P0748

P0748 WITH 4 SPEED AUTOMATIC

Diagnostic Chart:

Wiring Diagram:

CIRCUIT DESCRIPTION

The Powertrain Control Module (PCM) uses the Pressure Control Solenoid Valve (PC Sol. Valve) in order to regulate the transmission line pressure. The PC Sol. Valve consists of an electrical connector, a coil, an armature, a regulating spring, and a poppet valve. The PC Sol. Valve is attached to the upper control body. The PCM compares Throttle Position (TP) voltage, fluid temperature, gear states, and other inputs in order to determine the line pressure appropriate for a given load. The PCM regulates the pressure by applying a varying amperage to the PC Sol. Valve. The applied amperage can be 0.1-1.0 amps. The PCM then monitors the amperage at the return line.

When the PCM detects a continuous open or short to ground in the PC Sol. Valve circuit, or the PC Sol. Valve, then DTC P0748 sets. DTC P0748 is a type D DTC.

CONDITIONS FOR SETTING THE DTC

^ The actual return amperage is 0.16 amps higher or lower than the desired amperage.

^ The system voltage is 10-16 volts.

ACTION TAKEN WHEN THE DTC SETS

^ The PCM freezes shift adapts from being updated.

^ The PCM commands maximum line pressure (0 amps).

^ DTC P0748 is stored in the PCM history.

^ The PCM does not illuminate the Malfunction Indicator Lamp (MIL).

CONDITIONS FOR CLEARING THE DTC

^ A scan tool can clear the DTC from the PCM history. The PCM clears the DTC from the PCM history if the vehicle completes 40 warm-up cycles without a failure reported.

^ The PCM cancels the DTC default actions when the fault no longer exists and the ignition is OFF long enough in order to power down the PCM.

DIAGNOSTIC AIDS

^ Inspect the wiring for poor electrical connections at the PCM. Inspect the wiring for poor electrical connections at the transmission 20-way connector. Look for the following conditions:

- A bent terminal

- A backed out terminal

- A damaged terminal

- Poor terminal tension

- A chafed wire

- A broken wire inside the insulation

^ When diagnosing for an intermittent short or open condition, massage the wiring harness while watching the test equipment for a change.

^ Extended engine cranking with a weak battery could set DTC P0748.

TEST DESCRIPTION

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

2. This Step verifies the ability of the PCM to command the PC Sol. Valve.

3. This Step verifies that the Automatic Transmission Wiring Harness Assembly and the PC Sol. Valve have correct resistance.

8. This Step tests circuit 1229 of the A/T Wiring Harness Assy. for a short to ground.

9. This Step tests circuit 1228 from the engine 20-way connector to the PC Sol. Valve for a short to ground.

11. This Step tests circuit 1229 from the engine 20-way connector to the PCM for an open.

12. This Step tests circuit 1229 from the engine 20-way connector to the PCM for a short to ground.

16. This Step tests circuit 1228 from the engine 20-way connector to the PCM for a short to ground.

20. When DTC P0748 sets, the PCM commands maximum line pressure and does not inhibit TCC engagement. This can lead to excessive heat build-up, causing the Torque Converter to "balloon." You must replace the Torque Converter.