* PCM as the Central Controller: The PCM is the "brain" of your truck's engine and drivetrain. It controls fuel delivery, ignition timing, and various other systems, including the climate control. A malfunctioning PCM could cause a misfire (bogging), and a safety feature might switch the AC to defroster as a consequence of the overall system problem.
* Possible Scenarios:
* PCM Malfunction: The most likely culprit. A failing PCM can exhibit erratic behavior, affecting multiple systems at once.
* Wiring Harness Issues: Damaged or corroded wiring between the PCM, TPS (Throttle Position Sensor – which you already replaced), mass airflow sensor (MAF), and other sensors could cause inconsistent signals, leading to the symptoms you're experiencing. Look for chafed wires, especially near the engine and firewall.
* Faulty Sensor (other than TPS): Although you replaced the TPS, other sensors like the MAF sensor, crankshaft position sensor (CKP), or camshaft position sensor (CMP) can cause similar issues. A faulty sensor will send incorrect data to the PCM leading to incorrect fuel and ignition control.
* Vacuum Leaks: A significant vacuum leak could affect both engine performance (bogging) and climate control operation (switching to defroster).
* Troubleshooting Steps:
1. Check Engine Light: Is the check engine light on? If so, get the diagnostic trouble codes (DTCs) read using an OBD-II scanner. This will provide valuable clues about the potential problem.
2. Inspect Wiring: Carefully examine the wiring harness connecting the PCM to all relevant sensors and actuators. Look for any signs of damage, corrosion, or loose connections. Pay special attention to areas that experience vibration or heat.
3. Check Other Sensors (MAF, CKP, CMP): Have these sensors tested. A simple test for the MAF is to disconnect it and see if the engine runs differently. (Expect rough running, but it can point to a problem). Testing the others usually requires a scan tool or multimeter and specific knowledge of the sensor's operation.
4. Vacuum Leak Test: A mechanic can perform a vacuum leak test to identify any leaks in the intake system.
5. PCM Testing: This requires specialized equipment and is best left to a qualified mechanic or an automotive electrician. PCM testing or replacement is usually a last resort after thoroughly checking everything else.
Important Considerations:
* Professional Diagnosis: Given the complexity of the issue, it's highly recommended to take your truck to a qualified mechanic for diagnosis and repair. Troubleshooting this yourself without the proper tools and knowledge could lead to further damage or wasted time and money.
* Safety: Driving with these symptoms can be unsafe. Limit your driving until the problem is diagnosed and repaired.
By systematically checking these areas, you should be able to narrow down the cause of the problem. Remember, professional help is often the most efficient and safest approach to diagnose and repair complex automotive issues.
