to determine the appropriate line pressure for a given load. The VCM regulates the pressure by applying a varying duty cycle to the PC solenoid valve based on the resistance of the solenoid, and the amperage needed to obtain optimum line pressure. The applied amperage varies from 0.1 amps for maximum line pressure to 1.1 amps for minimum line pressure.

If the VCM detects a PC duty cycle that exceeds 95% or less than 1.9%, then DTC P0748 sets. DTC P0748 is a type C DTC.

CONDITIONS FOR RUNNING THE DTC
^ The system voltage is greater than 10 volts at -40°C TFT or 12.5 volts at 150°C TFT.
^ The engine must be running more than 475 RPM for at least 7 seconds, and not in fuel cutoff.

CONDITIONS FOR SETTING THE DTC
^ The PC duty cycle exceeds 95% or is less than 1.9%.
^ All conditions are met for 200 milliseconds (0.2 second).

ACTION TAKEN WHEN THE DTC SETS
^ The VCM does not illuminate the Malfunction Indicator Lamp (MIL).
^ The VCM disables the PC solenoid valve, defaulting the transmission to maximum line pressure.
^ The VCM freezes shift adapts.
^ The VCM records the operating conditions when the Conditions for Setting the DTC are met. The VCM stores this information as Failure Records.
^ The VCM stores DTC P0748 in VCM history.

CONDITIONS FOR CLEARING THE DTC
^ A scan tool clears the DTC from VCM history.
^ The VCM clears the DTC from VCM history if the vehicle completes 40 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
^ First diagnose and clear any engine DTCs or TP sensor codes that are present. Then inspect for any transmission DTCs that may have reset.
^ Inspect for improperly wired after-market equipment.
^ Inspect the connectors at the VCM, the AT wiring harness and all other circuit connecting points for an intermittent condition. Refer to Testing for Intermittent and Poor Connections in Diagrams.
^ Inspect the circuit wiring for an intermittent condition. Refer to Testing for Electrical Intermittents in Diagrams.

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table.
2. This step tests the ability of the VCM to command the PC solenoid valve.
3. This step tests the PC solenoid valve and the AT wiring harness assembly for incorrect resistance
6. This step tests the PC solenoid valve and the internal wiring harness for a short to ground.
10. This step tests the entire PC solenoid valve circuit up to the VCM for continuity.
11. This step tests for a short to ground in circuits 1228 and 1229, of the engine harness.