Owner's Problem Description
The owner reports a P0748 code on a 2001 Dodge Ram 1500 that was thrown about a year ago. The truck has been driven since then without repairs. The owner inspected the wiring harnesses and connectors outside the transmission, finding them intact. He swapped the transmission control relay with a working one, but the issue persists. Considering replacing the governor pressure solenoid and adding clean transmission fluid, but is uncertain whether the PCM/ECM might be at fault.
Professional Technical Analysis
The 2001 Dodge Ram 1500 equipped with a 4.7L V8 engine and a 545RFE transmission is showing a P0748 code, which indicates a governor pressure sensor or circuit malfunction. The vehicle has been driven approximately 150,000 miles, and the owner reports intermittent issues with shifting. Initial inspections of the wiring harnesses and connectors reveal no visible damage. The transmission control relay was swapped with a known good one, but the code remains. Given the nature of the P0748 code, a thorough investigation of the PCM connections and harness integrity is required. The governor pressure solenoid is a common failure point in this transmission system, especially in high-mileage vehicles. Testing the solenoid's resistance should yield readings between 10-20 ohms, while the voltage supply should be checked for consistent 12V when activated. Additionally, any frayed wires or poor connections in the harness can lead to erratic behavior, as evidenced by intermittent operation. Current vehicle conditions suggest a need for a deeper electrical diagnosis, including potential PCM failure, which may not be evident without further testing. Notably, internal transmission issues could also contribute to fault codes, and careful examination of fluid condition and internal components may uncover additional problems.
Possible Causes
Most common causes (ordered by frequency):
- Governor Pressure Solenoid Failure: The governor pressure solenoid can fail internally, causing incorrect pressure readings. Resistance should be checked; normal readings are between 10-20 ohms. If the solenoid is faulty, it may cause erratic shifting and trigger the P0748 code. Replacement is often necessary if testing reveals failure. - Intermittent shifting issues, P0748 code, resistance readings outside the normal range.
- Wiring Harness Integrity Issues: Inspect the wiring harness for frayed or damaged insulation, particularly near connectors. Poor connections can lead to voltage drops and intermittent signals, causing the P0748 code to be triggered. - Visual inspection reveals no damage; however, continuity testing may show intermittent connections.
- PCM Malfunction: The powertrain control module may be malfunctioning, causing it to misinterpret signals from the governor pressure sensor. Testing the PCM may require specialized tools to verify its performance and integrity. - Unlikely to visibly inspect; requires advanced diagnostic tools for verification.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Code Scan and Review: Use a professional OBD-II scanner or GeekOBD APP to retrieve and review any fault codes. Pay particular attention to the freeze frame data, which can provide insights into the vehicle's operating conditions when the fault was recorded.
- STEP 2 - Visual Inspection: Conduct a thorough visual inspection of the transmission harness and connectors for signs of wear or damage. Ensure all connectors are securely seated and check for any corrosion that may affect connectivity.
- STEP 3 - Resistance Testing: Test the governor pressure solenoid's resistance using a multimeter. Confirm that the reading is within the specified range (10-20 ohms). If the reading is outside this range, the solenoid likely needs replacement.
- STEP 4 - Check PCM Connections: Inspect the PCM connectors for any signs of damage or loose connections. Ensure that all pins are properly seated and free of corrosion. Testing continuity from the PCM to the governor solenoid may reveal issues.
